Milad Mousazadeh

Zohreh Naghdali

Işık Kabdaşlı

Miguel A Sandoval

Fatima Ezzahra Titchou

Farideh Malekdar

Mahmoud Nasr

Carlos A Martínez-Huitle

Eric Lichtfouse

Mohammad Mahdi Emamjomeh
email: m_emamjomeh@yahoo.com

Milad Contact

Mousazadeh

Reclamation of forward osmosis reject water containing hexavalent chromium via coupled electrochemical-physical processes

Keywords: Hexavalent chromium removal, Electrocoagulation, Forward osmosis concentrate, Response surface methodology, Sustainable treatment Highlights

Forward osmosis is a water separation process that uses the natural energy of osmotic pressure to separate water from dissolved solutes through a semipermeable membrane. One of the major challenges using this process is the rejection water which contains high content of pollutants, hindering its practical application. Herein, for the first time, this work introduces a coupled electrochemical-physical process including iron-electrocoagulation/filtration/sedimentation as a cost-effective treatment to the forward osmosis reject water containing hexavalent chromium to be reclaimed. The synergistic treatment was optimized through a central composite design and response surface methodology to enhance hexavalent Cr removal and minimize operating costs, electrical energy consumption, and settled sludge volume. A 90.0% chromium removal was achieved under optimized conditions: electrolysis time of 59.7 min and current of 1.24 A (J = 6.32 mA cm -2 ). In addition, operating costs of 0.014 USD m -3 , electrical energy consumption of 0.005 kWh m -3 , and settled sludge volume of 445 mL L -1 were obtained.

Introduction

Heavy metal pollution has been associated with mining and industrial activities [START_REF] Perraki | Tracing the origin of chromium in groundwater: current and new perspectives[END_REF][START_REF] Tünay | Chemical oxidation applications for industrial wastewaters 1st ed[END_REF][START_REF] Tünay | Treatment of metal finishing wastes[END_REF][START_REF] Karimi-Maleh | Recent advances in removal techniques of Cr (VI) toxic ion from aqueous solution: a comprehensive review[END_REF]. Among these metals, hexavalent chromium (Cr(VI)) is one of the most concerning pollutants due to its hazardous effects to human beings and environmental health. Cr(VI) can be present in water matrices as monomeric ions H 2 CrO o 4 , HCrO - 4 , and CrO 2- 4 ; or as the dimeric ion Cr 2 O 2- 7 [START_REF] Palmer | Natural attenuation of hexavalent chromium in groundwater and soils[END_REF]. These oxyanions are the most toxic species of chromium. They have been associated with irreparable harms including mutagenic, teratogenic, carcinogenic, reproductive and birth defects. Moreover, kidney and liver disorders, dermatitis, ulcers and other health effects have been linked to the chronic exposure of wastewater containing high concentrations of Cr(VI). Consequently, the World Health Organization advised a maximum concentration level of 0.05 mg L -1 [START_REF] Who | Guidelines for drinking-water quality: volume 1: recommendations[END_REF][START_REF]Chromium in drinking water[END_REF]. Wastewaters produced by different industrial processes may contain Cr(VI), therefore, they need to be strictly regulated and treated before discharge to the environment [START_REF] Tünay | Chemical oxidation applications for industrial wastewaters 1st ed[END_REF][START_REF] Tünay | Treatment of metal finishing wastes[END_REF]. Some technologies such as membrane-based processes and electrochemical techniques have been developed to meet permissible limit. However, as individual processes, some limitations are worth noting [START_REF] Perraki | Tracing the origin of chromium in groundwater: current and new perspectives[END_REF][START_REF] Karimi-Maleh | Recent advances in removal techniques of Cr (VI) toxic ion from aqueous solution: a comprehensive review[END_REF][START_REF] Gracepavithra | A review on cleaner strategies for chromium industrial wastewater: present research and future perspective[END_REF][START_REF] Kerur | Removal of hexavalent Chromium-Industry treated water and Wastewater: a review[END_REF][START_REF] Prasad | Chromium contamination and effect on environmental health and its remediation: a sustainable approaches[END_REF].

Forward osmosis (FO) is a membrane separation technology with increasing attention for water treatment [START_REF] Ang | Forward osmosis research trends in desalination and wastewater treatment: a review of research trends over the past decade[END_REF][START_REF] Chia | Sustainable membrane technology for resource recovery from wastewater: forward osmosis and pressure retarded osmosis[END_REF][START_REF] Coday | The sweet spot of forward osmosis: treatment of produced water, drilling wastewater, and other complex and difficult liquid streams[END_REF][START_REF] Singh | A comprehensive review of standalone and hybrid forward osmosis for water treatment: membranes and recovery strategies of draw solutions[END_REF]. FO is an osmotically driven membrane process which occurs in a spontaneous manner without requiring hydraulic pressure [START_REF] Ibrar | Challenges and potentials of forward osmosis process in the treatment of wastewater[END_REF]. The concentration gradient provides a osmotic pressure difference between concentrated draw solution (DS) and diluted feed solution (FS) across a semi-permeable membrane [START_REF] Ang | Forward osmosis research trends in desalination and wastewater treatment: a review of research trends over the past decade[END_REF][START_REF] Ezugbe | Membrane technologies in wastewater treatment: a review[END_REF]. Thus, FO requires lower energy than conventional reverse osmosis and barely experiences fouling issues [START_REF] Ibrar | Challenges and potentials of forward osmosis process in the treatment of wastewater[END_REF]. However, challenges arise from the reject water handling that contains a high content of pollutants. The removal of pollutants from the FS is an urgent issue to ensure competitive deployment of FO technology. Furthermore, highly concentrated impurities in the forward osmosis reject water (FOR) can cause fouling of the membrane, which requires frequent chemical cleaning. Even though electrocoagulation (EC) can experience challenges to treat diluted chromium effluents [START_REF] Garcia-Segura | Electrocoagulation and advanced electrocoagulation processes: a general review about the fundamentals, emerging applications and its association with other technologies[END_REF], it arises as an electrified process that can effectively treat FOR. Electrocoagulation is an electrochemical technology that removes target pollutants from water matrices based on coagulation principles. This technique relies on the use of sacrificial electrodes which provide coagulant species (i.e. Fe 2+ , Fe 3+ , Al 3+ ). The coagulant dose is controlled by the applied current. The electrochemical reactions occurring in an EC reactor, with iron material used as sacrificial electrode, are stated as follows [START_REF] Sandoval | Arsenic and fluoride removal by electrocoagulation process: a general review[END_REF]:

At the anode: Fe is oxidized to ferrous iron:

Fe (s) Fe 2+ (aq) + 2e - (1)
Meanwhile, at the cathode OH ─ and H 2 are generated from the water reduction reaction:

2H 2 O + 2e - H 2 + 2OH - (2)
In the reaction solution, hexavalent chromium is reduced to trivalent chromium by dissolved ferrous ions as follows [START_REF] Aoudj | Simultaneous removal of chromium(VI) and fluoride by electrocoagulation-electroflotation: application of a hybrid Fe-Al anode[END_REF][START_REF] Un | The treatment of chromium containing wastewater using electrocoagulation and the production of ceramic pigments from the resulting sludge[END_REF].

At pH < 6.5:

HCrO - 4 + 3Fe 2+ + 7H + Cr 3+ + 3Fe 3+ + 4H 2 O (3) Cr 2 O 2- 7 + 6Fe 2+ + 14H + 2Cr 3+ + 6Fe 3+ + 7H 2 O (4)
At 6.5 < pH < 7.5:

CrO 2- 4 + 3Fe 2+ + 4H 2 O 3Fe 3+ + Cr 3+ + 8OH - (5)
At pH > 7.5:

CrO 2- 4 + 3Fe(OH) 2 + 4H 2 O Cr(OH) 3 + 3Fe(OH) 3 + 2OH - (6)
Additionally, Fe 3+ may be generated when ferrous ions react with the dissolved oxygen:

4Fe 2+ + 4H + + O 2 4Fe 3+ + 2H 2 O, in acid media (7) 4Fe 2+ + 2H 2 O + O 2 4Fe 3+
+ 4OH -, in alkaline media [START_REF] Gracepavithra | A review on cleaner strategies for chromium industrial wastewater: present research and future perspective[END_REF] In the bulk solution, insoluble iron species Fe(OH) 2 , Fe (OH) 3 , and/or FeOOH (goethite) are produced according to Equations ( 9)- [START_REF] Ang | Forward osmosis research trends in desalination and wastewater treatment: a review of research trends over the past decade[END_REF], respectively.

4Fe 2+ + 8OH - 4Fe(OH) 2 (9) 4Fe(OH) 2 + 2H 2 O + O 2 4Fe(OH) 3 (10) Fe(OH) 3 FeOOH + H 2 O (11)
These dissoluble iron hydroxides favour the formation of flocs. However, ferric oxyhydroxide and goethite are considered the preferent coagulants to remove chromium because these species precipitate in a greater interval of the pH value [START_REF] Aoudj | Simultaneous removal of chromium(VI) and fluoride by electrocoagulation-electroflotation: application of a hybrid Fe-Al anode[END_REF][START_REF] Un | The treatment of chromium containing wastewater using electrocoagulation and the production of ceramic pigments from the resulting sludge[END_REF].

Trivalent chromium combines with hydroxyl ions to form chromium hydroxide (Cr(OH) 3 ) which precipitate as amorphous solids. In addition, negative chromium species might adsorb on the positively charged metal hydroxide [START_REF] Mouedhen | Electrochemical removal of Cr(VI) from aqueous media using iron and aluminum as electrode materials: towards a better understanding of the involved phenomena[END_REF]. Moreover, trivalent chromium removal may occur through other means as well, such as adsorption onto ferric oxyhydroxide and goethite.

Despite the potential benefits of the FO treatment process, this interesting technique suffers from reject water handling that contains a high content of pollutants. According to the literature, it is one of the main challenges that prevents its practical application. Here, for the first time, this work studies as proof of concept the performance of the iron-electrocoagulation/ filtration/sedimentation (Fe-EC/filtration/sedimentation) process to deal with the environmental issue caused by FOR. The Fe-EC module was optimized through experimental design and modelling by the response surface methodology to reclaim FOR with chromium residues. Engineering figures of merit were used to optimize technology competitiveness in terms of key indicators such as chromium removal, electrical energy consumption, operating cost and settled sludge volume.

Experimental

Forward osmosis reject water

FOR was obtained after the forward osmosis process which used a commercial aquaporin-based biomimetic polymeric membrane (Aquaporin Asia Pte. Ltd, Singapore). This process was performed in our previous work to treat a solution containing dichromate potassium [START_REF] Naghdali | Optimization of the forward osmosis process using aquaporin membranes in chromium removal[END_REF]. The composition of the obtained feed solution includes CaCl 2 , KCl and MgCl 2 were 0.012, 0.007 and 0.01 mg L -1 , respectively. The initial concentration of Cr(VI) in reject water (as feed solution or concentrated solution) was 69 mg L -1 . The pH value and the electrical conductivity were 7.1 and 18 mS m -1 . These initial values were maintained constant in all the experiment set-up. Thereupon, the obtained reject water was subjected to the Fe-EC reactor as mentioned in the following section.

Experimental set-up and procedure

The iron-electrocoagulation (Fe-EC) unit is schematized in Figure 1. It consisted of a cylindrical glass reactor (d: 9 cm, h: 12.5 cm) with an effective volume of 500 mL. To ensure a homogenous mixing during Fe-EC experiments, a magnetic stirrer (model SHA R-50, Iran) was placed at the bottom of the reactor at a constant rate of 250 rpm. Four iron electrodes (70 mm (width) × 70 mm (length) × 5 mm (depth)) were mounted on the EC reactor where the electrode gap was fixed at 10 mm. The electrodes were vertically placed in a monopolar-parallel connection using a power supply (model JPS303D Iran; Emax: 30 V) operated under galvanostatic mode. After each run, the electrodes were well polished with emery paper to avoid electrode passivation and then washed with acid reagent and distilled water to remove any impurities from the surface. All electrocoagulation runs were performed in triplicate showing a standard deviation below 3%; average data are reported.

The pH of the solution was monitored using a multiparameter analyzer (CONSORT C831, Belgium). The pH value was maintained at approximately 8.0-8.5 by manual addition of 0.01 M H 2 SO 4 and 0.1 M KOH to ensure minimum solubility of Cr(OH) 3 [START_REF] Tünay | Treatment of metal finishing wastes[END_REF]. A digital calibrated conductivity meter (Leybold 666222, Germany) was used for electrical conductivity measurements.

At the end of each Fe-EC run, samples were collected and filtered through a 0.45-µm membrane filter for chromium analysis. The treated solution was allowed to settle for about 60 min in a quiescent state. The settled sludge volume values were recorded after complete settlement.

Collected samples were analysed for total chromium content by atomic absorption spectrophotometry using a BRAIC WFX-130 (China) spectrophotometer at 620 nm according to Standard Methods for the Examination of Water and Wastewater [START_REF] Apha | Standard methods for the examination of water and wastewater[END_REF]. The chromium removal efficiency was calculated according to Equation (12) [START_REF] Naghdali | Optimization of the forward osmosis process using aquaporin membranes in chromium removal[END_REF]:

R(%) = C 0 -C t C 0 × 100 ( 12
)
where C 0 is the initial concentration of chromium and C t is the effluent chromium concentration measured at the end of each electrocoagulation run in mg Cr L -1 . The settled sludge volume (SSV) is one of the most important parameters for evaluating the performance of electrocoagulation. Sludge separation and dewatering is an essential part of coagulation-based treatment processes that can be costly. The settled sludge volume can be determined according to Equation [START_REF] Coday | The sweet spot of forward osmosis: treatment of produced water, drilling wastewater, and other complex and difficult liquid streams[END_REF]:

SSV (mL•L -1 ) = A × d V s ( 13
)
where A is the area of the cylindrical vessel (cm 2 ), d is the height occupied by the sludge (cm) and V s is the total volume of water treated (L). The operational cost (OPC), electrode material consumption (EMC) and electrical energy consumption (EEC) were the main parameters investigated in this case of study as techno-economic indicators for electrocoagulation competitiveness. These parameters were calculated according to the following equations [START_REF] Mousazadeh | Electrocoagulation as a promising defluoridation technology from water: a review of state of the Art of removal mechanisms and performance trends[END_REF][START_REF] Niazmand | Optimization of electrocoagulation conditions for the purification of table olive debittering wastewater using response surface methodology[END_REF][START_REF] Titchou | Batch elimination of cationic dye from aqueous solution by electrocoagulation process[END_REF][START_REF] Titchou | Removal of persistent organic pollutants (POPs) from water and wastewater by adsorption and electrocoagulation process[END_REF]:

OPC (USD • m -3 ) = a × EMC + b × EEC (14) EMC (kg • m -3 ) = I × t × M z × F × V s (15) EEC (kWh • m -3 ) = E Cell It 1000 × V s ( 16
)
where EMC and EEC present the consumption quantities of electrode material and electricity required for target pollutant removal. 'a' (USD•kW•h -1 ) and 'b' (USD•g -1 ) are the price of electricity and electrode material, respectively. The variable I is the applied current (A), t is the electrolysis time (h), M is the molecular mass of iron (55.85 g mol -1 ), z is the number of electrons transferred (z = 2), F is the Faraday's constant (96,487 C mol -1 ), V s is the volume of the solution (m 3 ) and E cell (V) is the average of the applied voltage in the cell.

Mathematical modelling for optimization

Traditionally, the most common practice in the study of operational parameters is the traditional single factor at time approach. It consists in looking for the effect of one factor, which must vary systematically, while keeping the other factors constant. The result of this univariate analysis does not guarantee the optimization of the parameters. This approach would only be valid if the factors to be optimized were totally independent of each other [START_REF] Geiger | Statistical methods for fermentation optimization. fermentation and biochemical engineering handbook[END_REF]. However, compared to single-factor and orthogonal experiments, where the optimization is find out through the study of multi-factor and level of design method by analysing each test point. Response surface methodology (RSM) constructs an approximate model in the design space using known experimental data to that meets the fitting accuracy between the design variables and the target response [START_REF] Yang | A structural optimization model of a biochemical detection micromixer based on RSM and MOEA/D[END_REF]. Generally, RSM has the ability to model as many responses to measure, allows identifying the interactions between the independent variables, to model the system mathematically and to save time and money by reducing the number of experiments [START_REF] Toor | Optimization of Fenton process for removing TOC and color from swine wastewater using response surface method (RSM)[END_REF]. In this work, RSM was employed as the optimization tool through experimental design to maximize pollutant removal while minimizing operational expenditure. The analysis was depicted using expert design 10 software (Stat-Ease, Inc., Minneapolis, USA) to perform a full central composite design with five levels (-1.68, -1, 0, +1, +1.68), consisting of 13 points (8 experiments and 5 central points). The selected independent variables were electrocoagulation time (A: 15.09, 24, 45.50, 67 and 75.91 min) and current (B: 0.51, 0.8, 1.50, 2.20 and 2.49 A). The appropriate high and low ranges were defined according to some preliminary tests. The selected responses for process optimization were Cr(VI) removal efficiency, operating costs, electrical energy consumption and settled sludge volume. The system was described by a numerical expression (Equation ( 17)) of the polynomial statistical model that allowed the mathematical description of the RSM [START_REF] Mousazadeh | Electrocoagulation as a promising defluoridation technology from water: a review of state of the Art of removal mechanisms and performance trends[END_REF][START_REF] Emamjomeh | Simultaneous removal of phenol and linear alkylbenzene sulfonate from automotive service station wastewater: optimization of coupled electrochemical and physical processes[END_REF]:

Y = b 0 + i=k i=1 b i x i + i=k i=1 b ii x 2 i + i=k 1≤i≤j b ij x i x j + 1 (17)
where Y represents the response and x i and x j are the coded independent variables. Then, β 0 is the constant coefficient of the model, β i is the linear coefficient, β ii is the quadratic effect coefficient, β ij is the interaction effect coefficient and ε is the residual term associated with random error. Three replicates of the central point were conducted to estimate the pure error of experiments. The validation of the model was confirmed through analysis of variance (ANOVA) to ensure statistical significance.

For multi-response optimization, the desirability method is one of the most widely used approaches to respond to multi-objectives [START_REF] Asante-Sackey | Ion exchange dialysis for aluminium transport through a face-centred central composite design approach[END_REF]. It associates an individual scale-free desirability (d i ), where 0 ≤ d i ≤ 1 [START_REF] Ano | Nitrate removal by electrocoagulation process using experimental design methodology: a techno-economic optimization[END_REF] with each response (Y i ). The values d i (0 and 1) present the complete refusal and maximum satisfaction of the response, respectively. The overall desirability (D) considers all response Y i desirability and their requirements. D can be determined according to Equation ( 18) [START_REF] Kerur | Removal of hexavalent Chromium-Industry treated water and Wastewater: a review[END_REF]:

D = n i=1 d W i i 1 W i (18)
where n and W i represent the number of responses and the weighting factors of the response Y i , respectively. 1, a complete chromium removal was achieved at 13th electrocoagulation run operated at a current of 2.49 A (J = 12.7 mA•cm -2 ) and an electrolysis time of 45.5 min. On contrary, the lowest current value (0.8 A) resulted in the cheapest operation cost (0.003 USD m -3 ) together the lowest settled sludge volume (180 mL L -1 ).

Results and discussions

Statistical analysis and model fitting via central composite design

According to the obtained results, the central composite design under response surface methodology suggested a quadratic model. The quadratic equations for study responses considering coded factors were obtained as follows: Cr(VI) removal efficiency(%)

= +90.69 + 1.97 × A + 6.41 × B -1.36 × A × B + 0.65 × A2 + 0.17 × B 2 (19) OPC (USD) = +0.013 + 6.578E-003 × A + 7.498E-003 × B + 3.650E-003 × A × B + 8.000E-005 × A 2 + 5.800E-004 × B 2 (20) EEC (kWh•m -3 ) = +5.233E-003 + 2.857E-003 × A + 4.157E-003 × B + 2.118E-003 × A × B + 1.064E-004 × A 2 + 8.000E-004 × B 2 (21)
SSV (mL•L -1 ) = +499.00 + 91.11 × A + 247.71

× B + 50.00 × A × B -17.94 × A 2 -6.69 × B 2 (22)
where A and B are the coded terms for the independent variables of the chosen response as mentioned in Table 1.

The ANOVA results are outlined in Table S1. As can be seen, all terms in the statistical quadratic model were highly significant (p , 0.0001). Meanwhile, F-values for Cr(VI) removal of 127.68, operating cost of 982.04, electrical energy consumption of 139.55 and settled sludge volume of 183.44 were non-significant, which implied that there were only 0.01% chance that the F-values were out of design due to noise. Accordingly, good fits to the quadratic model were confirmed by large Fvalues, small p-values, and large values of R 2 , R 2 adjusted, and R 2 predicted.

Based on the ANOVA results, Adeq precision (AP) measures the signal-to-noise ratio, in other words, a measure of predicted response value range at the design points relative to associated mean prediction error. A ratio greater than 4 is desirable. AP values of 35.417, 99.506, 37.035 and 42.992 for Cr removal, operating cost, electrical energy consumption and settled sludge volume, respectively, indicated an adequate signal and high capability of developed models in predicting the results. Coefficient of variance (CV) describes the precision and reproducibility of a model. A model with a ratio less than 10% is considered reproducible. CV values of acceptable ranges from 0.83 to 9.83% proved the reproducibility of the model and high precision of obtained results. The lack-of-fit test (LOF test) describes the variation of data around the fitted model. The LOF test would be non-significant if the model fits the data well. The LOF test for Cr(VI) removal (p = 0.1331), operating cost (p = 0.9253), electrical energy consumption (p = 0.9351) and settled sludge volume (p = 0.8008) were not statistically significant, reinforcing that those points were properly distributed around the fitted model.

The diagnostics of the experimental data yielded the graphical results given in Figures 2 and3. Four aspects of graphical model validation are (i) the normal probability plot of residuals (Figure 2(a-d)); (ii) externally studentized residuals vs. predicted (Figure 2(e-h)); (iii) externally studentized residuals vs. run number (Figure 3(ad)) and (iv) predicted vs. actual plots (Figure 3(e-h)). The residuals almost lied on or close to the normal probability line (Figure 2(e-h)), which indicated the presence of very minimal errors in experimentation and testified the reliability of the quadratic model. Moreover, as is visible in Figure 3(a-d), the residuals lied well within the acceptable range from -3 to +3 which further validated the quadratic model. The actual vs. predicted plots displayed that the experimental data approximately overlapped with the predicted data (Figure 3(eh)), contributing as the last nail in the coffin of model.

Interaction of electrolysis time and applied current on chromium removal

In the EC process, the amount of electro-dissolved iron ions generated by the anode depends on the current and electrolysis time in accordance with Faraday's Law (Equation [START_REF] Ibrar | Challenges and potentials of forward osmosis process in the treatment of wastewater[END_REF]). An increase in the current density (J; mA cm -2 ) or the current (I; A) accelerates the reaction rate due to an increase in the amount of iron dissolved at cathode [START_REF] Zhou | Comparison of Cr(VI) removal by direct and pulse current electrocoagulation: implications for energy consumption optimization, sludge reduction and floc magnetism[END_REF][START_REF] Heidmann | Removal of Cr(VI) from model wastewaters by electrocoagulation with Fe electrodes[END_REF][START_REF] Khan | Hexavalent chromium removal in an electrocoagulation column reactor: process optimization using CCD, adsorption kinetics and pH modulated sludge formation[END_REF]. At pH values between 7.5 and 8.1, this increase in the Cr(IV) reduction rate was explained by an increase in concentrations of FeOH + and Fe(OH) o 2 [START_REF] Sedlak | Reduction of hexavalent chromium by ferrous iron[END_REF][START_REF] Olmez | The optimization of Cr(VI) reduction and removal by electrocoagulation using response surface methodology[END_REF]. Increasing current or prolonging electrolysis time also induces the formation of OH -ions owing to the water reduction reaction occurring at the cathode (Equation ( 2)), resulting in the precipitation of amorphous Fe(OH) 3 solid [START_REF] Arroyo | Effect of pH and chloride concentration on the removal of hexavalent chromium in a batch electrocoagulation reactor[END_REF]. At slightly alkaline pH, Cr(III) ions are combined with OH -ions to produce insoluble Cr(OH) 3 in the bulk solution [START_REF] Mouedhen | Electrochemical removal of Cr(VI) from aqueous media using iron and aluminum as electrode materials: towards a better understanding of the involved phenomena[END_REF][START_REF] Arroyo | Effect of pH and chloride concentration on the removal of hexavalent chromium in a batch electrocoagulation reactor[END_REF][START_REF] Villalobos-Lara | Electrocoagulation treatment of industrial tannery wastewater employing a modified rotating cylinder electrode reactor[END_REF]. Furthermore, two Cr(VI) removal mechanisms were proposed by Heidmann and Calmano [START_REF] Heidmann | Removal of Cr(VI) from model wastewaters by electrocoagulation with Fe electrodes[END_REF] depending on the current value for the EC process utilizing iron electrodes. According to their approach, at high currents varying between 1.0 and 3.0 A, Cr(VI) might be directly reduced to Cr(III) at the cathode surface prior to precipitation as Cr(OH) 3 . At low currents from 0.05 to 0.1 A, Fe(II) produced by the dissolution of iron anodes worked as a reducing agent to reduce Cr(VI) to Cr(III) before precipitating as Cr (OH) 3 together with either Fe(OH) 2 or Fe(OH) 3 depending on the formation of oxygen at the cathode [START_REF] Heidmann | Removal of Cr(VI) from model wastewaters by electrocoagulation with Fe electrodes[END_REF]. As mentioned in the following subsection, in our case, the second seems to be responsible for the chromium removal mechanism even at high currents.

Both experimental and model data obtained in the present study are consistent with the relevant literature [START_REF] Zhou | Comparison of Cr(VI) removal by direct and pulse current electrocoagulation: implications for energy consumption optimization, sludge reduction and floc magnetism[END_REF][START_REF] Olmez | The optimization of Cr(VI) reduction and removal by electrocoagulation using response surface methodology[END_REF]. As can be seen in Table 1, chromium removal efficiency significantly improved with increasing current, and a complete chromium removal was achieved at the highest applied current (2.49 A; J = 12.7 mA cm -2 ). Similarly, an extension in electrocoagulation treatment time had an enhancement effect on process performance in terms of chromium removal for the current up to 2.2 A (J = 11.22 mA cm -2 ). Together with this observation, it is worth noting that an extending electrolysis time from 24 to 67 min resulted in only 0.5% additional chromium removal (cf. runs 1 and 2) for a current value of 2.2 A.

Figure 4(a-b) emphasizes the mutual interaction between current and electrolysis time. Data indicated that almost complete chromium removal could be obtained by arranging of either current or electrolysis time as (i) increasing the current from 0.5 (J = 2.55 mA cm -2 ) to 2.49 A (J = 12.7 mA cm -2 ) (100%) for an electrolysis time of 45 min or (ii) extending electrocoagulation treatment time to 69 min (98.4%) at a current value of 2.2 A (J = 11.22 mA cm -2 ).

In the present study, where the initial Cr(VI) concentration was 69 mg L -1 , the optimum operation conditions were determined as an electrolysis time of 59.7 min and a current of 1.24 A (J = 6.32 mA cm -2 ). About 89.7% chromium removal was attained at these conditions. This efficiency is similar with that obtained by Deveci et al. [START_REF] Deveci | Enhancing treatability of tannery wastewater by integrated process of electrocoagulation and fungal via using RSM in an economic perspective[END_REF]. They achieved a 86.27% Cr(VI) removal at electrocoagulation conditions: current of 0.675 A, initial pH of 8, an electrolysis time of 60 min, and a conductivity of 56.2 mS cm -1 at 20°C (± 0.1) for a raw tannery wastewater (Cr(VI): 22.3 mg L -1 and COD: 18,800 mg L -1 ). Borba et al. [START_REF] Borba | Desirability function applied to the optimization of the photoperoxi-electrocoagulation process conditions in the treatment of tannery industrial wastewater[END_REF] studied the chromium removal from real tannery wastewater. The authors reported a 96.2% of removal efficiency using the following optimal parameters: pH 4, current density of 68.4 mA cm -2 and electrolysis time of 120 min. Khan et al. [START_REF] Khan | Hexavalent chromium removal in an electrocoagulation column reactor: process optimization using CCD, adsorption kinetics and pH modulated sludge formation[END_REF] reported that (i) process removal efficiency was more sensitive to the applied current and (ii) complete chromium (Cr(IV) of 49.96 mg L -1 ) removal could be achieved at optimum conditions: an applied current of 1.48 A, pH of 3.0 and an electrolysis time of 21.47 min. Gong et al. [START_REF] Gong | Toxicity reduction of reverse osmosis concentrates from petrochemical wastewater by electrocoagulation and Fered-Fenton treatments[END_REF] obtained conflicting results in the treatment of reverse osmosis concentrates from petrochemical wastewater by the Fe-EC process, which found that less than 20% of chromium was removed within 20 min at 30 mA cm -2 . This difference may be due to the fact that with increasing EC time, the pH changes which directly affect the formation of Fe(III) hydroxides and Fe(II) hydroxides during the Fe-EC process that finally affects the Cr(IV) removal [START_REF] Moreno-Casillas | Electrocoagulation mechanism for COD removal[END_REF].

Interaction of electrolysis time and current on settled sludge volume

Figure 5(a,b) exhibits the settling behaviour of flocs as a function of EC time and current value. All SSV analyses were evaluated for 60 min settling period. This means that after stopping the stirring, we allow the flocs to settle for 60 min. As expected, increasing current as well as EC time induced an increase in the sludge production due to increase in the co-precipitation of iron hydroxides with Cr(OH) 3 . Depending on effluent pH reached at the end of EC operation, Cr(OH) 3 may co-precipitate together with Fe(OH) 3 flocs as aforementioned. In our case, because solution pH was kept at a range of 8.0-8.5, both solid phases assumed to be generated during EC treatment, resulting in high SSV values particularly at increasing currents. The formation of two solids at pH greater than 7.0 was also confirmed by literature data [START_REF] Khan | Hexavalent chromium removal in an electrocoagulation column reactor: process optimization using CCD, adsorption kinetics and pH modulated sludge formation[END_REF][START_REF] Olmez | The optimization of Cr(VI) reduction and removal by electrocoagulation using response surface methodology[END_REF].

The flocs produced at the highest current value were the most difficult to settle, particularly, for current value of 2 A (J = 10.2 mA cm -2 ). For instance, elevating current from 0.8 to 2.2 A (J = 4.08-11.22 mA cm -2 ), high SSV corresponding to 180 and 590 mL L -1 within 24 min of Fe-EC were produced. A similar trend was observed for all Fe-EC runs with different operating times. The SSV values ranged between 495 and 830 mL L -1 for Fe-EC performed during 45.5 min or longer. High SSV values were obtained at extended EC times or higher current intensities. Ölmez [START_REF] Olmez | The optimization of Cr(VI) reduction and removal by electrocoagulation using response surface methodology[END_REF] experimentally treated the chromium plating process (Cr(VI): 1470 mg L -1 ) by EC with stainless steel electrodes. In that study, SSV was obtained as 928 mL L -1 (sludge volume index = 80 mL g -1 , sludge = 11.6 g VSS•L -1 ) at optimum EC conditions: a current value of 7.4 A, an EC time of 70 min and an initial pH of 11.0. It is obvious that during Fe-EC, hydrogen bubbles are evolved at the cathode and can be trapped in the flocs consequently provoking them to float [START_REF] Jimenez | Study of the production of hydrogen bubbles at low current densities for electroflotation processes[END_REF]. As a result, the lower the current value of the FOR in the Fe-EC cell, the more bubbles formed and the more likely flotation occurring. As shown in Figure 5(a,b), this phenomenon was most notable when the current value was lower than 1.5 A (J = 7.65 mA cm -2 ) at the initial EC time (30 min).

In general, the SSV data was in line with the results of Brahmi et al. [START_REF] Brahmi | Use of electrocoagulation with aluminum electrodes to reduce hardness in Tunisian phosphate mining process water[END_REF] showing that the most important drawback of the EC process are the long settling time periods. Whilst the results were contrary to the opinion of Ashraf et al. [START_REF] Ashraf | Electrocoagulation for the purification of highly concentrated brine produced from reverse osmosis desalination of coal seam gas associated water[END_REF] who found that over time, the flocs settled well and reached a minimum of about 200 mL L -1 .

Interaction of electrocoagulation time and current on electrical energy consumption

Electrical energy consumption is defined as the amount of electrical energy consumed per unit volume (m 3 ) of wastewater to be treated. Electrical energy consumption together with electrode material consumption are generally used to estimate the operating cost of EC. Therefore, the effect of current and EC time on electrical energy consumption were explored at a range of 0.5-2.5 A (J = 2.55-12.75 mA cm -2 ) and 15-76 min, respectively. Figure 6 Extending Fe-EC operation time significantly increased the electrical energy consumption as well as improved process efficiency at all tested conditions. For instance, an extension in EC time from 24 to 67 min resulted in a threefold increase in electrical energy consumption in both current values of 0.8 and 2.2 A (J = 4.08-11.22 mA cm -2 ). It is obvious that the effect of EC time on electrical energy consumption became significant when the current increased. The highest electrical energy consumption was determined as 0.015 kWh m -3 at a current value of 2.2 A (J = 11.22 mA cm -2 ) and at an EC time of 67 min. For an EC time of 45.5 min, current values of 0.5, 1.5 and 2.49 A (J = 2.55, 7.65 and 12.7 mA cm -2 ) yielded electrical energy consumptions of 0.001, 0.005 and 0.013 kWh m -3 , respectively, indicating that this operating parameter also has a significant effect on electrical energy consumption. In these Fe-EC experiments, complete Cr(VI) removals were achieved. These findings were in good agreement with the recent literature [START_REF] Khan | Hexavalent chromium removal in an electrocoagulation column reactor: process optimization using CCD, adsorption kinetics and pH modulated sludge formation[END_REF][START_REF] Arslan-Alaton | Electrocoagulation of simulated reactive dyebath effluent with aluminum and stainless steel electrodes[END_REF][START_REF] Kabdaşlı | Complexing agent and heavy metal removals from metal plating effluent by electrocoagulation with stainless steel electrodes[END_REF][START_REF] Kabdaşlı | Electrocoagulation applications for industrial wastewaters: a critical review[END_REF][START_REF] Kabdaşlı | Treatment of phthalic acid esters by electrocoagulation with stainless steel electrodes using dimethyl phthalate as a model compound[END_REF] which reported that values of specific energy consumption were directly proportional to the applied current and inversely proportional to the amount of Cr(VI) removed.

Interaction of electrolysis time and current on operating cost

As given in Equation ( 16), the operating cost incurred in Fe-EC are associated with electrode and energy consumptions. Both consumptions are strictly dependent on cell voltage, current and EC time. An extended EC may cause the passivation of the working electrodes and subsequently high cell potentials would be required. These higher values will increase energy consumption and costs. However, the FOR used in this work showed high electrical conductivity (18 mS cm -1 ). This last avoided the formation of this passive film.

Figure 7 exhibits the combined effect of EC time and applied current on operating cost. The cost of the Fe-EC, when a complete abatement of chromium was achieved was 0.025 USD m -3 (0.013 kWh m -3 ). However, as mentioned above, the current and the EC time influenced these operating costs. For instance, a quasicomplete reduction in Cr(VI) concentrations (98.3% and 97.9%) observed operation costs of 0.031 USD m -3 (0.015 kWh m -3 ) and 0.011USD•m -3 (0.005 kWh•m -3 ). These last values strongly depended on the reaction time. In the same venue, poor removal efficiency (81.2%) was observed at 0.51 A (J = 2.6 mA•cm -2 ) and 45.5 min of current and EC time, respectively. These last conditions produced an Fe-EC treatment cost of 0.004 USD m -3 (0.001 kWh m -3 ). In this regard, both the percentage of pollutant removal and the operating costs must be assessed to consider the optimal operating conditions. In these experiments, the cell voltage varied from 2.9 to 3.4 V. Bhatti et al. [START_REF] Bhatti | Electrocoagulation removal of Cr(VI) from simulated wastewater using response surface methodology[END_REF] reported the energy consumption as 137.2 kWh m -3 to achieve a 90.4% of chromium removal efficiency for an electrocoagulation operated at pH of 5, a current of 24 V, and 24 min treatment time. In the same vein, Khan et al. [START_REF] Khan | Hexavalent chromium removal in an electrocoagulation column reactor: process optimization using CCD, adsorption kinetics and pH modulated sludge formation[END_REF] reported that energy consumption (12.97 kWh g -1 ) for a complete removal of Cr(VI) was reduced when solutions with higher initial concentration were treated at lower applied current for longer durations. Elabbas et al. [START_REF] Elabbas | Treatment of highly concentrated tannery wastewater using electrocoagulation: influence of the quality of aluminium used for the electrode[END_REF] stated that the efficient treatment to remove chromium by electrocoagulation (using aluminium electrodes) required around 20 kWh m -3 (initial concentration 7000 mg L -1 ). Oden and Sari-Erkan [START_REF] Oden | Treatment of metal plating wastewater using iron electrode by electrocoagulation process: optimization and process performance[END_REF] calculated the operational cost of electrocoagulation for a complete chromium removal from a metal plating wastewater as 5.34 USD m -3 . Additional literature for hexavalent chromium removal by EC utilizing either aluminium (Al) or iron (Fe) electrodes are also given in Table 2.

Electrical energy consumption revealed an exponential increase with applied current at the same EC time (45 min). These last two variables had a synergistic effect on the operating costs in terms of energy consumption. However, additional experiments using different electrode material, inter-electrode gap, as well as the mode of operation would be needed to enhance chromium removal efficiency in terms of energy consumption. Finally, it is important to note that the operating costs may fluctuate based on the geographical area and domestic values.

Desirability function, optimization and costeffectiveness estimation

The desirability function aims to find the optimized conditions that satisfy multi-objective responses, with values between 0 and 1, where 0 represents an undesirable value and 1 a desirable value [START_REF] De Oliveira | Removal of natural organic matter from aqueous solutions using electrocoagulation pulsed current: optimization using response surface methodology[END_REF]. In the present study, the desirability function was used to maximize chromium removal efficiency while minimizing electrical energy consumption, operating cost and settled sludge volume. Same importance was attributed to each response (weighting factor 1). The optimum conditions for Fe-EC were predicted by the response surface methodology and central composite design model as an EC time of 59.7 min and a current of 1.24 A (J = 6.32 mA cm -2 ). As per the model predictions, Cr(VI) removal efficiency, operating cost, electrical energy consumption and settled sludge volume were calculated as 90%, 0.014 USD m -3 , 0.005 kWh m -3 and 445 mL L -1 , respectively. To control the applicability of the optimum conditions, an affirmative Fe-EC experiment was performed. The experimental results demonstrated Cr(VI) removal of 89.7%, operating cost of 0.017 USD m -3 , electrical energy consumption of 0.008 kWh m -3 and settled sludge volume of 515 mL L -1 were attainable under such optimum conditions. These results agreed with the predicted values, suggesting the appropriateness and accuracy of these optimization tools. However, even when lower EC costs were estimated, renewable energy-driven electrochemical technologies from laboratory to industrial scale is a crucial issue in commercialization scope as well as in the autonomy of the process. In this way, on-grid solar photovoltaic energy could benefit the scale-up and industrialization of the electrochemical technology, as already is studied by other research groups [START_REF] Garcia-Segura | Electrocoagulation and advanced electrocoagulation processes: a general review about the fundamentals, emerging applications and its association with other technologies[END_REF][START_REF] Ganiyu | Renewable energies driven electrochemical wastewater/soil decontamination technologies: a critical review of fundamental concepts and applications[END_REF].

Conclusion

In the present study, Fe-EC/filtration/sedimentation proved to be an efficient and promising Cr(VI) removal method for the FOR. The following conclusions could be drawn from the present study:

. Complete Cr(VI) removal could be achieved at a current density of 16.94 mA cm -2 and an EC time of 45.5 min. However, the lowest current density [40] Present study c a The removed Cr(VI); b produced aluminium; c experimental study results.

(5.44 mA cm -2 ) resulted in the cheapest operation cost (0.003 USD m -3 ) together the lowest SSV (180 mL L -1 ) as well as 82.8% Cr(VI) removal percentage.

. The optimal predicted conditions of the response surface methodology and central composite design based on the highest Cr(VI) removal while minimizing operation cost and SSV were an EC time of 59.7 min and a current density of 6.32 mA cm -2 . According to optimization, satisfactory performance was achieved for Cr(VI) removal efficiency, electrical energy consumption, OC, and SSV of 90%, 0.005 kWh m -3 , 0.014 USD m -3 , and 445 mL L -1 , respectively. This prediction agreed with the experimental results (89.7%, 0.008 kWh m -3 , 0.017 USD m -3 , and 515 mL L -1 , respectively). . Although FO has proved its potential in wastewater treatment, the integration of other treatment technologies such as EC as a post-treatment can diminish substantially the volume of reject water and maximize water reuse. In addition, treatment and regeneration of FOR would be economically feasible. In this sense, a research into renewable and cleaner energy sources to efficient energy management practices and optimize resource utilization must be emphasized. . In future research, the reduction kinetics analysis which indicates the reduction behaviour and the reduction mechanism of Cr(VI) via electrochemical process will be discussed. Beyond that, a new kinetic model will be developed to simulate for the reduction behaviour of Cr(VI) under both isothermal and non-isothermal conditions.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Figure 1 .

 1 Figure 1. Fe-EC set-up for the removal of Cr(VI) from FOR in a monopolar-parallel connection (1 A; an applied current density (J) = 5.1 mA cm -2 ).

Figure 2 .

 2 Figure 2. Validation of the model. Normal plot of residuals: (a) Cr removal, (b) operating cost, (c) electrical energy consumption and (d) settled sludge volume. Residuals vs predicted: (e) Cr removal, (f) operating cost, (g) electrical energy consumption and (h) settled sludge volume.

Figure 3 .

 3 Figure 3. Validation of the model. Residuals vs run: (a) Cr removal, (b) operating cost, (c) electrical energy consumption and (d) settled sludge volume. Predicted vs actual plots: (e) Cr removal, (f) operating cost, (g) electrical energy consumption and (h) settled sludge volume.

Figure 4 .

 4 Figure 4. (a) 3D plot and (b) 2D contours for chromium removal: effect of current and electrolysis time.

  depicts (a) 3D plots and (b) 2D counters as a function of current and EC time on electrical energy consumption.

Figure 5 .

 5 Figure 5. (a) 3D plot and (b) 2D contours for settled sludge volume: effect of current and electrolysis time.

Figure 6 .

 6 Figure 6. (a) 3D plot and (b) 2D contours for electrical energy consumption: effect of current and electrolysis time.

Figure 7 .

 7 Figure 7. (a) 3D plot and (b) 2D contours for operating cost: effect of current and electrolysis time.

Table 1

 1 displays the experimental results of the central composite design matrix and the predicted responses from the fitted model for Cr(VI) removal by Fe-EC. The predicted values were close enough to those experimentally obtained, indicating good fitness. As shown in Table

Table 1 .

 1 Design matrix of experiments and results of CCD.

		Std	Experimental matrix	Cr removal (%)	Responses OPC (USD•m -3 )	EEC (kWh•m -3 )	SSV (mL•L -1 )
	Run		A	B	Actual	Predicted	Actual	Predicted	Actual	Predicted	Actual	Predicted
	1	3	-1	+1	97.9	97.3	0.011	0.011	0.005	0.005	590	580
	2	4	+1	+1	98.4	98.6	0.031	0.031	0.015	0.015	860	863
	3	11	0	0	90.6	90.7	0.013	0.013	0.006	0.005	495	499
	4	6	+1.68	0	94.9	94.8	0.022	0.022	0.009	0.009	605	591
	5	5	-1.68	0	88.3	89.2	0.004	0.004	0.002	0.001	330	334
	6	9	0	0	91.3	90.7	0.013	0.012	0.006	0.005	535	499
	7	10	0	0	91.0	90.7	0.013	0.013	0.006	0.005	520	499
	8	7	0	-1.68	81.9	82	0.004	0.003	0.001	0.001	150	135
	9	13	0	0	89.9	90.7	0.012	0.013	0.004	0.005	485	499
	10	12	0	0	90.1	90.3	0.013	0.012	0.005	0.005	460	499
	11	2	+1	-1	88.7	88.4	0.009	0.009	0.003	0.003	250	267
	12	1	-1	-1	82.8	81.8	0.003	0.003	0.001	0.001	180	185
	13	8	0	+1.68	100	100.1	0.025	0.025	0.013	0.013	830

Table 2 .

 2 Comparison with the literature data. , 1.48 A, 21.47 min Cr(VI) o = 49.96 ppm 100% 12.97 Wh.(g Cr a ) -1 -[36] Al Al-AlAl-Al,11.57 A•m -2 , electrode gap 1 cm 77.6%-97.9% -3.148USD.m -3 [57] Fe 20 mA•cm -2 , pH 2.4, 0.05 M NaCl electrolyte, Cr(VI) o = 1000 mg.L -1

	Electrodes	Operating parameters	Cr(VI) removal	Energy consumption Operating costs	Reference
	Fe	pH 3.0100%	2.68 kWh.m -3	-	[20]
	Fe + Al Al Fe	pH 9, t = 20 min 6 mA•cm -2 100 rpm 6.32 mA•cm -2 ; 59.7 min	100% 89.7%	0.37-2.78 kWh.m -3 1.98 kWh.m -3 0.008 kWh•m -3	0.7 USD m -3 b 0.017 USD•m -3	[58]

Funding

The support of the Research Vice-Chancellor of Qazvin University of Medical Sciences is wholeheartedly appreciated (IR.QUMS.REC.1400.122). Milad Mousazadeh is thankful to the Iran's National Elites Foundation (INEF) for granting the Elite Award, No. 15/11529. Miguel Angel Sandoval is grateful to Agencia Nacional de Investigación y Desarrollo (ANID-FONDE-CYT, Chile) for granting the postdoctoral scholarship, No. 3200274.

Data availability statement

The authors confirm that the data supporting the findings of this study are available within the article.


Jean Kempf

The Ambiguous Image of New Deal "Model Towns"

When Boorstin wrote his seminal book, The Image or What Happened To The American Dream (New York : Atheneum, 1961), he formulated a theory that post-Depression America was a world totally constructed around representation-although, of course, very real things did happen ; they had ceased to be, however, the alpha and omega of politics.

The Ambiguous Image of New Deal «Model Towns»

The topic of the form and structure of the urban space and its relation with the « country » environment is one which was prominent in New Deal culture. It forms a strong intellectual and political link with previous periods and the evolution of the conception of the city in the 1933-1943 decade is especially interesting because of its ambiguity.

This paper suggests that by following the conceptualization of the urban space-more specifically of small towns-through New Deal photography one can perceive a shift from a modernist, constructivist New Deal (roughly between 1934-35 and 1938) to a conservative, « organic » New Deal, after 1938. And the visible shift that took place at that moment is indicative of changing ideologies and public strategies in what constitutes a preparation for a cultural levelling by the middle class in the post-war years. It looks as if this « regressive » period worked as a countervailing force against the standardization required in the new mass capitalist society whose structure had evolved since the mid-19th century with the ever widening web of exchanges. The « return to Arcadia » acted in the ubiquitous field of visual communication as a means to try to overcome the impossible contradictions of a society where the « common man » was to make way for the « average man ».

FSA photographs provide a convenient and significant access to those questions because they are consistenly organized and were made by professional-or almost professional-photographers, thus ensuring a rather strong adequation of form to function. 1 In 1914, Walter Lippman said of Woodrow Wilson : « He knows that there is a new world demanding new methods, but he dreams of an older world. He is torn between the two. It is a very deep conflict in him between what he knows and what he feels ». 2 In his usual pungent way, the famous American ntellectual (if there were any)perfectly characterized the 1920s by defining it as a tension between « progress and nostalgia » as Lawrence Levine posits in his important article But such contradiction may even be truer of the 1930s, because the Depression can be seen as a crisis of adjustement to fundamentally new 1 Studying the many images made by countless amateurs in the agencies, especially at local level, and even in the WPA programs, is hardly significant as the visual incompetence and illiteracy of the operators makes analysis of little significance. paradigms, and first and foremost that of exchanges.3 But, contrary to what one might expect, FSA images when taken from the bulk of the file do not show a growing adjustement to « modernity », starting from what was available-the vernacular structure of the « small town »-in order to gradually introduce the « new town », presumably more adapted to the world of tomorrow. Actually rather the contrary happens, as images exemplifies a complex return to vernacular forms after a foray into the avenues of modernity. This paper will only concentrate on the first stage of this process.

Under the leadership of Rexford Tugwell, the Resettement Administration started in 1935 a whole series of projects, temporary relief ones of course -the RA/FSA camps -but also an ambitious building project of garden cities, the Green Towns. 4 The detailed history of the four Green Towns ever built5 has been made elsewhere ; we will rather concentrate here on their photographic representation. 6 Indeed, the central feature in the project was the creation of a model, in a mass society where the image already occupied a key position. Thus one may see those cities as belonging as much to the « publicity » sphere as to that of urban planning. Just as priming up the pump was one of the tenets of keynesian economics, priming up the symbolic pump was one of Tugwell's central ideas and tools. 7 But as results proved disappointing, both in visual and real terms, and as « rampant collectivism » came under regular attacks, the symbolic/visual center shifted from a modern(ist) model to the archaic form of the « real » homegrown traditional American small town that could be found all over the continent. What took place, however, was more than the replacement of a « public-sector » version of utopia by a private-sector one, as James Curtis suggests in his sensitive-and sensible-study of FSA photography. 8 Roy Stryker and his FSA crew were fascinated with the vernacular and indisputably searched for the sort of « middle landscape » theorized by Leo Marx, the small town as pastoral space of the Nation. The real reason for this shift, however, was the insurmontable contradictions of the Green Towns themselves, as their image in official photographs perfectly demonstrated.9

I -Fabricating the environment

Most of the existing images of the Green Towns are of their construction and of partially completed sections. This is not mere historical coincidence. It is as if the new town was first and foremost a process, a space in the making.

This « new » space begins by dis-location. Just as the American experiment began both symbolically and in actuality by a transportation and a cesura, for New Deal planners the reinvention of the city was less a matter of rehabilitation than of creation, less a matter of repairing buildings or structures than of planning new and better ones. It is even present in the very name of the agency, the « Resettlement Administration ». The territory was in need of reorganization, which meant planning and rearrangement according to « higher principles », those of rational analysis and efficiency. Thus, came the idea of building cities in the country, as Alphonse Allais, the French 19th century humorist once said, because cities were necessary and the country was indispensable.

The Green Town must first and foremost look good from above. The aerial view or the map/blue-print is the first step conveying the harmony of the whole, as well as its main characteristic, that of being turned inward, insulated from the rest of the territory by a green belt, hence their generic name where the stress lies less on « green » (the greenery within the town limits) than on « belt » (the greenery protecting it from the outside world). The master plan, the curving streets as well as the radiating streets visualize-but only in the image and never for the pedestrian-the flower or clover-leaf type of layout that signify the town's true nature. Unsurprisingly, this very image of the blueprint and the drawing board became, in the case of Jersey Homesteads an emblem-albeit a controversial one-of the whole operation. Ben Shahn ended his huge triptych mural for the town (where he and his wife moved in) by a scene summing up, in perfect socialist realistic mode, the spirit of the operation [illustration ?]. The map of the new town is the connecting link between people, as well as between the people and the leaders. Supervising the whole, exactly where the crucifix would have hung in other settings, a poster of FDR, « the gallant leader », between the image of a coöp and that of a mother and child, puts the crowning touch to the message.

The same can be said of the use by photographers of the Bauhaus-like architecture of some of the towns (especially Greenbelt, Md). The geometrical lines and the flat white surfaces as well as the row alignments are used to suggest the coherence of the whole, the integration of the general design more than the sheer beauty of each of the individual pieces. This emphasis on planning is obviously not without its own paradox. Of course planning means rationality, a basic tenet of capitalistic economy, and those photographs and their accompanying comments try to establish precisely that against the complaints at inefficiency especially of building procedures. But here planning also points at the fact that everything here is a creation of government (thus the high cost of buildings) and the operation is cooperative. Paradoxically then, it is the communal -or communal -aspect that came under attack while at the same time it was heralded by writers and ideologues alike as a fundamental component of the American character.

And indeed these towns were ambivalent: deeply embedded in an American tradition through their names (Jersey Homesteads and Subsistence Homesteads, even the generic Greenbelt, Greenhill, Greenville sound like traditional American toponyms) they remained an imported concept. For although clearly many if not all of the original American settlements (Jamestown, Williamsburgh, Philadelphia to name but a few) as well as the Federal Capital were perforce-or at least understandably-« planned communities », the « garden city » is on the contrary a deeply European (and specifically British) concept. It entails a peculiar conception of the notion of « community » as well as a specific relation to the occupation of the land which was born out of European experience and history.

The images which dominate the iconography of those projects are those of construction sites, or of buildings in the making. These are not mere historical coincidence. Part of the social and cultural project of those homesteads, and partially of the new towns, was to involve the residents in the making of their own dwellings. Also, showing « work » in the 1930s was generally thought to be a form of self-stimulation (and of pro domo propaganda) towards recovery. In the process, however, the construction site is established as a metaphor of society, and beyond of America. The machine as icon, emblem and even God, is displaced here towards the object : the fruit, the product, the new structure being erected (see frame and beams images) -the same images are to be found with skyscrapers -but also the bowels of the earth repeatedly exposed, a classic late 19th century image. In that context they are the signs of a « society that works », as well as one which cannot satisfy itself with merely the inherited but must set up its legitimity on the will to transform. This peculiar dominance of the construction site over the factory also connects, in representation, the « winning of the West » by civil engineers and the postmodern fascination for foundations and buildings as metaphor of the workings of discourse. 10 Where modernism streamlined its objects (that is to say make them as smooth, as unobtrusive, as perfectly fitting and efficient as possible), post-modern sensibility build itself on rasping and grinding, on asperities and disjunctions, on breaks rather than the seamlessness of modernist forms.

II -A Safe World

Those menacing disjunctions are precisely visible in the permanently transient character of « making-of » images. This is why their documentary value notwithstanding, they were actually of little use for promotion purposes. 11 The main staple of new town publicity was therefore elsewhere, in the patient construction of the image of a safe world. Lawrence Levine aptly reminds us that one of the central concerns of those dark years was a yearning for security. 12 Green Towns images give a lot of that, as do Subsistence Homesteads images and FSA migrant camps-and that is probably their salient rhetorical feature. The streets of the Green Towns, as opposed to those from which many of the residents came, are safe for children and adults alike, broad, clean and white, and sheltered from automobile traffic : this is how one should understand these repeated views of underpasses whose single endlessly repeated message is « safety, safety, safety ». 13 In fact the images of New Towns and FSA camps are an exact reprise -only inverted -of the traditional « reform» photography whose paradigm was established by Jacob Riis. 14 Where Riis' inquisitive eye and flash light relentlessly explored the darkest corners of urban squalor metaphorized into social rot, New Town images are bright and sunny as the weather they were made in. Photographers in the Green Towns hardly ever used artificial lighting, 10 A typical contemporary example is Lewis Baltz, Park City (New York/Albuquerque : Aperture/Artspace Press, 1980). 11 As James Curtis (94-95) notes.

12 Levine, Unpredictable Past, 207-230. 13 When residents of the FSA relief camps were displaced farmers, residents of the new towns were for the most part former tenement residents, at least as far as Greenbelt and Jersey Homestead were concerned. 14 Probably more than by Lewis Hine whose case is somewhat different although his photographs shared some of the same characteristics as Hine's. For a relevant analysis, see Maren Stange, Symbols of Ideal Life. Social Documentary Photography 1890-1950 (Cambridge : Cambridge University Press, 1989). and for a very good reason : rooms were so well lit that such device was often unnecessary in the first place. And even when flash lights were used (as in FSA labor camps for instance or in some interior shots) it is done in such a way as to simulate natural light and merely to enhance the volumes and the large flat white spaces of the appartments.

There is, however, a deeper motive for that. Despite the regular presence of people -mostly man and women at work and children at play, a significant bias in itself -most images show the enveloppe, the architecture or the interior design of the houses and apartments. The reason is to be found in the deep behaviorist ideology pervading those projects : changing life by changing the living environment, a position inherited from the generalization of social darwinism in American reform movements (as opposed to political reform or revolutionary movements) to which the New Deal was the direct heir. 15 But in the context of mass society, the engineering of space must serve a double function, that of « instrument » and that of « monument » as well, which calls in photography as the indispensable partner in the construction of reform as a tangible -that is to say visible -fact. In that respect the design of houses and interiors functionned as sign of change almost as much as actual change. 16 The relationship between human beings and the built environment is also typically inherited and contrasted from urban reform photography, once more making the New Towns the natural continuators of America's changing urban experience, and the migrant camps the (temporary stage of) transition to the city for displaced farmers. First there are very few people in those images, partly because those were new towns, and -actually -not quite inhabited/lived-in yet (as testified by the numerous model houses and moving-in scenes), and because the enveloppe was probably more important at that stage. But the rarity of people also gives a strong sense of space, of an antidote to the essential overcrowding of the metropolis. This was complemented by the visual use of the wide spaces in between houses, 17 the broad streets where children could play and women could walk their strollers, a total inversion of the common images of antlike figures swarming the big city.

Even the way the characters are arranged in the images is significant. While the standard presentation in turn-of-the-century reform photography was the pose, facing the camera, for both technical and rhetorical reasons, the 15 The filiation also involves literature with the novel ; see Simon J. Bronner (ed.), Consuming Visions. Accumulation and Display of Goods in America, 1880-1920 (New York : W.W. Norton, 1989), 14-15. 16 The limitations in that field are also patent, as noted by Walter Benjamin in his famous remark about the Krupp factory (Literarische Welt, September 18,25, October 2, 1931) : after all, the images of the Jersey Homesteads cooperative garment factory or the Maryland cooperative gas station, look terribly like that of a normal clothes workshop or of a gas station. Only the caption may suggest a difference of any sort. 17 Greenhills, 21-40. Greendale, 1-20.

presence now evolved towards a much more natural attitude, almost as in casual snaphsot images. In reform photographs, it was as if moments of « real life » were arrested for the photographer, the « subjects » presenting themselves to the photographer, and beyond to society forced to look upon them, often not as victims but as proud individuals despite their dire circumstances. 18 On the contrary, here, the photographer with the complicity of the subjects tries to stage « use », « usage » and « usefulness » in a completely artificial -or at least fabricated -context. Paradoxically those new happier individuals appear as « models » in a showroom or catalogue for the good life, and somewhat lose their status by their instrumentalization.

As for those not working at constructing the new dwellings, or in the cooperative garment factory, they are mostly portrayed in their leisure activity : lying with friends on the lawn in a typically campus attitude (Greenbelt 21-40), camping in their huge back lawn (Greendale, Wisconsin, 1-20), or enjoying the swimming pool. All these images are traditional modernist icons of healthy living. For health, and its attendant signified safety, constitutes the core of New Town as well as FSA camp rhetoric.

Countless images show us medical centers, doctors and dentists at work, and of course pediatric care. Such easy unheard-of access to health care, despite its strong connotation of anti-liberal economics, worked as a general, overarching symbolism for the cure of society's illness, a cure that merely required organization and knowledge, a cure that was based upon the fight against ignorance, the eventual source of all illnesses. These images have to be seen next to the companion pieces in the numerous educational/learning activities (attending the library, or various classes in home economics or family skills) that literally punctate the files, not only of the FSA but also of all New Deal agencies. 19 There is, however, another dimension to the health metaphor, namely whiteness. The whiteness of walls, of interiors as well as exteriors, the whiteness of sheets in the nursery (FSA camp 199), the whiteness of kitchen utensils (Greenbelt 21-40) are reinforced by the see-through perspectives and the immaculate emptiness of model interiors. This ever present paradigm connects those projects, in a more than perfunctory way, to the 1893 Columbian Exposition « White City ». 20 For as Warren Susman suggests in « The City in American Culture » here lies the founding paradigm of a « first great national synthesis », in the form of « a city that is ordered » (247), that « is central to American experience . . . down to the 1930s » as part of a « millenial hope » (247). [START_REF] Susman | The City in American Culture » in Culture as History[END_REF] In other words it can only be understood in the context of the other combined metaphors of order. While the whole nation exhibited its brokenness -nothing worked, cars, farms, people -those New Deal Projects displayed their nicely aligned rows of houses -or salads for that matter [illustration : Documenting America 194b] -those square Bauhaus windows and nice flat roofs, and eventually the quiet order of the home. [START_REF]See in particular Russell Lee, Farm Couple[END_REF] If this was not bliss, at least things were shown to be back on track. This people is a safe, happy one. Such presentation was not, however, without a serious ulterior political motive : in the field of « welfare » the Federal governement was then taking over a great deal of the traditional constitutional prerogatives of the local and state governments to initiate a new form of constructed, imported, imposed and bureaucratic (it was not yet called technocratic) government. And the bottom line was nothing less than a redefinition of federalism which entailed a new form of economic integration, through expanded communication. But such an agenda was not without contradictions.

III -The Paradox of community

One of the ideas upon which both migrant camps and New Towns were built was that of attempted self-sufficiency. Obviously it could remain but an objective, a moral and intellectual principle and could never really hope to become a real economic aim. This may be seen in the creation of a prefabrication unit at Jersey Homesteads, and of course in the use of settlers or camp clients as manpower for the construction of the dwellings. The most symbolic element, however, present in the name « subsistence homestead », was that of the vegetable garden, the indispensable companion of the lodging unit both as economic supplement and as educational, even moral and symbolic tool. Jersey Homesteads was a slightly different community in that it chose its members on very select criteria, such as their ability at self-help, and at paying the sum of $500. It was not conceived as a sleeper suburb, but as part of a backto-the-land project, privately initiated by Benjamin Brown and supported by such prestigious immigrants as Albert Einstein, then taken over in 1935 by the RA. 23 The idea was to build a self-sustaining subsistence farm, combined with seasonal employment in a cooperative garment factory. 24 There are indeed a few images showing potatoe and wheat fields as well as portraits of those « Jewish-American farmers » complaining that « Jews are said not to be able to farm » (JH 241-260, 261-75). But besides those of the buildings most of the images of the file are of the garment factory. They show the necessary exchanges with the city, taking the form of delivery trucks (one of them displays a « moving » sign on the side) and of the fitting room where outside customers came to have clothes tailored. Those were also present in the store pictures, as in the famous image of a mother shopping for « co-op fancy cornflakes ». 25 Home pickles and jars were more typical of the intended message, and they are numerous in the files, but rather in the Subsistence Homesteads and the camps than in the model towns, an indication of their slightly different economic bases. 26 This attempt at reviving the pioneer spirit of self-reliance was simultaneoulsy deeply American and problematic in a context of a capitalistic economy which based its recovery on the stimulation of consumption. It all boiled down to a matter of finding a way out of the dependence on consumerism which had ruined many families, and at the same time of enhancing or at least reinventing it. This tension was indeed probably more serious because it was aporetic than that resulting from public management of the projects, as the original charters planned a progressive buying back of the houses by families, the governement merely acting as promoter/banker. The great ambiguity of the New Deal, can be seen at work, at a microscopic level in those town projects. [START_REF]A contradiction also deeply informing the rise of the gangster and the gangster movie in the 1930s[END_REF] The contradiction became visible in the project of keeping cars out of the New Towns while making it technically the necessary instrument of mobility (to commute to the neigboring centers) and ideologically ultimately of individual freedom.

Even more telling is the total disappearance of any center in the photographic representation. The garden city conceived by Ebenezer Howard was seen as a wheel whose center was the « central park ». [START_REF] Howard | Garden Cities of Tomorrow[END_REF] Despite the existence of centers of community life such as the store and the school, the new settlements appear to have a lot of common space, even of public space, but hardly no -if any -meeting space. In that respect they were not a reproduction of older settlements -in short the New England village which is always present as historical model (a question that needs separate investigation 29 ). As a paradigmatic instance of such representation, we may take the Pie Town series by Russell Lee in 1940. 30 The contrast with the Green Towns is striking. Pie Town, NM, is seen first as a general store which forms a genuine center of economic life and a series of political and cultural meetings which materialize the town less as place than as community. Togetherness is also expressed through pictures of family dinners making the sense of unity even more palpable, or of the town square which functions as the local agora. 31 The Green Towns, on the contrary, had appeared as a new space « miraculously » and costlessly liberated from this constraint by the regulating force of planning and of economic activities. While politics, or even polity, became the most important defining characteristic of small towns as seen by the FSA, it was here strangely absent, evidencing a shift, a return to values borrowed from a past made usable which constitutes the most important marker of change of the late 1930s.

The New Towns, and the camps as well, 32 seem to be regulated as if by magic, all trace of government having disappeared. 33 In its place we are left with the image of individual units, individual colonists, individual clients. One series seems to me particularly revealing of that state of affairs. Arthur Rothstein, imitating the visual formula Walker Evans used for his work on the sharecroppers in Hale County, Alabama, in the summer of 1936, did a large series of individual close-up portraits of camp residents all against the same horizontal boards of the FSA office in the camp (the DoA / FSA sign is visible in some shots). Those images, often praised for their rich portraitural qualities, do have a striking impact on the viewer, as have all such extremely neutral and frontal portraits. 34 Together with the presentation of standardized, individual dwellings and interiors, they shift the meaning of representation from that of the 29 See a partial answer in James L. Machor, Pastoral Cities. Urban Ideals and the Symbolic Landscape of America (Madison : The University of Wisconsin Press, 1987). 30 Curtis ch 5. See also Russell Lee in Vale, Oregon in 1941 (Documenting America, 206-225), the series by John Collier in New Mexico in 1943 (ibid., 294-311) to name but two of the most accessible images among many others in the FSA/RA file. 31 Curtis, 108. 32 The case of the camps is both similar and different. In particular, the center is signified by the bureaucratic organization of the FSA, raising questions as to the democratic status of those units. But one must keep in mind that they were seen as temporary structures. 33 To be quite honest one must mention in the several hundred images one of the chief of police of Greendale, Wisconsin, two of meetings not surprisingly in the most cooperative town, Greenbelt, Md. 34 This « straight » device will become standard in later portrait photography. See for instance Richard Avedon's portraits. common man -as in the collective portait of small towns -to the « average man ». This very useful concept, borrowed from Olivier Zunz in Why The American Century, allows us to see a deep shift in the conceptualization of the American. 35 Individualized, categorized and anthropometrically reconstructed by photography-after the social sciences-, that new man was made for the new consumer and bureaucratic society that emerged from the catastrophy of the Depression. This image, however, was not quite acceptable yet in actual facts.

What we are witnessing in the infinite reproduction of individual units (houses as well as people) is of course the America of the next decade, geographically located in what was not yet called « suburbia ». This is why the photographers of the FSA turned to the construction of a « usable past » as a way out of this paradox. They searched relentlessly -and quite successfully as a matter of fact -for an older America, for the synthetic space of the small town, for the organic and homegrown community, which no longer existed but could still be visualized and even met in some preserved reservations that could give the soothing illusion that the middle space existed. But it was merely a detour into an imagined past which made the jump into yet another modernity possible if not totally painlessly.

Thus the very modernist -and forceful -New Deal gave way (with for instance the stepping out of the emblematic figure of Rex Tugwell) to a more cunning one, one that embraced archaic regression, and even conservatism, to better impose a new economic and ideological order. One will have to wait until the 1960s to see the early 1930s modernist imagery resurface, albeit with a very critical stance. But that is another story. 35 Olivier Zunz, Why The American Century (Chicago : The University of Chicago Press, 1998), 65-69, 74-75, and passim, especially Part Two.

  See however Peter Daniel, Merry A. Forresta, Maren Stange and Sally Stein, Official Images. New Deal Photography (Washington, D.C. : Smithsonian Instituion Press, 1987).

2

In Lawrence Levine, « Progress and Nostalgia », in The Unpredictable Past. Explorations in American Cultural History (New York : Oxford University Press, 1993), 200.

I am therefore fully subscribing to Levine's opening statement that :« the central paradox in American history [...] has been a belief in progress coupled with a dread of change ; an urge towards the inevitable future combined with a longing for the irretrievable past. » (191).

The other big New Deal projects in that respect were that of the Subsistence Homesteads. Most of the things I am developing here about the « Green Towns » are applicable to those projects which replicate many of the same features, only on a smaller scale.

Greenbelt, Maryland ; Greenhills, Ohio ; Greendale, Wisconsin ; Jersey Homesteads, New Jersey, plus about a hundred Subsistence Homesteads and other communities built by various agencies, most notably Cumberland Homesteads, Crossville, Tennessee and Arthurdale

James Curtis, Mind's Eye, Mind's Truth. FSA Photography Reconsidered (Philadelphia : Temple University Press, 1989), 101.

I will compare the New Town images with that of the migrant camps which are rhetorically very similar.

This has been quite convincingly demonstrated by Stange, Symbols of Ideal Life.

See Official Images.

See Alan Trachtenberg, The Incorporation of America. Culture and Society in the Gilded Age (New York : Hill & Wang, 1982), ch. 7 and passim.