In Artsʿakh (today's Karabagh), in the Middle Ages also known as Khachʿen after the name of the river crossing its central district, and in the province of Nakhichevan, as in the rest of historical Armenia, from the fourth century AD until modern times, cult architecture has been the main manifestation of an art almost entirely devoted to the Christian religion. Despite the tragic trials of history, and in particular the considerable destruction suffered between the end of the 19th and the beginning of the 21st century 1 -and up until today in these two provinces-, hundreds of monastic complexes, churches and chapels, probably around a thousand heritage units in total, have either survived to the present day or are fairly well attested in the sources available. We do not include here the innumerable "minor" monuments corresponding primarily to the cross plaques called khachʿkʿar, whose initial number of several thousands was drastically reduced by the destruction of the very large cemetery of Julfa, Nakhichevan, at the beginning of this century 2 .

Muslim worship is also represented in these provinces by several dozen monuments. The oldest are in the lower areas of Nakhichevan, where an Arab emirate was established during the Caliphate (8th-10th centuries), followed by Seljuk, Mongol and Turkmen political constituencies (11th-15th centuries). The latter were accompanied by the settlement of Muslim populations on the edges of the mountains that remained under Armenian rule, both in Nakhichevan and in eastern Artsʿakh. There are several Muslim mausoleums of the 12th-15th centuries, some of which are of remarkable quality, in brick in Nakhichevan, and one in stone, in the eastern corner of Artsʿakh, dated to 1314, which will [p. 66] be discussed later. A few other Islamic monuments reflect the establishment, in the second half of the 18th century, of a Turkic-Tatar power in the heart of the mountainous massif of Artsʿakh, then called Karabakh: two mosques in the town of Shushi are particularly noteworthy: the "upper" one, built in 1883, and the "lower" one, built in 1875.

The civil and military architecture is illustrated in particular by the remains of medieval fortresses, one hundred and twenty-four in total for the two provinces, caravanserais and numerous bridges (fifty-three in what was, until the autumn of 2020, the territory of the republic of Artsʿakh; and twenty-six in Nakhichevan), reflecting the volume of trade throughout the centuries. Artsʿakh, as the neighbouring 1 . See H. Khatchadourian's chapter in this volume. 2 . See A. Ayvazyan's chapters in this volume. province of Siunikʿ, also preserves the remains of several princely palaces from the modern period: the residences of the meliks of the 17th and 18th centuries.

Also in Artsʿakh, despite the social upheavals of the Soviet period, the folk architecture still includes some ancient specimens of wooden peasant houses, of the gĕlkhatun type, with a form of dome called hazarashēn with beams staggered in relation to each other, and a central skylight called erdik. It is likely that this very ancient tradition was one of the sources of monumental architecture, particularly for monastic buildings with a truncated dome. More numerous are the large stone houses covered with tiles (or metal sheets), built from the end of the 19th century onwards, which have features common to regions with a relatively mild climate, from the Balkans to the Caucasus, in particular long balconies with openwork wooden balustrades on one or more façades.

In Nakhichevan and Artsʿakh, the cities of Agulis and Shushi experienced a great economic and cultural boom, the former in the 17th and 18th centuries, the latter in the 19th century. In addition to the construction of numerous Christian buildings, this development resulted in the flourishing of urban architecture, which was one of the few examples in the Armenian world to be preserved until after the First World War. Both cities were largely destroyed when they were taken by the Turkish and (newly formed) Azerbaijani armies in 1918 and 1920 respectively.

Building Material and Techniques

The main material is volcanic stone, particularly basalt in the southeastern and eastern provinces; limestone, felsite, granite and schist are also used in construction, while tuff is less common here than in the more central regions around the Ararat plain. Although very present in the forests of [p. 67] Artsʿakh-but rarer in Nakhichevan, as in the country as a whole-wood is hardly used in monumental construction. Earth-based composite materials are also rare. The province of Nakhichevan is characterised by a fairly extensive use of brick in the late Middle Ages, especially for domes and the drums supporting them, probably to reduce loads and facilitate construction.

Subject to the same requirements, to be not only static but also seismically resistant, as in the whole of Armenia, the vast majority of the church buildings in the south-east and the east of the country are built employing the traditional technique of concrete between facings. This technique is similar to the opus caementicium used by the Romans. The stone is broken into small blocks and mixed with sand and lime mortar (not clay, as in antiquity) to form the wall core, while on both sides of the walls, the blocks, more or less regularly cut, form a covering. On the large buildings of the cities and monasteries, this cladding is perfectly rectilinear and the façades are bonded with great care. On the other hand, in Artsʿakh, as in many other provinces of Armenia, more modest monuments use stones that are less suitable for regular cutting, which gives the walls a more rustic appearance. Atypical cases can also be observed: for example, at Bĕṙi Ełtsʿi in Artsʿakh a "pseudo-cyclopean" masonry is juxtaposed with horizontal courses of thin limestone plates. In churches that were intended to be decorated with paintings, for example in the Middle Ages in Dadivankʿ in Artsʿakh, and even more so during the modern period in many buildings in Nakhichevan, the inner walls were covered with plaster.

On the roofs-in those parts of Armenia where ancient roofs are preservedceramic tiles, heir to the ancient Roman tegula, are attested up to the end of the 7th century; from the 9th century onwards, they were replaced by stone slabs. These slabs, fitted in principle like the tiles, give the domes a flat surface and characteristic Armenian shapes: pyramidal, conical or umbrella-like. Artsʿakh, however, is one of the few provinces of Armenia to maintain, here and there, the use of tiles throughout the Middle Ages. The picturesque umbrella dome, which is one of the remarkable creations of medieval Armenian architecture, present in Artsʿakh in the 13th century-for example in Gandzasar and in Gĕtchʿavankʿ (Monastery of Gĕtichʿ)enjoyed great popularity in Nakhichevan in the modern period. Until the war in the autumn of 2020, this type of dome displayed a particular elegance on Shushi Cathedral.

Taking advantage of the stones' chromatic richness, builders often resorted, from the 13th century onwards, to the use of bicolour contrasts to embellish monuments, which they systematised at the end of the Middle Ages, particularly in alternating horizontal bands and in a chequered pattern. The churches [p. 68] [fig. 3.1] of Nakhichevan provided many examples of this (Figure 3.2). They also sometimes used contrasting materials, stone and brick (Figure 3.4). The church of St John the Baptist (Surb Yovhannēs Mĕkĕrtichʿ), known as Kanachʿ Zham (i.e. 'Green Church'), in Shushi, also illustrates a 19th-century decorative use of different materials and masonries-in this case, probably inspired by Western Europe.

Volcanic facing stones are relatively easy to hew and they lend themselves well to bas-relief sculpture, often with a flat surface, carved with wedge-shaped furrows rich in contrasts of light and shadow. On them, the great diversity of the bands of vegetal and geometric ornaments is effectively highlighted. This same technique is also employed on the countless khachʿkʿars. However, more modelled and more prominent sculptures are also used, especially in Artsʿakh on the great monuments of the 13th century, such as Dadivankʿ and Gandzasar; as well as, sometimes, during the modern period in Nakhichevan, for example in Tsʿĕłna. The three-dimensional angels on the bell tower of the Cathedral of the Holy Saviour (Łazanchʿetsʿotsʿ Surb Amenapʿĕrkichʿ) in Shushi, set in the round, represent the culmination of this trend in the 19 th century.

[ p. 69] [fig. 3.2, 3.3] [p. 70] [fig. 3.4] In addition to the technical field, the omnipresence and the importance of inscriptions engraved on the stone of the façades and the tympana of the doors should be underlined (Figure 3.5). Inherited from classical antiquity, the principle of lapidary inscriptions has been maintained throughout the centuries, up to the present day, on a large number of monuments throughout Armenia, and in particular in Artsʿakh and Nakhichevan. Citing the sponsors, and occasionally the craftsmen, relating the motivations and circumstances of the foundations, and recording their date, these engraved texts constitute invaluable historical documents. [p. 71] [fig. 3.5] In the ecclesiastical architecture of Armenia, the compositions are divided into two groups: those without a dome, longitudinal, with one or three naves, inherited from the pre-Christian past and relatively easy to build; and those with a dome, better adapted to the Christian faith, in particular to the Christology adopted by the Armenian Church, founded on the dogma of the Council of Ephesus (431), the Formulary of Reunion of 433 and the writings of Cyril of Alexandria who defined the person of Jesus Christ as follows: "One is the incarnate nature of God the Word". The unity of the space covered by the dome is the architectural translation of this (Figure 3.6).

Semantics and Symbolism of Forms

The domed churches are, in turn, subdivided into compositions: a) with a centred, cruciform or radiating plan, b) with a longitudinal plan of an inscribed cross, with supports attached to the walls or with four free supports.

According to the 5th-century Armenian author Agathangełos, during the Christianisation of the country in the early 4th century, a dome that appeared to Saint Gregory the Illuminator in a vision prefigured the mother church, the Ēdjmiatsin Cathedral3 . Agathangełos describes the forms shown to him in [p. 72] [fig. 3.6] the vision: "And on the crosses of these four columns, amazing arches joined. And over this I saw a building made of clouds in the form of a dome-shaped cube4 , an astonishing divine creation" (Agathangełos, 102, 16); then he gives the explanation: "The arches that connected the columns are the sign of the equality and unity of the universal Church; and the dome [khoran5 ] above is the image of the supreme city, the gathering place of the kingdom of heaven" (Agathangełos,102,(53)(54).

This text, which probably served as a guide for medieval builders, sets out the essential principle of Armenian architecture, the domed baldachin, and its meaning, the Church united under the symbol of the kingdom of God. It helps us to understand why the dome, standing at the intersection of the arms of the cross or above a radiance, crowns so many church buildings in Armenia. In Artsʿakh, this paradigm is present both on chapels, such as the one at Vankʿasar [p. 73] (7th century), and on the main monastic churches of the 13th century: Dadivankʿ, Gandzasar and Gĕtchʿavankʿ, as well as on some notable sanctuaries of later periods.

In Nakhichevan, too, this same principle was of course applied throughout the centuries, with several variations, but the buildings rarely retained their former state. This principle was applied again during the revival of the 17th-19th centuries, in a large number of sanctuaries, which were preserved until the end of the 20th century. The builders and their patrons had a clear preference for one type of church: they built churches in the form of an inscribed cross, with a dome on four free supports, and they covered the entire province with them.

The dome is always preceded by a drum-when it crowns a church and not a narthex or another conventual building. Depending on the configuration of its base, the drum may be polygonal or cylindrical. The drum is a multifunctional element: it raises the dome, reinforcing its symbolic significance and underlining the vertical axis of the building; it also allows the interior of the church to be illuminated by a quasizenithal light, which contributes to this same semantics.

Churches-places where parishioners gather for liturgy-can also be more modest buildings where the dome and the cross are not present in the architectural structures. In Artsʿakh, in the ordinary communities of the mountain villages and in the humble hermitages, there is a large number of elongated, undomed churches, especially with a single nave. In Nakhichevan, as in the whole country, three-aisled basilicas without a dome reappeared in large numbers during the modern period. In these simple compositions, deprived of the hemisphere of the dome, it is the quarter sphere (the conch) covering the apse, at the eastern end of the sanctuary, that evokes the celestial vault and the kingdom of God.

Whatever their composition, the buildings of worship in all the provinces of Armenia, as in principle in the whole Christian world, are oriented, that is to say are turned towards the east (sometimes with a slight deviation). Indeed, the east, from which the sun spreads its rays over the world every morning, is, according to the Gospel (Matthew 24:27), the direction from which Jesus Christ will return for the Last Judgment. At the eastern end of the church, which is called apse (khoran in Armenian), on an elevation of a few steps (bem in Armenian), whose height increases over centuries, is the altar table (sełan = table). This is the most important part of the sanctuary, where the Eucharist is celebrated. In the Armenian tradition, the apse is separated from the nave by a curtain which is drawn back at important moments of the liturgy, especially during the preparation of the Eucharistic species.

[p. 74] The apse is usually rounded on the inside (sometimes horseshoe-shaped). In Artsʿakh, a peculiarity can be observed in many single-nave chapels of the 13th century: the inner contour of the apse is rectangular. We will consider later a possible link of this type with a funerary function, attested in several of these chapels, given that Armenian mausoleums often have a flat eastern wall inside. In Armenian churches, the rectangular apse appears very rarely during the early Christian period, and only once at the beginning of the 10th century in Biurakan; it is a rare sight in Nakhichevan as well. On the other hand, in the 17th century there are many apses in this province with a polygonal internal contour: pentagonal or heptagonal. This feature can also be observed in the province of Vaspurakan (today in eastern Turkey) and in the Armenian communities of north-western Iran.

Early Christian and Pre-Arab Periods (Fourth-Seventh Centuries)

Before briefly mentioning the main stages in the history of Armenian sacred architecture in the following sections, it should be noted that the provinces of interest to us here suffered greatly from the strong shocks caused by the Arab occupation (8th-9th centuries), and then by the Turkish, Mongol and Turkmen invasions (11th-15th centuries). This is particularly true for the province of Nakhichevan, where the low-lying part of the Araxes valley was particularly vulnerable. To this must be added, still for Nakhichevan, the harsh ordeal of the deportation ordered at the very beginning of the 17th century by Shah Abbas of Persia. This is why very few buildings dating from the ancient periods (prior to the 13th century for Artsʿakh and to the 17th century for Nakhichevan) are preserved or attested and documented in these provinces.

The first stage in the history of Christian architecture in Armenia, the early Christian period (4th-6th centuries), is marked by the constitution of two architectural categories, that with a funerary function and that with a strictly ecclesiastical function.

In the first group, for the whole country, we know of a dozen of martyria and mausoleums, small rectangular hypogeal rooms, oriented and barrel-vaulted; several of them constituted the lower part of a two-level structure; most of them have an apse on the east side, which can be rounded or rectangular; arcosolia or funerary recesses can be dug within the side walls. Three of them are located in Artsʿakh, at Amaras, Tigranakert (Tigranocerta of Classical authors) and Vachaṙ, and have an eastern entrance, a feature unknown elsewhere and probably inspired by the aedicula of the Holy Sepulchre in Jerusalem.

In Nakhichevan, in the homonymous city, the remains of a two-storey dark tuff

[p. 75] [fig. 3.7]
mausoleum with an octagonal hypogeal vault and an oratory were reported until 1948. This small construction with a rough masonry, the dating of which is unknown, was considered by the population to be the mausoleum of the patriarch Noah 6 .

In the second category are numerous chapels and churches with a single nave and about ten three-nave basilicas, as well as the first chapels and churches with a dome. In Artsʿakh, the ruins of two single-nave sanctuaries, certainly early Christian (4th-6th centuries), were uncovered during the excavations of the city of Tigranakert/Tigranocerta (2005-2020) directed by Hamlet Petrosyan. Close and parallel to each other, these two churches constituted an ensemble of great interest (Figure 3.7), united by the attractiveness of the cruciferous stele monument that stood between them and by the presence of a mausoleum-reliquary under one of them. The traditional tripartite technique mentioned above was employed here, with lime mortar blocking between two faces of stone carefully cut on the outside. The church to the south was the [p. 76] [fig. 3.8] largest single nave in the area, and it retained two carved imposts (engaged capitals), probably from its western portal (Figure 3.8). It may have been flanked on the south by a baptistery, a very rare form in Armenia. Its north wall was pierced by two doors, also a rare feature, to open up to the common memorial space. The chapel to the north was probably a memorial shrine, as it contained a mausoleum under its apse, which was probably intended to preserve relics. This is one of the 6 . On the ancient tradition associating Nakhichevan with the site of the ark's landing, and on the apocryphal sources speaking of a city built by Noah after his exit from the ark, see M.E. Stone, 'Mount Ararat and the Ark', in M.E. Stone et al. (eds), Noah and His Book(s), Atlanta, 2010, pp. 307-316; M.E. Stone and A. Topchyan, Jews in Ancient and Medieval Armenia. First Century BCE to Fourteenth Century CE, Oxford, 2022, pp. 1-18. The Armenian apocryphal account providing an aetiological explanation of the name of the city of Nakhichevan-nax ('primary') + iĵewan ('place of descent', < [iĵ-], 'to descend') -as the Place of Noah's First Descent has been edited by W.L. Lipscomb, The Armenian Apocryphal Adam Literature, Atlanta, 1990, p. 204. three cases, unique to Artsʿakh, of a mausoleum opened to the east, which will be discussed in chapter 5.

In Nakhichevan, the single-nave, barrel-vaulted chapels and churches that we know of are certainly more recent. It should be noted, however, that of the fifteen or so single-nave churches in Nakhichevan, three had a configuration close to the early Christian types, with an annex chapel flanking the chevet (Holy Mother of God of Agulis, St John of Berdak and St George of Erĕndjak), which could betray an early origin.

[p. 77] A three-aisled basilica, probably partly early Christian, can be mentioned in these territories: that of Tsitseṙnavankʿ, which is well preserved thanks to several restorations. The chapter 5 gives an account of it.

As mentioned above, since its introduction during the early Christian period, the dome has tended to gather all the parts of the building under the central space that it crowned. Thus, from the end of the 6th and during the 7th century, many different dome compositions were created in Armenia, cruciform and radiating, centred and longitudinal, which are very important in the history of its architecture. These typologies are also present in Artsʿakh and Nakhichevan. In the former, the two main examples dating from this period will be presented in chapter 5. These are the monuments of Vankʿasar, with a dome on a free cross, and Ōkhtĕ Dĕṙni, with a dome on a radiation of four axial conches and four diagonal niches.

In Nakhichevan, a few structures with a dome on inscribed cross, monoconch and triconch, also existed in Julfa, Kukʿi and Nakhichevan, but their dating is uncertain and several late remodellings are attested. The above-mentioned Cathedral of St George in the city of Nakhichevan, rebuilt in 1869, possibly on the remains of an inscribed cross church with a dome on four free supports from the 7th century, will be commented upon later (see Figures 3.3 and 3.4).

Period of the Kingdoms (Tenth-Eleventh Centuries)

The post-Arab period (10th-11th centuries), during which several kingdoms more or less vassal to the Bagratid kingdom of Ani were formed, is illustrated throughout the country by a brilliant architectural flourishing. One has merely to recall the remarkable churches of Ałtʿamar in Vaspurakan and of Ani in Shirak. In the province of Artsʿakh, this period is represented by a few buildings. They are not precisely dated and are poorly preserved, but various clues allow us to link them to the same period. These include the cruciform domed chapels of Varazgom, Kʿashatʿał/Lachʿin district (see chapter 5) and, further north, St Sergius (Surb Sargis) of Getabek/Gədəbəy and St Sergius of Gag (on the border of the provinces of Gugarkʿ and Utikʿ, in the district of Kazakh/Gazakh/Kasāl [Qazax]), as well as the single-nave churches of Poladlı and Paṙisos in the district of Getabek/Gədəbəy.

Within this group, we should note the curious domed chapel of St Sergius of Gag, an inscribed monoconch cross, flanked on the north by a mononave chapel (Figure 3.9 and Figure 3.10). It is noteworthy in particular for two features: a) its unusual position, on a summit dominating the north-eastern limits of the Armenian Plateau, like the chapel at Vankʿasar; b) as at Varazgom, the [p. 78] [fig. 3.9, fig. 3.10] [p. 79] curious atrophy of its western arm, a tendency which is not entirely foreign to the region, but is generally much less marked than here. We shall return to these points later on.

Several sanctuaries in Nakhichevan, restored or rebuilt later, are supposed to have been founded during this period, but no tangible traces of them have been found. Today, unfortunately, seeking ancient evidence is no longer possible there.

The Feudal Period (End of Twelfth-Middle of Fourteenth Century)

There are many more works that attest, particularly in Artsʿakh, then called Khachʿen after the name of the river crossing its central district, to the architectural boom that Armenia experienced in the post-Seljuk period, under Queen Tamar of Georgia and her successors, and during the Mongol period (from the end of the 12th to the first half of the 14th century). As in the rest of the country, the princely families who dominated the province multiplied the number of foundations, particularly monastic ones. The monasteries were the main centres of spiritual, cultural and artistic life; they housed scriptoria in which numerous manuscripts were copied and illuminated. Most often fortified, these complexes reflect a search for geometric rigour in their composition and for balance in the layout of their components, as well as a concern for harmony with the surrounding nature. The monastery of Dadivankʿ (12th-13th century), which we will examine in chapter 5, is one of the largest conventual complexes in medieval Armenia, consisting of some twenty buildings divided into three groups, each with its own functional specificity: a) worship, b) activities of the brotherhood, c) housing.

Among the monastic churches in Artsʿakh, one can still find many single-nave, undomed, barrel-vaulted compositions under a saddle (two-pitched) roof. The reason for this is probably the ease of execution. The most characteristic example is found in the monastery of the Holy Apostle Elisaeus (Surb Ełishē or Ełisha Aṙakʿeal, district of Martakert), which will be discussed later, with no less than eight single-nave chapels arranged from north to south. As mentioned above, many of the Artsʿakh single-nave churches have a flat sanctuary, which, as we shall see, may be related to their funerary function. Murad Hasratyan and Jean-Michel Thierry have suggested that this is a remnant of the pagan past or a borrowing from the Nestorian churches of upper Mesopotamia (later, however). Another reason, as in medieval Europe, where the phenomenon is not uncommon, is the simplicity of construction. A large, wide church with a single nave and a rounded apse on the inside, whose superstructures have not [p. 80] been preserved, stood to the north of the monastery of Dadivankʿ. It probably housed the martyrium of the saintly dedicatee.

Apart from the particular phenomenon of single naves, the most common model in Armenian monasteries of the time is the domed church on a partitioned inscribed cross. This is a composition deriving from the "domed hall" of the 7th century, in which the interior cruciform volume is inscribed in a rectangular perimeter, the supports of the dome are attached to the wall perimeter, and two or four twostorey sacristy-chapels are housed in the corners.

This type can be seen in the churches of three monasteries in Artsʿakh: Dadivankʿ (1214), Gandzasar (1216Gandzasar ( -1238) ) and Gĕtichʿ (1241Gĕtichʿ ( -1246)). In the latter two sites, the dome adopts the picturesque shape of a pleated cone (or an umbrella dome) on the outside, which was widespread in Armenia from the 10th-11th centuries. These three monastic sanctuaries are among the best examples of medieval Armenian art, owing to their elegant proportions, the care taken in their masonry and the abundance of sculptural, notably figurative, decoration, to which in Dadivankʿ was added a painted decoration of high artistic quality.

Characteristic of Armenian monastic architecture, especially in the 13th century, the gawitʿ (identical from the 13th century onwards to zhamatun), a narthex used mainly as a mausoleum and probably also as a meeting room, is almost omnipresent in front of the west façade of the main churches. In Artsʿakh it takes various forms. Some archaic gawitʿs are reminiscent of early experiments in Siunikʿ (south-eastern Armenia) in the 9th-10th centuries, such as the simple barrel-vaulted room or the open gallery to the south. Examples can be seen at Dadivankʿ and St James of Mets Aṙankʿ (Mets Irankʿ). Some others are atypical, such as the asymmetrical hall of Gĕtichʿ, with two naves, a wide central one and a very narrow northern one.

The gawitʿ (or zhamatun) can also adopt the quadrangular hall composition with four pillars in the centre supporting a dome with a central skylight, which is attested for the first time in 1038 in Hoṙomos near Ani and was widespread throughout the country: this is the case in Dadivankʿ in 1224 and in Khatʿravankʿ in 1225 (Figure 3.11). Finally, the remarkable formula where the vault is supported by two pairs of intersecting arches can be observed at Bĕṙi Ełtsʿi (Figure 3.12), Gandzasar (1261) and Hoṙekavankʿ (1284). The Gandzasar gawitʿ, which has the same composition as those of Hałbat and Mĕshkavankʿ (northern Armenia), offers, in its centre, an example of a vault cut with stalactites (Figure 3.13), close to those seen at Haṙich and Gełard (early 13th century), as well as on Seljuk constructions in Asia Minor and Western Armenia.

In the large complex of Dadivankʿ, the group of southern buildings seems to include some spaces important for the life of the monastic community: a refectory and kitchen, a large room called tachar (a kind of chapter house) and a library with a scriptorium (functions remaining uncertain). At the south-[p. 81] [fig. 3.11] western end of the complex, a large two-storey building housed a monastic hostelry; it adjoins buildings that probably served as a prelature and a residence for the abbot, as well as an oil press and a cellar.

In this chapter on Armenian religious buildings of the 13th-14th centuries, a place must also be reserved for a Muslim monument, that of Khachʿen-Dorbatlı, located on the eastern fringe of Nagornȳĭ Karabakh, today in the district of Ağdam (Figures 3.14 and 3.15). It belongs to the numerous category of türbes, medieval Muslim mausoleums in Asia Minor and the South Caucasus, which are based on the principle of a cylinder with conical dome, surmounting a vault. These monuments are clearly related to the drum and dome of Armenian churches; but the one at Khachʿen-Dorbatlı has an even closer connection to Armenian architecture. An Arabic inscription states that it was built in 1314 by a master builder named Shahik. With its refined sculptural decoration, technique, numerous animal themes and ornamental repertoire, including remarkable stalactites, this mausoleum closely resembles a Christian [p. 82] [fig. 3.12] [p. 83] [fig. 3.13] burial chapel built in the first decades of the 14th century, further west in the small town of Yeghvard/Ełuard (Republic of Armenia), by the same architect (vardpet) Shahik who engraved his name in Armenian. A third mausoleum, a Muslim one, located in Yerevan, dated 1319 and visible only from the inside, decorated with stalactites (muqarnas) identical to those of the two previous ones, also bears the signature of Shahik, again in Arabic. It should be noted that, as indicated in H. Achaṙian's dictionary of Armenian anthroponyms, the first name Shahik, a diminutive of Shahēn, of Persian or Pahlavi origin, is attested several times in Armenian sources from the 11th to the 17th centuries 7 . These accounts of the execution of three projects by a builder who was probably an Armenian, for both Christian and Muslim patrons, are precious because they shed light on one of the main modalities of the interactions, which were numerous at that time, between Armenian and Islamic arts 8 .

[p. 84] [fig. 3.14] Several authors (A. Ayvazyan, P. Cuneo, M. Hasratyan, S. Karapetyan) agree that a series of churches were founded in Nakhichevan during the period dis-[p. 85] [fig. 3.15] [p. 86] cussed (and notably: St Thomas of Agulis, St John the Baptist of Abrakunis, Holy Sign of Kukʿi, Holy Mother of God of Ōtsʿop). Indeed, the available photographs show here and there masonries and decorative details that could confirm this hypothesis. However, the extensive alterations in the modern period, the lack of detailed documentation and the destruction of the monuments make it impossible to ascertain their dating.

Modern Period (Seventeenth to Nineteenth Centuries)

After a long interruption due to the invasions and wars in the 14th to the 16th centuries, the restoration of peace between the Ottoman Empire and Safavid Persia by the treaty of 1639 allowed architectural life to flourish again throughout Armenia. In all parts of the country, monasteries were restored, enlarged, and new complexes were built. In Nakhichevan, where a large part of the Armenian population had been deported to Persia and where the destruction had been considerable, vast campaigns of integral reconstruction were carried out.

In the new monasteries, according to the most frequent architectural formula of the time, the church is isolated in the centre of the quadrangular enclosure along which the cells and other monastic premises are arranged. The narthex, which in earlier periods almost always stood attached to the west façade of the church, is now often replaced by a three-arched gallery or a porch-belltower. The porch-belfry, leaning against the façade (usually western, sometimes southern) of the church, is one of the emblematic creations of this period. Sometimes it has two, but more often three levels; the first two as a tetrapod canopy and the last one as a rotunda, generally octastyle.

Many monastic and parish churches adopted the relatively easy-to-build composition of the dome-less basilica, with three naves separated by one, two or three pairs of pillars. A two-pitched roof covers the whole building, often with a small lantern at the top, over a small opening for the rope that operates the bell. Large churches are also built in the form of an inscribed cross, with a dome on four free supports. In Artsʿakh, examples include St Grigoris of Herher (1667-1676) and the church of the monastery of the Three Youths in the Fiery Furnace (Eritsʿ Mankantsʿ, 1691).

In Nakhichevan, such examples were numerous. Two of them merit our special attention. First, the large church of the Holy Mother of God (Surb Astuatsatsin) in Tsʿĕłna (Tsʿĕghna), built in 1683, was impressive for its size, the height of its south porch-belfry, with two rotundas on a tetrapod baldachin (the opposite of the usual formula), the originality of its figurative sculpted [p. 87] decoration and the care with which its bicolour bond (light grey basalt and reddish tuff) is created. We will examine it in the following chapter (n° 4). Secondly, the elegant church of St Stephen of Agulis (Surb Stʿepanos), whose chevet had the particularity of having three deep apses almost equal in width (see Figures 3. 1 and 3.2). Supported by four free cruciform pillars, the high octagonal drum was crowned by an elegant umbrella-like dome. The façades of light colour were enlivened with a continuous horizontal band of dark stone, decorated with a "Seljuk chain" that went around the portals and rose on the gables with one or two crosses. The whole was further enriched by the corner quoins with alternating long and short blocks of dark stone on a light background, as well as by the orange tuff window frames.

A new planimetric formula (new for Armenia: in Byzantium it had been practised for a long time), almost antinomic with tradition, had been chosen for several churches in Nakhichevan, of the 17th century or slightly later: St John the Baptist (Surb Yovhannēs Mĕkĕrtichʿ), The Holy Sign (Surb Nĕshan) and The Holy Trinity (Surb Errordutʿiun) of Agulis (in the quarter of Nerkʿin Agulis), The Holy Mother of God (Surb Astuatsatsin) of Shahkert, St Gregory of Kʿeolkʿ, The Holy Mother of God (Surb Astuatsatsin) of Tanakert, St John (Surb Yovhannēs) of Chahuk. These were "domed basilicas" and not inscribed cross churches: the drum and the dome surmounting it were supported by four pillars placed in the centre of the basilica's inner space comprising three parallel, barrel-vaulted naves without a transept. This resulted in the absence of a cross in the volumetry and in the external image of the building, particularly the roof; the dome rose above a flat or a two-pitched roof. This solution was perhaps dictated by a concern for simplification and economy, but was less resistant, since the balance of buttresses is a great advantage of the inscribed cross, with the crossing of the transept and the nave. This is probably the reason why on the photographs of most of these buildings we see the domes made of brick, which is unusual for Armenia, or collapsed.

Another peculiarity of Nakhichevan in this period is that, whatever the composition of the churches, the two sacristies flanking the apse often had a rectangular shape without the usual small apse. It should be noted that these sacristies, which comprised two levels, were partly hypogeal on the lower level, which suggests a funerary function. This brings us back to the hypothesis of a link between flat sanctuary and funerary function.

Finally, the 19th century is also marked here by a combination of references to earlier sanctuaries and innovations. Let us mention two characteristic cases.

In the city of Nakhichevan, St George Cathedral, built in 1869 in ochre tuff and surmounted by a brick dome (Figs. 1.3, 1.4), was erected on the remains of an earlier church (of the 7th century?), thanks to a public fundraising. It was a [p. 88] large inscribed cross-shaped construction with a dome on four free supports, which had a series of original features inspired in part by the heritage of the 7th century: a) the chevet, slightly lowered in relation to the general volume, had an unusual breadth which led to a strong offset of the dome towards the west-a common offset in the 7th century, but very marked here; on the other hand, the shape of the apse and sacristies in three deep rectangular compartments is a rare formula; b) each of the central compartments of the four façades was slightly projecting, as on the east and north façades of St Hṙipʿsimē in the 7th century; c) in the centre of the gable of each arm of the cross, an oculus was open, as in the Tʿalin and Zuartʿnotsʿ Cathedrals, 7th century, and in that of Ani (1001 AD); d) the apse was pierced by two windows, instead of the traditional single window of the Armenian Apostolic churches (i.e. non-Chalcedonian), perhaps approaching the three windows of the Chalcedonian churches; e) the façades were quite plain, with a wide, high window in each bay, topped by a small arched cornice with linear mouldings and short horizontal wings, reminiscent of the golden age of the 7th century; f) in the centre of the east façade, at the spandrel of the double arcade of the pair of windows, a rectangular plaque bore, in flat bas-relief, the image of Saint George slaying a dragon; g) a large cylindrical drum topped by a dodecagonal dome was pierced by twelve windows alternating with twelve flat niches; h) this vigorous rhythm was underlined, at the top of the drum, by a horizontal band decorated with twenty-four small triangles, a sort of allusive reference to the gables bearing an umbrella.

The second, even more innovative, example is the Holy Saviour Cathedral in Shushi (1868-1887) in Artsʿakh, one of the largest religious buildings in modern Armenia, which will be discussed in chapter 5. Here we will merely point out that it borrows its plan from the cathedral of Ēdjmiatsin, in one of its oldest states, but is distinguished by the slender proportions of its umbrella-like dome and by the breadth of its openings, which transforms its conches into a kind of porches. The high campanile that stands next to it-and echoes the sharpness of its umbrella-stands out for its statues of angels in the round, a great novelty in the history of Armenian art. Bretanitskiĭ, 1966, p. 192, fig. 117

List of illustrations

Fig. 3 . 1 .

 31 Fig. 3.1. Church of Surb Stepʿanos (17th century), Agulis, region of Nakhichevan. Plan: A. Ayvazyan 1990, p. 128, fig. 66b Fig. 3.2. Church of Surb Stepʿanos (17th century), Agulis, region of Nakhichevan, south façade. Photo: A. Ayvazyan (1980s), UIOTC V-21867, Argam Ayvazyan's Fund, 'Land and Culture' Organisations Fig. 3.3. Church of Surb Gēworg (1869), city of Nakhichevan. Plan: A. Ayvazyan 1990, p. 215, fig. 161b Fig. 3.4. Church of Surb Gēworg (1869), city of Nakhichevan, view from the northwest. Photo: A. Ayvazyan (1960), UIOTC V-21391, Argam Ayvazyan's Fund Fig. 3.5. Monastery of Eritsʿ Mankantsʿ, Artsʿakh. South façade, door, tympanum with dedicatory inscription dated 1691. Photo: H.H. Khatcherian Fig. 3.6. Dadivankʿ monastery, Artsʿakh. Main church (1214). Internal view towards the apse and the dome. Photo: H.H. Khatcherian Fig. 3.7. Tigranocerta/Tigranakert, Artsʿakh. Remains of two single-nave churches (ca 5th century). View from the northeast. Photo: H.H. Khatcherian Fig. 3.8. Tigranocerta/Tigranakert, Artsʿakh. Large single-nave church. Two imposts (engaged capitals) of the portal. Photo: H.H. Khatcherian Fig. 3.9. Chapel of Surb Sargis, Gag, Utikʿ (probably 10th-11th centuries). Plan and longitudinal section after Akhundov 1986, p. 214, fig. 238 and 239 Fig. 3.10. Surb Sargis, Gag, Utikʿ (probably 10th-11th centuries). Chapel on top of Mount Gag. Photo: 'Ethnos' (online open source) Fig. 3.11. Khatʿravankʿ monastery, Artsʿakh. Narthex (gawitʿ), 1225. Internal view towards the west. Photo: R. Kortoshyan Fig. 3.12. Bĕṙi Ełtsʿi, Artsʿakh (13th century). The common narthex (gawitʿ) of two chapels. Internal view towards the east. Vault with pairs of crossed arches. Photo: H.H. Khatcherian Fig. 3.13. Monastery of Gandzasar, Artsʿakh. Narthex (gawitʿ), 1261. Central part of the vault on crossed arches. Photo: H.H. Khatcherian Fig. 3.14. Khachʿen-Dorbatlı, Artsʿakh. Mausoleum (türbe), 1314, architect: Shahik. Plan afterBretanitskiĭ, 1966, p. 192, fig. 117

Fig. 3 .

 3 Fig. 3.15. Khachʿen-Dorbatlı, Artsʿakh. Mausoleum (türbe), 1314, architect: Shahik. North view. Photo: Ch. & J.-C. Hotellier

Fig. 3

 3 Fig. 3.3. Church of Surb Gēworg (1869), city of Nakhichevan. Plan: A. Ayvazyan 1990, p. 215, fig. 161b

Patrick Donabédian, "Monumental Heritage of Artsʻakh and Nakhichevan", 2023

. Ēdjmiatsin (Էջﬕածին), lit. 'The Only-Begotten Has Descended'.

. It is difficult to give an accurate translation of the Armenian gĕmbetadzev khoranard, which can be also understood as "vaulted cube", or interpreted as "domed baldachin".

. The term khoran, whose primary meanings are tent, tabernacle, altar, vault, etc., and which is also used to translate "apse", clearly designates here the space above the four columns described in the Vision, which could be rendered as "vault", but which we propose to translate, as well as gĕmbet, as "dome".

Patrick Donabédian, "Monumental Heritage of Artsʻakh and Nakhichevan", 2023 [p. 173] Appendix

Select Bibliography on the Armenian Medieval Architectural Heritage: Artsʿakh and Nakhichevan

(including the articles published in the book Monuments and Identities in the Caucasus. Karabagh, Nakhichevan and Azerbaijan in Contemporary Geopolitical Conflict, Brill, 2023)


Until the First World War, Agulis was a small town with a predominantly Armenian population. Thanks to the silk and cotton trade, taking over from Julfa (Arm. Djuła) which had just been ruined, Agulis had experienced a great expansion in the 17th and first half of the 18th century. It had a dozen monasteries [page 90] [fig. 4.2] and churches, which were looted when the city was taken by the Turkish army in 1918 and then destroyed by the Azerbaijani authorities at the end of the 20th and beginning of the 21st century.

The main complex in the centre of the city was the Monastery of St Thomas. According to the inscription on the western door of its church, it was founded at the very beginning of the Christian period, but its first documented mention is in the colophon of a manuscript from 1375. The monastery was restored in the 17th century and, as the inscription attests, its church was rebuilt in 1694.

[page 91] [fig. 4.3] It was the episcopal seat of the canton and had an active scriptorium where a large number of manuscripts were copied.

As in many Armenian monasteries of the modern period, the church stood isolated, without a narthex, in the centre of a rectangular enclosure against the inner wall of which the utilitarian buildings and monks' cells were arranged. Archival photographs show a large fortified complex with two powerful rounded corner towers. The church was built in a very careful masonry bond, with alternating layers of light felsite and grey basalt, following a bicolour fashion common in Armenia in the 17th century. It illustrated the revival of a typology which was widespread in the 7th century but which during the later centuries had only been used once, in the year 1000, in the cathedral of Ani: the [page 92] [fig. 4.4] composition of an inscribed cross with a dome on four free supports. In the early examples of this type, the dome is placed approximately in the centre of the nave, not including the apse, so that in the overall volume of the building it is clearly offset to the west. The same was true of St Thomas, where this shift was accentuated by the relative narrowness of the western bay.

In addition to the bicolour façades, St Thomas had several features specific to the Armenian 17th century, in particular the interior configuration of the chevet: an apse with a polygonal outline (here heptagonal) flanked by two transversally elongated sacristies, whose width supersedes their length, with a flat eastern wall (without an apse) on two levels. The passage from the four cruciform pillars to the circular base of the drum was performed, as was very often the case in Armenia at the time, by means of reticulated [page 93] [fig. 4.5] pendentives. Borrowed from Persian architecture, this device was popularised by the Armenian churches of Isfahan. The drum was externally dodecagonal and had an umbrella-shaped dome, a picturesque shape that was widespread in Armenia since the 10 th -11th centuries and was still popular during the modern period, especially among the architects of Nakhichevan.

The sculpted decoration of the church included crude bas-reliefs inspired by Western iconography, placed above the three doors. On the west side was the scene of the Incredulity of Saint Thomas-the holy dedicatee of the church-enhanced with the stone colours and accompanied by paintings (God at the top, the Holy Mother of God on the right and Mary Magdalene on the left), while the Apostles Peter and Paul were carved on the south and north doors. In accordance with the norm of the time, the sculpted decor also included geometric ornamental bands: the angular interlacing known as "Seljuk chain" (drawing a large cross at the top of the façades), a row of stalactites, a cable and a braid. The abundant painted decor inside was rich in arabesques and Iranian vegetal motifs; it reflected the influence, noticeable at the time in Armenia, of the splendid churches of the Armenian district of New Julfa, in Isfahan. This decor was the work of the famous Armenian painter, poet and bard Naghash Hovnatʿan (Nałash Yovnatʿan) (1661-1722), founder of the Hovnatʿanian dynasty of painters, who taught in Agulis.

[page 94] [fig. 4.6] The tetrapod porches that stood in front of the north and west doors of the church (the latter rebuilt in 1825, 1831 and 1904) were topped by a small lantern-as shown in 19th-century photographs. A third, similar, aedicula, dated 1666, slightly to the west of the south door, housed a mausoleum.

Agulis, Church of St Christopher (Surb Kʿristapʿor) (1671-1675)

The church was located in the Verin Tʿał (Upper District) of Agulis. A church with this dedication already existed here in the 15th century, as we know from a colophon of 1447. After it had collapsed in 1670, it was rebuilt in 1671-1675 on the initiative of the district's inhabitants, who brought in teams of masons from Vaspurakan.

It had the same plan as St Thomas and was also carefully built, but was distinguished by its dome, which was made of brick. This material, often used here during the modern period, made it possible to lighten the weight of the superstructures and facilitate construction. The drum of the dome was octagonal on the outside and its cap had consequently the shape of an eight-sided pyramid. The outer faces of the drum were hollowed out with flat niches terminated by a [page 95] pointed arch in the Iranian style. The reticulated pendentives that ensured the interior support of the drum followed the same trend.

On the other hand, as in St Thomas, the figurative bas-reliefs on the exterior reflected the Western influence that was penetrating Armenian culture during the modern period. Thus, above the western doorway, the Madonna wore a crown. The plastered interior of the church was also decorated with paintings by Naghash Hovnatʿan in the late 17th century. There were some figurative subjects, including portraits of prominent figures of the Armenian Church and angels. And similarly to New Julfa, two foreign sources of inspiration joined here the Armenian tradition: the iconographic models of Western engravings, on the one hand, and the vegetal and geometric ornaments of the Iranian repertoire, on the other.

Tsʿĕłna, Church of the Holy Mother of God (Surb Astuatsatsin) (1683)

Current location: Republic of Azerbaijan, Nakhichevan Autonomous Republic, Ordubad district, village called Çənnəb in Azeri (Çənənəb until 2003). Medieval location: province of Vaspurakan (then Siunikʿ in the 10th century), canton of Gołtʿĕn.

The Armenian village of Tsʿĕłna, attested since the 14th century, expanded on both banks of the river bearing the same name. According to the colophons, three churches stood there, including the church of the monastery of the Holy Mother of God. This church was the only surviving part of a monastery which had been a fairly active scriptural centre; the oldest Gospel book copied there is dated at 1349.

As the inscription on the lintel of its western door indicated, the large church was rebuilt in 1683 by five merchants-on the site of a sanctuary that had already been restored several times. Closely related to the above-mentioned monuments of Agulis, it was built of grey basalt combined with reddish tuff and had the composition of an inscribed cross with a dome on four free supports (cruciform pillars). Also, here the apse had a heptagonal interior contour, and the large two-level rectangular sacristies flanking it had no apse. The external dodecagonal drum was topped by a pyramid.

As in Agulis and in many other Armenian monuments of the period, the sculpted decor combined occidentalising figurative images with Islamicising ornamentation. Above the western door, on a plaque dated by inscription to [page 96] [fig. 4.7] 1683, the Madonna was shown seated, crowned by two angels-a feature borrowed from Western European iconography. Above the southern doorway were the Apostles Peter and Paul holding a cross pattée in a medallion-a reminder of the early Christian centuries. A band of two intersecting rods with angular links, conventionally known as a "Seljuk chain", encircled the entire building, joining the mouldings that framed the doors and the windows, in a formula developed in the 13th-14th centuries.

The decoration of the doors retained a structure developed in the 10th-11th centuries, with a rectangular frame surmounted by an arched casing, but at the same time their opulent ornamentation followed a new formula, widespread in the 17th century, with Islamic affinities and some references to Europe. The high and wide arched doorframe included, from top to bottom, a braided chain, a cable and either a scroll with small foliage (west door) or a band with stalactites (south door). This "outer" frame surrounded the two wide bands immediately bordering the rectangular doorway; one bore a very [page 97] [fig. 4.8] elaborate interlacing pattern, rounded on the west side, angular on the south side; the other was decorated with elaborate arabesques on the west side, and with a row of vases pouring out an abundance of flowers on the south. The latter motif, popular throughout Armenia at the time, was a borrowing from Baroque art, reflecting the links established by Armenian merchants with Western Europe. As in Agulis, the interior of the church of Tsʿĕłna was decorated with paintings, which were badly damaged in the late 20 th century.

A particular attention was paid to the high porch-bell tower that stood in front of the southern door of the church of Tsʿĕłna. First of all, while respecting the principle of the two-or three-storey porch-bell tower, leaning against the church façade, which is typical of the modern period, its structure was singular. Unlike most representatives of this type, the first tetrapod level was here surmounted by two superimposed octopod rotundas. The last one was crowned with a very tapered umbrella. Secondly, the figurative carved décor used unusual formulas. On the top of each of the two southern pillars of the porch, a niche was carved. In the left niche, the crucified Christ was sculpted,

[page 98] [fig. 4.9] with the Armenian initials of "Jesus of Nazareth, King of the Jews" on the three upper arms of the cross. The right-hand niche was occupied by the figure of the Mother of God, with her arms folded and her hands crossed over her chest, with two small angels sculpted on the sides of the niche (the one on the right-hand side is very eroded and is barely visible), holding a crown on her head. It is interesting to note that, despite the Western European imprint present on these sculptures (for example in the crown worn by the Virgin), the feet of Christ crucified are shown parallel, attached to the cross by a single nail, as on several Armenian sculptures from the end of the 13th century, and not crossed, as is common in the West. At the foot of the cross, however, the photographs seem to show that the widespread Western pattern of the skull between two crossed bones (based on the Gospels) was chosen instead of the [page 99] [fig. 4.10] Armenian tradition of the head of Adam (based on an old Christian belief). 1 Finally, the strong projection of these two sculptures and their position constituted an interesting novelty. Mirroring its period and milieu, the sculpted décor of Tsʿĕłna achieved a synthesis of three traditions, Armenian, Islamic and West European. --------------------1. The ancient tradition of Christ's Crucifixion above Adam's burial site has been examined in two recent works: E. Grypeou and H. Spurling, The Book of Genesis in Late Antiquity, Leiden, 2013, p. 230, and, in

  Current location: Republic of Azerbaijan, Nakhichevan Autonomous Republic, Ordubad district, village called Yuxarı Əylis in Azeri, a few km from the district capital. Medieval location: province of Vaspurakan (then Siunikʿ in the 10th century), canton of Gołtʿĕn.

  Fig. 4.1. Monastery of Surb Tʿovma (1694), Agulis, region of Nakhichevan. West view. Photo: M. Smbatiants (1883), UIOTC V-34139, 'Land and Culture' Fund

Fig. 4

 4 Fig. 4.1. Monastery of Surb Tʿovma (1694), Agulis, region of Nakhichevan. West view. Photo: M. Smbatiants (1883), UIOTC V-34139, 'Land and Culture' Fund

Fig. 4 .

 4 Fig. 4.11. Church of Surb Astuatsatsin (1683), Tsʿĕłna, region of Nakhichevan. Porchbell tower leaning against the south façade. West view. Photo: A. Ayvazyan (1980s), UIOTC V-22329, Argam Ayvazyan's Fund

  . Church of Surb Tʿovma (1694), Agulis, region of Nakhichevan. West door.

	Photo: A. Ayvazyan (1980s), UIOTC V-21882, Argam Ayvazyan's Fund, 'Land and
	Culture' Organisations
	Fig. 4.5. Church of Surb Tʿovma (1694), Agulis, region of Nakhichevan. Zenithal view
	of the interior. Photo: Z. Sargsyan (1987), UIOTC V-21881, Argam Ayvazyan's Fund
	Fig. 4.6. Church of Surb Kʿristapʿor (1671-1675), Agulis, region of Nakhichevan. South
	view. Photo: A. Ayvazyan (1980s), UIOTC V-21843_01, Argam Ayvazyan's Fund
	Fig. 4.7. Church of Surb Astuatsatsin (1683), Tsʿĕłna, region of Nakhichevan. Plan
	after A. Ayvazyan 1990, p. 144, fig. 84
	Fig. 4.8. Church of Surb Astuatsatsin (1683), Tsʿĕłna, region of Nakhichevan. View
	from the northwest. Photos: A. Ayvazyan (1980s), UIOTC V-22322 and 22324, Argam
	Ayvazyan's Fund. Fragmentary photos assembled into a single image by P.
	Donabédian
	Fig. 4.9. Church of Surb Astuatsatsin (1683), Tsʿĕłna, region of Nakhichevan. Plaque
	above the west door representing the Virgin and the Child crowned. Photo: A.
	Ayvazyan (1970s) and UIOTC V-18796, Argam Ayvazyan's Fund
	Fig. 4.10. Church of Surb Astuatsatsin (1683), Tsʿĕłna, region of Nakhichevan.
	Sculpted decoration of the west and south portals. Photos: A. Ayvazyan (1980s), UIOTC
	V-22169, 22328 and 22309, Argam Ayvazyan's Fund. Fragmentary photos assembled
	into a single image by P. Donabédian
	Fig. 4.11. Church of Surb Astuatsatsin (1683), Tsʿĕłna, region of Nakhichevan. Porch-
	bell tower leaning against the south façade. West view. Photo: A. Ayvazyan (1980s),
	UIOTC V-22329, Argam Ayvazyan's Fund
	Fig. 4.12. Church of Surb Astuatsatsin (1683), Tsʿĕłna, region of Nakhichevan. Porch-
	bell tower leaning against the south façade. South view. Niches with sculpted images
	of Christ and the Virgin. Photos: A. Ayvazyan (1980s), UIOTC V-23983, 23993 and
	23994, Argam Ayvazyan's Fund. Fragmentary photos assembled into a single image
	by P. Donabédian

Patrick Donabédian, "Three Important Monuments of Nakhichevan", 2023

Select Bibliography on the Armenian Medieval Architectural Heritage of Nakhichevan

(including the articles published in the book Monuments and Identities in the Caucasus. Karabagh, Nakhichevan and Azerbaijan in Contemporary Geopolitical Conflict, Brill, 2023)


Chengyang Wu

Jingzhou Zhou

Si Pang

Lin Yang

Xiaodi Li

Eric Lichtfouse

Siqing Xia

Hongbo Liu

Yujie Men

Enhanced removal of 2,4-dichlorophenol by coupling of Pd nanoparticles with biofilm

Keywords:

The persistence of chlorinated phenols in the environment is a major concern due to their persistence and toxicity, yet advanced remediation methods are actually limited. For instance, reductive dechlorination of chlorophenols utilizing palladium nanoparticles (PdNPs) supported on hydrogen-transfer membranes is not fully efficient. Here we tested the simultaneous microbial-driven and Pd-catalyzed reduction of 2,4-dichlorophenol (2,4-DCP) for removal rate, selectivity of reduction products, and optimal reaction conditions. For that we compared three hydrogen-based membrane reactors: a 'Pd-biofilm reactor' with Pd nanoparticles and a biofilm, with two controls: a 'biofilm reactor' with biofilm alone, and a 'Pd-film reactor' with Pd nanoparticles alone. Results show that the Pd-biofilm reactor removed 89.3% of total phenols, versus 15.7% for the biofilm reactor and 19.5% for the Pd-film reactor. The strong adsorption capacity of Pd-biofilm enhanced dechlorination. This elevated local chlorophenols concentration led to a 2-4 times higher reduction rate within Pd-biofilm compared to abiotic Pd-film. The presence of nitrate enhances phenol removal over Pd-biofilm but inhibited 2,4-DCP dechlorination. Similarly, high concentrations of nitrite, above 250 μM, inhibited 2,4-DCP reduction, predominantly in microbial processes rather than in catalytic reduction. Increasing hydrogen pressure facilitated the reduction of both 2,4-DCP and nitrate by Pd-biofilm, with an optimal pH of 7.0 for Pd-biofilm. The Pd-biofilm reactor decreased total phenols from 5-10 mg L -1 to the recommended threshold of 0.1 mg L -1 in 55 days, thus appearing as an efficient technique to clean chlorophenol-contaminated waters.

Introduction

With large usage in production of preservatives and herbicides, 2,4dichlorophenol (2,4-DCP) is emerging as one of the most prevalent chlorophenols found in aquatic environments. 2,4-DCP has been detected at ppm levels in raw water [START_REF] Gonzalez | Surface and groundwater pollution by organochlorine compounds in a typical soybean system from the south Pampa, Argentina[END_REF] and is classified as a priority pollutant by the United States Environmental Protection Agency (EPA) (2012) due to its carcinogenicity, cytotoxicity, and slow biodegradation. The maximum contaminant level for 2,4-DCP in drinking water is set at 0.1 mg L -1 by the EPA.

The strong carbon-chlorine bond (~ 400 kJ mole -1 ) (Luo and Yu-Ran, 2007) renders chlorinated phenols resistant to biological degradation under ambient conditions [START_REF] Long | Complete dechlorination and mineralization of para-chlorophenol (4-CP) in a hydrogen-based membrane biofilm reactor (MBfR)[END_REF]. Conventional destructive methods, such as advanced oxidation [START_REF] Wang | Perdisulfate-assisted advanced oxidation of 2,4-dichlorophenol by bioinspired iron encapsulated biochar catalyst[END_REF], electrochemical processes [START_REF] Samarghandi | Application of a fluidized three-dimensional electrochemical reactor with Ti/SnO2-Sb/β-PbO2 anode and granular activated carbon particles for degradation and mineralization of 2,4-dichlorophenol: Process optimization and degradation pathway[END_REF], and photocatalytic degradation [START_REF] Ding | Boosting visible photocatalytic degradation of 2,4-dichlorophenol and phenol efficiency by stable core@shell hybrid Ag3PO4@polypyrrole[END_REF], demand significant energy inputs.

Nondestructive removal methods, including adsorption, ion exchange, liquid-liquid extraction, are considered less environmentally friendly due to their tendency to produce harmful byproducts [START_REF] Garba | An overview of chlorophenols as contaminants and their removal from wastewater by adsorption: a review[END_REF]. Catalytic hydrodechlorination (HDC) using platinum group catalysts offers a promising approach for dechlorination, because it only requires hydrogen gas (H 2 ) as the chemical input and provides fast kinetics, mild operating conditions, and no secondary waste stream. Among the catalysts, palladium (Pd) is the most studied catalyst due to its great H 2 -absorption capacity and ability to form highly reductive atomic hydrogen on the catalyst's surface, which is activated to substitute the Cl atoms in chlorophenols. However, Pd-based reductive dechlorination by conventional catalytic approaches have encountered challenges: poor efficiency of the H 2 supply and catalyst deactivation due to its loss.

A palladized membrane biofilm reactor (Pd-MBfR) tackles the challenges of low efficiency in H 2 supply and maintaining long-lifespan batch mode under an H 2 pressure of 15 psig (2.0 atm, absolute pressure) to produce PdNPs within the biofilm (Pd-biofilm) and on the surface of membranes (Pd-film), respectively. The composition of the Pd(II)containing medium is detailed in a previous study [START_REF] Wu | A kinetic model for 2,4-dichlorophenol adsorption and hydrodechlorination over a palladized biofilm[END_REF]. The Pd (0)-deposition process lasted for 48 h until dark precipitates were observed on the fiber bundles of both reactors, and more than 99% of the input Pd(II) was recovered after each addition. A total of 31 mg of Pd was retained, either through direct deposition onto the fiber surfaces in the Pd-film (establishing the membrane Pd-film reactor) or through association with the biofilm in the Pd-biofilm (establishing the Pd-MBfR).

Continuous operation for 2,4-dichlorophenol removal

During Stage I (Day 1-12), the reactors were fed with the medium containing 360 μM NO 3 -and 30.7 μM 2,4-DCP. In Stage II (Day 13-35), the influent medium was modified to include 61.3 μM of 2,4-DCP. In Stage III, the applied H 2 pressure was increased from 7 psig to 15 psig. Specific operational parameters for each stage are summarized in Table 1. Steady-state conditions were reached in each stage when the effluent concentrations of chlorophenols and NO 3 -remained stable for at least 3 days with a variation of less than 10%. The influent medium was degassed with N 2 for 15 min before use, resulting in a dissolved oxygen (DO) concentration in the influent medium of less than 0.15 mg L -1 . 5-mM 2,4-DCP stock solutions were prepared by dissolving them in deoxygenated water and stored at 4 ℃, which were added into influent water during the experiments. 2-mM phosphate buffer was added to the influent, and the pH values of the influent were maintained within the range of 7.2 ± 0.2.

Optimal conditions for 2,4-DCP removal

After operating the three reactors for 55 days, short-term experiments were conducted to investigate how 2,4-DCP loading, NO 3 -loading, NO 2 -loading, H 2 availability, and pH affected 2,4-DCP removal systematically. The experiments were organized into five series (Table S2), with hydraulic retention time (HRT) maintained at 4.5 h. Prior to each test, the three reactors were returned to the steady-state condition with an influent of 100 μM 2,4-DCP, 85 μM NO 3 -, 15 psig H 2 pressure, and pH 7.0. For each short-term test, the change of system conditions lasted for 5 HRTs to reach steady state, before samples were taken. Replicate samples were collected at 6, 7, and 8 HRTs and the results are presented as the average values.

Analytical methods

The liquid samples in the three reactors were collected using 10-mL syringes and filtered through 0.22-μm PTFE membrane filters. The concentrations of Pd(II) was determined using inductively coupled plasma-mass spectrometry (ICP-MS, Agilent 7700, Agilent Technologies, USA). For the analysis of 2,4-DCP, ortho-chlorophenol (2-CP), parachlorophenol (4-CP), and phenol, the filtered samples were subjected to high-performance liquid chromatography with an Agilent C18 column (HPLC, Shimadzu LC-20A, Japan). NO 3 -and NO 2 -were analyzed by an ion chromatograph (IC, Dionex Aquion, USA) with an AS-19 column. pH and DO were measured using a multi-Parameter Meter (HACH HQ40d, USA).

Characterization of solid state

At the end of Stage III, solid samples from the three reactors were collected by excising fiber segments approximately 5 cm in length from the coupon fibers, and the remaining fiber was sealed by tying the end into a knot. After chemical fixation and ultra-microtome slicing, these fiber sections were characterized by transmission electron microscopy (FEI Tecnai G2 F20, USA) equipped with energy-dispersive X-ray stability of catalysts [START_REF] Wu | Co-removal of 2,4dichlorophenol and nitrate using a palladized biofilm: denitrification-promoted microbial mineralization following catalytic dechlorination[END_REF]. The Pd-MBfR enables complete H 2 utilization and the formation of a robust Pd-catalyst by efficient bubble-free delivery of H 2 through nonporous membrane fibers, which is precisely controlled by the H 2 pressure. The Pd-MBfR also facilitates microbially driven Pd recovery to form palladium nanoparticles (PdNPs) associate with the biofilm (Pd-biofilm), thereby efficiently immobilizing and stabilizing them to enhance hydrodechlorination.

Nitrate often accompanies 2,4-DCP as a common co-contaminant in water [START_REF] Yang | Challenges and opportunities for the biodegradation of chlorophenols: aerobic, anaerobic and bioelectrochemical processes[END_REF]. The reduction of nitrate over the Pd-biofilm can have dual effects-stimulating the mineralization of dechlorination product phenol on one hand, and competitively inhibiting catalytic dechlorination on the other [START_REF] Wu | Co-removal of 2,4dichlorophenol and nitrate using a palladized biofilm: denitrification-promoted microbial mineralization following catalytic dechlorination[END_REF]. Moreover, factors such as the influent surface loading, pH, and electron donor supplying flux can influence the dechlorination performance of chlorinated solvents and the selectivity of the products [START_REF] Luo | Long-term continuous co-reduction of 1,1,1-trichloroethane and trichloroethene over palladium nanoparticles spontaneously deposited on H2transfer membranes[END_REF]. However, the effect of these factors on the synergistic 2,4-DCP removal within the Pd-MBfR remains unexplored.

In a recent study, we developed a Pd-MBfR by coupling a biofilm with in-situ generated PdNPs and documented that the Pd-MBfR facilitated hydrodechlorination and mineralization of 2,4-DCP [START_REF] Wu | Co-removal of 2,4dichlorophenol and nitrate using a palladized biofilm: denitrification-promoted microbial mineralization following catalytic dechlorination[END_REF]. We proposed distinct pathways for 2,4-DCP dechlorination on PdNP surfaces, followed by microbial mineralization via denitrification. Due to the lack of optimized operational conditions for the Pd-MBfR, the dechlorination performance needs improvement. Here, we conducted a comprehensive characterization of the removal performances for 2, 4-DCP and its intermediate products in three membrane reactor systems. We compared biofilm-only, PdNP-only, and Pd-biofilm setups under varying surface loadings of 2,4-DCP over approximately 55 days of continuous operation. To identify the optimal operational conditions for 2,4-DCP removal by the Pd-MBfR, we also investigated the effect of 2,4-DCP loading, nitrate loading, nitrite loading, H 2 pressure, and pH on 2,4-DCP removal. The aim of this study is to discover the mechanisms underlying the improved removal performance of phenols and shed light on the potential benefits of the synergetic platform.

Materials and methods

Configuration and setup of the reactors

Experiments were carried out with three reactors in parallel, which were similar to that used in a previous study (Fig. S1, Supporting Information) [START_REF] Wu | Dechlorination of 2,4dichlorophenol in a hydrogen-based membrane palladium-film reactor: Performance, mechanisms, and model development[END_REF]. Each reactor consisted of two interconnected tubes via Viton tubing (Saint-Gobain, USA), one containing a bundle of 32 nonporous polypropylene hollow-fiber membranes (Teijin, Japan) with both ends connected to the H 2 gas pipeline, while the other contained a coupon bundle of 10 identical fibers, with upper end connected to the pipeline. With a working volume of 65 mL and a total membrane surface area of 54 cm 2 . The liquid in the reactor was well mixed by a recirculation rate of 150 mL min -1 provided by a peristaltic pump (Longer, China). Throughout the experiments, the temperature was consistently maintained at 25 ± 2 • C.

Inoculation, feeding, and startup of the membrane biofilm reactors

We inoculated two of the three reactors ("MBfR" and "Pd-MBfR") with anoxic sludge from Wusong wastewater treatment plant (Shanghai, China) and fed the reactors continuously with a 0.7-mM NO 3 -medium to accumulate biofilms on the membranes. The components of the foundational medium are summarized in supplementary Table S1. After two months, mature biofilms had successfully developed on the membrane fibers. The specifics of the inoculation and feeding procedures were described in a previous study [START_REF] Wu | Co-removal of 2,4dichlorophenol and nitrate using a palladized biofilm: denitrification-promoted microbial mineralization following catalytic dechlorination[END_REF].

Synthesis and retention of Pd nanoparticles on membrane surfaces

We introduced Pd(II) medium (160 mg L -1 Pd(II) as Na 2 PdCl 4 ) into one of the MBfRs and a parallel non-inoculated reactor, operating in spectroscopy (EDX). Solid was scraped from the fibers in the rectors and then freeze-dried them for subsequent X-ray diffraction (XRD) using an X' Pert PRO MPD diffractometer with Cu-Kα radiation. Detailed protocols for sample fixation and slicing, as well as instrument specifications, can be found in previously published studies [START_REF] Wu | Co-removal of 2,4dichlorophenol and nitrate using a palladized biofilm: denitrification-promoted microbial mineralization following catalytic dechlorination[END_REF][START_REF] Wu | Reduction and precipitation of chromium(VI) using a palladized membrane biofilm reactor[END_REF].

Analysis of microbial communities

At the end of Stage III, a 10-cm section was excised from a coupon fiber in the Pd-MBfR or MBfR. We followed the procedures of biofilm separation and DNA extraction described by Xia et al. [START_REF] Xia | Direct delivery of CO 2 into a hydrogen-based membrane biofilm reactor and model development[END_REF]. After amplifying and purifying, the bacterial 16 S rRNA gene were sent to Majorbio Technology (Shanghai, China) for processing Illumina MiSeq sequencing, conducted in accordance with established protocols.

Fluxes calculations

The fluxes of the substrates (J) were calculated by

J = Q(C 0 -C e ) A ( 1
)
where J is the flux, mmol m -2 d -1 ; Q is the influent flow rate, m 3 d -1 ; C 0 and C e are influent and effluent concentrations of each substrate (chlorophenols and NO 3 -), mmol m -3 ; and A is the effective membrane surface area, m 2 . The dechlorination flux in electron equivalents (e -meq m -2 d -1 ) was calculated based on released Cl -concentrations according to reaction stoichiometry:

2, 4 -DCP + 4e -+ 2H + →phenol + 2Cl - (2)
The NO 3 -flux in electron equivalents (e --meq m -2 d -1 ) was calculated from Eqs. ( 3)-( 5):

NO - 3 +5e -+6H + →0.5N 2 +3H 2 O (3)
NO - 3 +2e -+2H + →NO - 2 +H 2 O (4) NO - 3 +8e -+10H + →NH + 4 +3H 2 O (5)
To indicate if the H 2 supply was limiting, the maximum H 2 -supply flux at applied H 2 pressure of 15 psig (i.e., 300 e --meq m -2 d -1 ) was calculated according to Tang et al. [START_REF] Tang | Hydrogen permeability of the hollow fibers used in H2-based membrane biofilm reactors[END_REF].

Results and discussion

Characterization of the membrane-supported Pd-biofilm, Pd-film, and biofilm

Dried powder samples were collected from the coupon bundle in the Pd-MBfR and the Pd-film reactor for TEM, and XRD analyses. Fig. 1a Table 1 Parameters for operations and average performance for each stage of the palladized membrane biofilm reactor (Pd-MBfR), membrane biofilm reactor (MBfR), and membrane palladium-film rector (Pd-film reactor) at steady state. ± 1.1 nm (Fig. 1f), indicating mild aggregation and clustering of Pd(0) deposits due to insufficient stabilization [START_REF] Zhou | Biofilm-enhanced continuous synthesis and stabilization of palladium nanoparticles (PdNPs)[END_REF]. The XRD spectra of freeze-dried sample collected from the fibers (Fig. S2 in Supporting Information) exhibited four distinct diffraction peaks at 2θ angles of 40. Parallel operation of the MBfR and the Pd-film reactor was conducted to compare their performance in 2,4-DCP removal and to understand the advantages of the Pd-MBfR. In the MBfR, the intermediate product, 4-CP, generated from the reduction of 2,4-DCP was apparently accumulated (Fig. 2b). The concentration of 2,4-DCP in the effluent continued to decrease and eventually stabilized around 4.91 μM in Stage III, resulting in a 2,4-DCP removal of 92.0%, which was comparable to that achieved by the Pd-MBfR. However, the significant accumulation of 4-CP led to a total removal of phenols of merely 15.7%. These results demonstrate that the MBfR exhibited efficient removal for 2,4-DCP, but the removal of intermediate 4-CP is deficient, leading to 4-CP accumulation and the low removal rate of total phenols.

As shown in Fig. 2c, the intermediate products of 2-CP, 4-CP, and phenol were detected in the Pd-film reactor effluent. The 2,4-DCP removal achieved by the Pd-film reactor was 93.4%, which is lower than the average removal rate of the Pd-MBfR (96.0%). Moreover, a significant accumulation of phenol led to a total removal of phenols of less than 20%. When the influent concentration of 2,4-DCP doubled in Stage III, the concentrations of all phenolic products in the effluent increased accordingly, and their proportions remained almost unchanged. The ratio of 2,4-DCP, 4-CP, 2-CP, and phenol in the effluent was 28.3%, 2.3%, 9.3%, and 60.4%, respectively. The H 2 supplying flux had a distinct impact on the reduction of 2,4-DCP in the Pd-film reactor. When H 2 pressure was doubled in Stage III, the ratio of 2,4-DCP, 4-CP, 2-CP, and phenol in the effluent changed to 10.4%, 2.0%, 6.3%, and 81.3%, respectively. These results demonstrate that the Pd-film reactor exhibited strong reduction potential for 2,4-DCP but had a limitation in removal of the dechlorination product, phenol.

In summary, the MBfR struggled to reduce 4-CP, while the Pd-film reactor faced challenges in phenol removal. The Pd-MBfR demonstrated the effective overall removal of 2,4-DCP and its intermediate products. The complete removal of 2,4-DCP by Pd-MBfR is the result of a synergy between catalytic dechlorination and microbial mineralization. The Pd-MBfR efficiently hydrodechlorinated 2,4-DCP to phenol, which was utilized as a more favorable electron donor than H 2 for denitrification and was mineralized to CO 2 [START_REF] Wu | Co-removal of 2,4dichlorophenol and nitrate using a palladized biofilm: denitrification-promoted microbial mineralization following catalytic dechlorination[END_REF]. presents the thickness of the Pd-biofilm attached to the hollow fiber was approximately 40 µm. The microbial cells exhibit elliptical or circular shapes, with a denser arrangement closer to one side of the fiber (within 20 µm distance from the fiber surface). Fig. 1b demonstrates that black PdNPs were mainly distributed on the surface of microbial cells and within the extracellular polymeric substances (EPS), with a small portion present inside the microbial cells. The particle size distribution of the PdNPs in Fig. 1c reveals that the average size is approximately 4.37 ± 0.27 nm. Compared to the Pd-biofilm sample, the thickness of the PdNPs-film attached to the outer surface of the fiber was approximately 1.75 µm (Fig. 1d), and the majority of PdNPs were aggregated near the fiber. The statistic average size of PdNPs is approximately 13.0 (Fig. 4c), the influent concentration increased from 45 μM to 70 μM resulted in a sharp drop in 2,4-DCP removal rate from 88.7% to 77.4%.

In the Pd-film reactor (Fig. 4e), the effluent concentrations of 2,4-DCP and its intermediate products increased with higher influent concentrations of 2,4-DCP, and the removal rate remained relatively stable at around 91.9%. It became apparent that when treating influent concentrations below 70 μM (surface loading of 2,4-DCP was< 4.7 mmol m -2 d - 1 ), the Pd-MBfR exhibited the highest removal rate for 2,4-DCP and total phenols. The 2,4-DCP-adsorption capacity of Pd-biofilm was reported to be 2-to 5-fold greater than that of abiotic Pd-film [START_REF] Wu | A kinetic model for 2,4-dichlorophenol adsorption and hydrodechlorination over a palladized biofilm[END_REF]. This enhanced adsorption had the effect of accelerating dechlorination by Pd-biofilm, especially its selectivity towards phenol rather than mono-chlorophenols. The Pd-MBfR gave a greater reduction rate than that did the Pd-film reactor, particularly for lower 2,4-DCP loadings, because adsorption of 2,4-DCP by the biofilm enhanced the catalytic contact of 2,4-DCP with activated hydrogen on Pd surface.

To estimate the reaction order, log J versus log C (Rittmann and McCarty, 2020) is plotted in the Fig. S3, and the results of reaction kinetics parameters for 2,4-DCP reduction are presented in Table 2. The reaction order for 2,4-DCP reduction in the MBfR was 0.52, which is close to the well-known half-order kinetics that was for deep biofilms in which the reaction is zero order in substrate concentration (Rittmann and McCarty, 2020). Chung et al. reported comparable findings in their research on the reduction of selenate and chromate using an MBfR [START_REF] Chung | Bio-reduction of soluble chromate using a hydrogen-based membrane biofilm reactor[END_REF][START_REF] Chung | Bioreduction of selenate using a hydrogenbased membrane biofilm reactor[END_REF], where the reaction orders in the biofilm were 0.41 and 0.49, respectively. When the reaction order is less than 1, the increase in the removal flux of 2,4-DCP is smaller than the increase in influent concentration, which resulted in a decrease in the removal rate. In the Pd-MBfR, the reaction order for 2,4-DCP reduction (0.48) indicates half-order kinetics. The Pd-MBfR exhibited a higher removal rate of 2,4-DCP compared to the MBfR, since the reaction constant approximately twice as large. This suggests that the reduction of 2,4-DCP in the Pd-MBfR primarily driven by the catalytic reduction over PdNPs distributed within EPS. Due to the MBfR and MBfR at the genus level. In Stage III of the Pd-MBfR, Methylocystis, Dechloromonas, Chryseobacterium, and Bradyrhizobium are the dominant genera, with relative abundances of 25.1%, 15.3%, 11.9%, and 5.7%, respectively. Methylocystis belongs to type II methanotrophic bacteria [START_REF] Wei | Screening and culture condition of a Type II methanotroph[END_REF] and can release soluble organic compounds as electron donors for denitrifying bacteria during methane oxidation [START_REF] Modin | Denitrification with methane as external carbon source[END_REF]. Studies have also shown that Chryseobacterium can efficiently degrade methane [START_REF] Zhao | Kinetics of affinity to methane oxidation by Chryseobacterium sp.from aged-refuse[END_REF]. The increase in the relative abundances of these two bacteria indicates the production of a significant amount of methane through the monooxygenation pathway [START_REF] Liu | Accurate O2 delivery enabled benzene biodegradation through aerobic activation followed by denitrification-coupled mineralization[END_REF] during the reduction of 2,4-DCP in the Pd-MBfR, which was observed in our previous study [START_REF] Wu | Co-removal of 2,4dichlorophenol and nitrate using a palladized biofilm: denitrification-promoted microbial mineralization following catalytic dechlorination[END_REF]. Dechloromonas is a common denitrifying bacterium that can degrade benzene under anaerobic conditions, with phenol being an intermediate in the degradation process [START_REF] Coates | Anaerobic benzene oxidation coupled to nitrate reduction in pure culture by two strains of Dechloromonas[END_REF][START_REF] Coates | Anaerobic benzene biodegradationa new era[END_REF]. Bradyrhizobium is a denitrifying bacterium [START_REF] Bedmar | The complete denitrification pathway of the symbiotic, nitrogen-fixing bacterium Bradyrhizobium japonicum[END_REF].

Effects of typical conditions on dechlorination 3.3.1. Optimal 2,4-dichlorophenol loading effectuated full dechlorination in the Pd-MBfR

We conducted separate batch tests to investigate the kinetics of 2,4-DCP reduction in the three reactors. The influent concentrations used in these tests were 25, 45, 70, 100, and 140 μM, respectively (Table S2).

The supply of electron donors was sufficient, as the dechlorination fluxes were significantly lower than the maximum H 2 flux (300 e --meq m -2 d -1 ). For the three reactors (Fig. 4a), the effluent concentrations of 2,4-DCP and its intermediate products (2-CP, 4-CP, and phenol) increased with higher influent concentrations of 2,4-DCP, and the electron equivalents flux consumed by dechlorination also increased with higher influent concentrations. Conversely, the removal showed an inverse relationship with the influent concentrations. In the Pd-MBfR, as the influent concentration increased from 45 μM to 70 μM, the removal of 2,4-DCP dropped abruptly from 97.6% to 90.2%. Similarly, in the MBfR thicker Pd-biofilm compared to the PdNPs-film (~30 µm vs. ~1.75 µm), the diffusion of 2,4-DCP within the thick biofilm to reach the PdNPs surface is limited, resulting in a decrease in the reaction order.

Higher nitrate loading enhanced phenol removal

NO 3 -is a common contaminant coexisted with 2,4-DCP in water and numerous studies have demonstrated that NO 3 -is a preferred electron acceptor in hydrogen autotrophic reduction [START_REF] Xia | Simultaneous removal of selected oxidized contaminants in groundwater using a continuously stirred hydrogen-based membrane biofilm reactor[END_REF][START_REF] Zhao | Effects of multiple electron acceptors on microbial interactions in a hydrogen-based biofilm[END_REF], and it can also serve as a nitrogen source for microbial metabolism. Series B investigated the impact of NO 3 -loading, with influent concentrations of 120, 220, 420, 820, and 1160 μM, respectively (Table S2).

In the Pd-MBfR (Figs. 5a and5b), as the influent concentration of NO 3 -increased from 120 μM to 420 μM, the effluent concentrations of and further increases resulted in no improvement in 2,4-DCP removal.

In the MBfR, increasing the H 2 pressure promoted the reduction of both 2,4-DCP and NO 3 -. However, the impact on NO 3 -reduction was greater than that on 2,4-DCP reduction, indicating that NO 3 -was the favorable electron acceptor. In the Pd-film reactor, where there was no electron competition between 2,4-DCP and NO 3 -reduction, the removal of 2,4-DCP significantly improved with increasing H 2 pressure, reaching saturation at around 10 psig. At the same H 2 pressure, the Pd-MBfR outperformed the MBfR for 2,4-DCP reduction, but at H 2 pressures higher than 10 psig, a large amount of NH 4 + accumulated. Therefore, it is recommended to select an optimal H 2 pressure of around 10 psig for all three reactors.

pH had a significant effect on the dechlorination flux and product selectivity

The pH plays a crucial role in microbial growth [START_REF] Lee | Effects of pH and precipitation on autohydrogenotrophic denitrification using the hollow-fiber membrane-biofilm reactor[END_REF] and significantly influences the adsorption, dechlorination, and reduction processes of 2, 4-DCP [START_REF] Xu | Dechlorination of 2,4-dichlorophenol by nanoscale magnetic Pd/Fe particles: Effects of pH, temperature, common dissolved ions and humic acid[END_REF]. We conducted series E to investigate the pH influence by varying it at 5.0, 6.0, 7.0, 8.0, and 9.0, with applied H 2 and influent 2, 4-DCP at the steady-state values of 15 psig and 100 μM, respectively (Table S2).

In the Pd-MBfR, the most effective 2,4-DCP reduction occurred at pH 7.0, and alkaline conditions had a stronger inhibitory effect on 2,4-DCP reduction compared to acidic conditions. In the MBfR, the optimum pH for 2,4-DCP reduction was 7.0, while at pH 9.0, 2,4-DCP reduction was almost negligible. Alkaline conditions had a stronger inhibitory effect on microbial reduction of 2,4-DCP. In the Pd-film reactor, the effect of pH on 2,4-DCP reduction was relatively minor, with the optimal pH range being 6.0 to 7.0. The pKa values of 2,4-DCP, 2-CP, 4-CP, and phenol are 7.80, 9.20, 8.55, and 9.89, respectively. When the pH is higher than the pKa value, chlorophenols act as weak acids and dissociate H + , resulting in a negatively charged state [START_REF] Xu | Dechlorination of 2,4-dichlorophenol by nanoscale magnetic Pd/Fe particles: Effects of pH, temperature, common dissolved ions and humic acid[END_REF]. Microbial cell surfaces typically possess negative charges, resulting in electrostatic repulsion between these surfaces and negatively charged chlorophenols. This repulsion can concentrations of 0, 150, 250, 550, and 750 μM, respectively (Table S2).

In the Pd-MBfR (Figs. 6a and6b), with an increase in influent NO

Accuracy control of H 2 supply accelerated dechlorination

The effects of H 2 -delivery flux on dechlorination flux and product selectivity were investigated by adjusting the H 2 pressure in the hollowfiber membrane. In the Pd-MBfR, when the maximum H 2 flux was 60% of the total demand (3 psig H 2 ), 2,4-DCP removal was only 55% (Fig. 7a). Applying 6 psig H 2 resulted in the maximum H 2 flux, which stoichiometrically matched the total demand for complete dechlorination of 100 μM 2,4-DCP to phenol. Despite this, the removal was still incomplete at 68%, indicating that the actual H 2 delivery to 2,4-DCP hydrodechlorination was less than the theoretical maximum capacity. A reason is that the actual H 2 concentration at the exterior of the membrane was greater than 0, which is what gives the maximum flux. As a result, 2,4-DCP removal stepwise reached ~96% only when the H 2 pressure was increased to 15 psig (2-fold the theoretical H 2 demand),

Conclusion

In general, in-situ generated PdNPs associated with the biofilm to form a stable Pd-biofilm could enhance 2,4-DCP removal in the Pd-MBfR system. We investigated the effect factors on 2,4-DCP removal in the Pd-MBfR, including influent 2,4-DCP loading, NO 3 -loading, NO 2 -loading, hydrogen partial pressure, and pH. Comparative tests were also carried out using the biotic MBfR and the abiotic Pd-film reactor contain PdNPs alone in parallel, and the major summary points are as follows:

(1) Compared to sole biofilm and PdNPs, coupling of PdNPs with biofilm improved the removal of 2,4-DCP and total phenols. The Pd-MBfR showed optimal 2,4-DCP removal rate (96%) for effluent concentrations below 70 μM.

(2) In the Pd-MBfR and the MBfR, there was competition between NO 3 -reduction and 2,4-DCP reduction, and NO 3 -reduction was preferred. The reduction of 2,4-DCP would be suppressed when the influent NO 3 -concentration exceed 220 μM. (5) In the Pd-MBfR and the MBfR, the optimal pH was 7.0 and 6.0, respectively, with alkaline conditions exhibiting a stronger inhibition on 2,4-DCP reduction compared to acidic conditions. In the Pd-film reactor, the pH had a relatively small effect on 2,4-DCP reduction, with the optimal pH range being 6.0 to 7.0.

Fig. 1 .

 1 Fig. 1. Transmission electron microscopy (TEM) images of selected areas in cross sections of Pd-biofilm (a) and Pd-film (d). The grey shapes are bacterial cells, and the dark spots are Pd(0). High resolution transmission electron microscopy (HRTEM) images of Pd(0) crystallites in Pd-biofilm (b) and Pd-film (e). Size distribution histograms of the Pd nanoparticles on the edge of the aggregated Pd-film (c) and deposited on the microbial cell surface of Pd-biofilm (f).

Fig. 3 Fig. 2 .Fig. 3 .

 323 Fig. 2. The variation in influent and effluent concentrations of 2,4-DCP during the reduction stage for the palladized membrane biofilm reactor (Pd-MBfR) (a), membrane biofilm reactor (MBfR) (b), and membrane palladium-film rector (Pd-film reactor) (c). 2,4-DCP, 2-CP, 4-CP denotes 2,4-dichlorophenol, orthochlorophenol, and para-chlorophenol, respectively.

Fig. 4 .

 4 Fig. 4. The effect of influent 2,4-dichlorophenol loading on 2,4-dichlorophenol reduction in the Pd-biofilm reactor (a, b), the biofilm reactor (c, d), and the Pd-film reactor (e, f).

2, 4 - 4 +

 44 DCP and 2-CP increased, while the concentrations of phenol, 2,4-DCP removal rate, and dechlorination flux decreased. Low concentrations of NO 3 -(≤ 420 μM) slightly inhibited the reduction of 2,4-DCP, while higher concentrations of NO 3 -(≥ 820 μM) led to an inadequate supply of electron donors, moderately impacting the reduction of 2,4-DCP. On the other hand, the effluent concentrations of NO 3 -, NO 2 -, and NH , as well as denitrification flux increased with higher influent concentrations of NO 3 -. The removal of NO 3 -remained stable at 82.8% in series B1-B3 but decreased to 65.6% in series B4-B5. The process of NO 3 reduction includes two steps: the reduction of NO 3 -to NO 2 -, which is a relatively slow reaction, and the subsequent reduction of NO 2 -to N 2 , which is a faster reaction. Only when electron donor was limited, NO 2 -accumulated in the effluent. Thus, the concentration of NO 2 -can serve as an indicator to assess the availability of electron donors [5,11]. In series B4-B5, the accumulation of NO 2 -indicates a relatively insufficient supply of electron donors (H 2 ) [35]. In the MBfR, low concentrations of NO 3 -(≤ 220 μM) had minor impact on the reduction of 2,4-DCP, while higher concentrations of NO -(≥ 420 μM) inhibited the reduction of 2,4-DCP. NO 3 -concentrations of 820 μM led to an inadequate supply of electron donors, resulting in a decline in 2,4-DCP removal. Since PdNPs in the Pd-film reactor were almost incapable of reducing NO 3 -, the reduction of 2,4-DCP was not influenced by the influent NO 3 -loading. In summary, for the Pd-MBfR to achieve overall removal of both 2,4-DCP and NO 3 -, the influent NO 3 -loading should not exceed 220 μM. The differences between the Pd-MBfR and the MBfR were attributed to the competition between NO 2 -, produced by the biological reduction of NO via denitrification, and the reduction of 2,4-DCP, which was catalyzed by PdNPs. Subsequent experiments investigating the effect of NO loading and H 2 pressure confirmed this phenomenon. 3.3.3. Higher nitrite loading lowered the selectivity to phenol NO 2 -is an intermediate product in the reduction of NO 3 -and is known to be toxic, exerting inhibitory effects on dechlorinating microorganisms [2,15,20,21]. Additionally, NO 2 -reduction can be catalyzed by Pd, which competed with the reduction of 2,4-DCP. Series C investigated the influence of NO 2 -loading on 2,4-DCP removal, with influent

Fig. 5 .

 5 Fig. 5. The effect of influent nitrate on 2,4-dichlorophenol reduction in the Pd-biofilm reactor (a, b), the biofilm reactor (c, d), and the Pd-film reactor (e, f).

Fig. 6 .

 6 Fig. 6. The effect of influent nitrite on 2,4-dichlorophenol reduction in the Pd-biofilm reactor (a, b), the biofilm reactor (c, d), and the Pd-film reactor (e, f).

Fig. 7 .

 7 Fig. 7. The effect of hydrogen pressure on 2,4-dichlorophenol reduction in the Pd-biofilm reactor (a, b), the biofilm reactor (c, d), and the Pd-film reactor (e, f).

Fig. 8 .

 8 Fig. 8. The effect of pH on 2,4-dichlorophenol reduction in the Pd-biofilm reactor (a, b), the biofilm reactor (c, d), and the Pd-film reactor (e, f).

( 3 ) 4 +.

 34 NO 2 -inhibited 2,4-DCP reduction in all three reactors, but the inhibitory effect on bioreduction of 2,4-DCP is greater than on Pdcatalyzed reduction. When electron donors were sufficient, PdNPs rapidly catalyzed NO 2 -reduction, reducing the impact of NO 2 -on 2,4-DCP reduction. (4) Increasing hydrogen pressure promoted the reduction of 2,4-DCP and NO 3 -in the Pd-MBfR. While at higher hydrogen pressure (15 psig), the reduction of 2,4-DCP was promoted, but excessive NO 3 -reduction led to the accumulation of NH When NO 3 -was present, the hydrogen partial pressure should not exceed 10 psig.

  After feeding the Pd-MBfR with the medium containing 30.7 μM of 2,4-DCP, intermediate products were undetected in the effluent during the first two days (Fig.2a). Then, phenol started to accumulate in the effluent, along with the appearance of 2-CP on day 3. Increasing the influent concentration of 2,4-DCP to 61.3 μM resulted in in a subsequent phenol accumulation, reaching a maximum concentration of 20.6 μM (Stage II). After day 20, Pd-MBfR achieved stable removal for both 2,4-DCP and the intermediate products. To further improve the removal rate, the hydrogen pressure was increased to 15 psig (1.0 atm) (Stage III). In Stage III, the effluent concentrations of 2,4-DCP, 2-CP, and phenol were approximately 1.8 μM, 1.0 μM, and 3.6 μM, respectively, resulting in a total phenols removal of approximately 89.3%. These results demonstrate that Pd-MBfR exhibited complete removal of 2,4-DCP with minimal phenolic intermediates.

	1 •
	(1 1 1), 46.7 • (2 0 0), 68.1 • (2 2 0), and 82.1 • (3 1 1) in the XRD spectra,
	confirming the predominant presence of the Pd(0) crystal structure with
	dominant facets of (1 1 1) in both reactors.
	3.2. Continuous dechlorination and mineralization of 2,4-dichlorophenol

Table 2

 2 Parameters of reduction kinetics in steady state under various influent concentrations of 2,4-dichlorophenol.

	Reactors	Effluent concentration (μM)			η 2,4-DCP (%)	n	k	R 2
		2,4-DCP	2-CP	4-CP	Phenol
	Pd-MBfR	0.8-33.7	0-4.5	0-1.7	1.3-46.7	97.0-82.6	0.48	0.1090	0.9872
	MBfR	3.6-74.1	\	6.2-45.1	\	85.5-62.0	0.52	0.0488	0.9501
	Pd-film reactor	1.7-15.6	0.6-3.8	0-0.7	21.7-167.8	92.5	0.95	0.0519	0.9956

2 -

 2 concentration from 0 to 150 μM, there were minimal changes in the effluent concentrations, removal rate, and dechlorination flux. As the influent NO 2 -concentration continued to rise, the removal of 2,4-DCP increased, while the concentrations of 2-CP and 4-CP initially increased and then decreased. NO 2 -inhibited the 2,4-DCP reduction, which resulted in a decrease in the removal rate of the MBfR and the Pdfilm reactor. Low concentrations of NO 2

	-(≤ 150 μM) had minor impact -(≥ 250 μM) on the 2,4-DCP removal, while high concentrations of NO 2 inhibited 2,4-DCP reduction. Increasing the influent NO 2 -loading also led to a decrease in NO 2 -removal. Among the three reactors, the inhib-itory effect of NO 2 -on the reduction of 2,4-DCP followed the order of MBfR > > Pd-MBfR > Pd-film reactor. This indicates that NO 2 -had a
	greater impact on bioreduction of 2,4-DCP. PdNPs can rapidly catalyze NO 2 -reduction, reducing its toxicity to microorganisms and minimizing
	its impact on the reduction of 2,4-DCP. Considering the overall removal of 2,4-DCP and NO , the influent NO 2 -concentration should not exceed
	250 μM.

Acknowledgements

The authors would like to acknowledge the co-funding of this work by the Natural Science Foundation of Shanghai (No.22ZR1443200) and the National Natural Science Foundation of China (No.52070130).

Data Availability

Data will be made available on request.

CRediT authorship contribution statement

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Appendix A. Supporting information

Supplementary data associated with this article can be found in the online version at doi:10.1016/j.jece.2024.112176.

Enhanced removal of 2,4-dichlorophenol by coupling of Pd nanoparticles with biofilm

Chengyang Wu a , Jingzhou Zhou b , Si Pang b , Lin Yang b , Xiaodi Li b , Eric Lichtfouse c , Siqing Xia b , Hongbo Liu