Mohammad Noori Sepehr

Mansur Zarrabi
email: mansor62@gmail.com

Hossein Kazemian

Abdeltif Amrane

Kamiar Yaghmaian

Hamid Reza Ghaffari

Removal of hardness agents, calcium and magnesium, by natural and alkaline modified pumice stones in single and binary systems

Keywords: Calcium, Magnesium, Adsorption, Modification, Batch system

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Introduction

Most of the water resources should be treated for purification before consumption. In some countries, groundwater is the main safe drinking water resource [START_REF] Bruggen | Removal of pollutants from surface water and groundwater by nanofiltration: overview of possible applications in the drinking water industry[END_REF]. In some cases, the resource does not satisfy to the desirable levels regarding their chemical properties, such as hardness, nitrate contamination, heavy metals, soluble iron, etc. [START_REF] Teixeira | The impact of the water background inorganic matrix on the natural organic matter removal by nanofiltration[END_REF]. Among them, water hardness can appear problematic in some cases; it can also be considered as an important aesthetic parameter. However, because public acceptance of hardness differs remarkably according to local conditions, a maximum acceptable level has not been defined. In general, water supplies with total hardness higher than 200 mg/L can be tolerated by consumers but are considered as poor resources; while values higher than 500 mg/L are not acceptable for most of the domestic consumptions. Hardness in water refers to existing divalent ions, such as iron, manganese, calcium and magnesium. Among them, calcium and magnesium are known as the dominant species for water hardening [START_REF] Yan | Effect of polyaluminum chloride on enhanced softening for the typical organic-polluted high hardness North-China surface waters[END_REF]. Although it has been shown that water hardness doesn't have serious health impact, it has been demonstrated that hard water is responsible for the formation of deposits in boiler and household facilities, as well as diverse influence on cleaning performances of detergents [START_REF] Suzuki | Removal of phosphate, magnesium and calcium from swine wastewater through crystallization enhanced by aeration[END_REF]. Formation of precipitates may cause a decrease of heat transfer in boiler, a decrease of fluid rate, bursting of water pipeline, the formation of stain in dishes and clothes. Furthermore, high concentrations of magnesium in drinking water may induce a bitter taste [START_REF] Dimirkou | Use of clinoptilolite and an Fe-overexchanged clinoptilolite in Zn 2+ and Mn 2+ removal from drinking water[END_REF]. In water purification and treatment plant, lime and soda ash are used for the removal of hardness. One of the main drawbacks of this process is the resulting waste, namely the large amount of liquid sludge produced, as well as the need for re-carbonation of the softened water [START_REF] Sheikholeslami | Composite scale formation and assessment by the theoretical Scaling Potential Index (SPI) proposed previously for a single salt[END_REF][START_REF] Fu | Removal of low concentrations of hardness ions from aqueous solutions using electrodeionization process[END_REF]. In addition, the use of additional chemicals to prevent sludge production is restricted and hence in most cases water hardness species cannot be fully removed [START_REF] Park | Removal of hardness ions from tap water using electromembrane processes[END_REF]. Ion exchange processes have been considered as an alternative approach which is commercially in use worldwide. However, high power consumption and periodical regeneration of saturated ion-exchanger resins are needed. Potential hazards of the effluent solutions resulting from the regeneration process should also be taken into account [START_REF] Seo | Investigation on removal of hardness ions by capacitive deionization (CDI) for water softening applications[END_REF].

Recently, various methods including electrodeionization process [START_REF] Fu | Removal of low concentrations of hardness ions from aqueous solutions using electrodeionization process[END_REF],

electromembrane processes [START_REF] Park | Removal of hardness ions from tap water using electromembrane processes[END_REF], capacitive deionization [START_REF] Seo | Investigation on removal of hardness ions by capacitive deionization (CDI) for water softening applications[END_REF], membrane and fluidized pellet reactor [START_REF] Li | Integrating a membrane and a fluidized pellet reactor for removing hardness: effects of NOM and phosphate[END_REF], ion exchange process [START_REF] Apell | Combined ion exchange treatment for removal of dissolved organic matter and hardness[END_REF][START_REF] Faghihian | The use of clinoptilolite and it's sodium form for removal of radioactive caesium, and strontium from nuclear wastewater and Pb ++ , Ni ++ , Cd ++ , Ba ++ from municipal wastewater[END_REF][START_REF] Kazemian | Evaluating the performance of an Iranian natural clinoptilolite and its synthetic zeolite P for removal of Cerium and Thorium from nuclear wastewaters[END_REF] and adsorption [START_REF] Torabian | Removal of Petroleum Aromatic Hydrocarbons by Surfactant-Modified Natural Zeolite[END_REF][START_REF] Seifi | Adsorption of BTEX on surfactant modified granulated natural zeolite nanoparticles: parameters optimizing by applying Taguchi experimental design method[END_REF] have been studied for the removal of a wide variety of ionic and molecular species from various water streams, including those responsible for hardness (e.g. Ca +2 and Mg +2 cations). Amongst the developed processes, adsorption has been widely studied for the uptake of various ionic and molecular species form water. Although the efficiency of activated carbon (AC) as an adsorbent, it cost remains its main drawback. Hence, finding inexpensive and efficient adsorbents remains a topical matter to water treatment industries [START_REF] Feng | Biosorption of heavy metals from aqueous solutions by chemically modified orange peel[END_REF]. Pumice stone is a volcanic rock which corresponds to solidified frothy lava formed when highly pressurized and super-heated rock is vigorously erupted from a volcano. Pumice can also be formed from the mixing of hot lava with water. This unnatural formation is considered as a result of simultaneous rapid cooling and depressurization. The depressurization creates bubbles (i.e. pores) by lowering the solubility of water and CO 2 molecules dissolved in the lava, causing a rapid release of gases. The simultaneous cooling and depressurization trap the bubbles inside the pumice matrix; resulting in highly porous pumice with a very low density (0.35-0.65 g/cm 3 ), a high water absorption capacity (20-30%) and mainly composed of SiO 2 and Al 2 O 3 .

Considering these physicochemical characteristics, it can be considered as a potential adsorbent for environmental applications [START_REF] Ersoy | Characterization of acidic pumice and determination of its electrokinetic properties in water[END_REF]. Pumice has been tested for the removal of sulfur dioxide [START_REF] Ozturk | Investigation of sorption capacity of pumice for SO 2 capture[END_REF], chromium [START_REF] Panuccio | Cadmium adsorption on vermiculite, zeolite and pumice: Batch experimental studies[END_REF], phenol and 4-chlorophenol [START_REF]Sorption of phenol and 4-chlorophenol onto pumice treated with cationic surfactant[END_REF]. The main objective of this research was to evaluate a raw pumice stone and its alkali-modified form (i.e. NaOH-treated) for the removal of calcium and magnesium from a synthetic solution simulating the Karaj Water Supply System (Iran).

Materials and Methods

Chemicals and solutions

All chemicals used in this work were obtained from Merck (Merck Co. Germany). 0.05 M H 2 SO 4 and 0.1 M NaOH were used for pH adjustment (Jenway, model 3510). MgSO 4 .7H 2 O and CaCl 2 were used for the preparation of stock solutions using deionized water. The electrical conductivity of the stock solutions was in the range of 21-26 µs/cm (Jenway, model 4520). A solution containing chloride (450 mg/L), nitrate (25 mg/L) and sulfate (325 mg/L) was prepared. Conductivity and total dissolved solid (TDS) of water solutions were adjusted by adding 1150 mg/L sodium chloride leading to a conductivity of 1850 µs/cm, which corresponded to that of the Karaj water supply system. Raw pumice stone was obtained from Tikmeh Dash Reign of East Azerbaijan (Azerbaijan, Iran). All experiments were conducted according to the Standard Methods for the Examination of Water and Wastewater [START_REF] Awwa | Standard Methods for the Examination of Water and Wastewater[END_REF].

Preparation of the Adsorbent

Raw pumice stone was washed several times with distilled water before use in order to remove any impurities until the turbidity value became lower than 0.1 NTU. The adsorbent was then treated with 1 M solution of HCl for further purification by dissolving acid soluble components of the sample. It was then washed with deionized water to remove the excess of acid (pH= 7). Pumice was then dried at 55 o C for 24 hours to evaporate the remaining water molecules. The dried pumice was then milled and sieved to achieve 10-30 meshes (2000 to 841 microns). In order to modify the pumice stone with sodium hydroxide, it was treated with 2 M NaOH for 24 hours at room temperature. During modification, the solution was stirred at 200 rpm. The modified pumice was then filtered and calcined at 750°C for 6 h in an electrical furnace (Lenton, England). The calcined sample was then washed several times with deionized water to remove the excess of NaOH. The modified adsorbent was then dried at 55 o C for 24 h before use.

Batch Experiments

All experiments were conducted in batch mode in 250 mL conical flasks. Several operating parameters including pH (2-10), temperature (10-50 o C), adsorbent mass (2-10 g/L), initial calcium and magnesium concentrations (25-150 mg/L) and contact time were investigated. Optimized adsorption times for natural and modified pumice adsorbents were first examined by varying the contact time at room temperature, pH= 6.0 and for an adsorbent mass of 6 g per liter of solution. For this purpose, 6 g of adsorbent were added to 1 L of solution in a conical flask containing Ca +2 or Mg +2 cations at a concentration in the range of 25 to 150 mg/L. The mixture was then shaken at 200 rpm (Hanna-Hi 190M, Singapore).

Samples were taken at predetermined time intervals, filtered (0.45 µ, Wathman), centrifuged (Sigma-301, Germany) and the ion concentration was measured by a titration method [START_REF] Awwa | Standard Methods for the Examination of Water and Wastewater[END_REF].

The removal efficiency (RE) was determined as follows (Eq. 1):

0 0 100 ) ( C C C RE e    (1)
After investigation of the effect of the contact time and the initial ion concentration, the effects of pH and adsorbent mass were examined. The effect of temperature was then investigated in the optimal conditions for pH, adsorbent mass and contact time, for three initial concentrations of Ca +2 or Mg +2 cations (50, 100 and 150 mg/L).

Adsorbent effect on electrical conductivity (EC) and turbidity

In order to measure the influence of adsorbents on EC and turbidity of water, 6 g/L of natural or modified adsorbent was poured into 250 mL of deionized water ( i.e. free from calcium and magnesium) at pH=6.0 and shaken for 200 min at 200 rpm. At predetermined time intervals, samples were taken for conductivity and turbidity measurements (Jenway, Model 4520).

Isotherm study

Equilibrium experiments were conducted in 250 mL conical flasks by adding 6 g/L of natural or alkali modified pumice stone to 250 mL of magnesium or calcium solutions at concentrations ranging from 25 to 150 mg/L. The mixture was shaken at 200 rpm for 6 hours at 25 o C to ensure that maximum sorption was achieved. The adsorption results were then analyzed by Langmuir, Freundlich and Temkin isotherm models.

The Langmuir isotherm model is based on the assumption of a homogenous surface energy distribution. The non-linear (Eq. 2) and linear (Eq. 3) forms of the Langmuir model are described as follows:

) 1 ( ) ( e e m e bC bC q q   (2)
) ( 1 b q q C q C m m e e e   (3)
Where is the equilibrium amount of adsorbate (mg/g), the equilibrium concentration of adsorbate (mg/L), the maximum adsorption capacity (mg/g) and b (L/mg) the Langmuir constant. The important feature of the Langmuir model can be described based on the R L parameter expressed by Eq. ( 4):

e q e C m q ) 1 ( 1 0 bC R L   (4)
Adsorption is unfavorable for R L > 1, linear for R L = 1, favorable for 0 < R L < 1 and irreversible for R L = 0.

The Freundlich isotherm model that is appropriate for heterogeneous systems is expressed by equations ( 5) and [START_REF] Sheikholeslami | Composite scale formation and assessment by the theoretical Scaling Potential Index (SPI) proposed previously for a single salt[END_REF].

n e f e C K q 1  (5) e f e C n K q log / 1 log ) log(   (6)
Where q e is the amount of adsorbate (mg/g), C e is the equilibrium concentration of adsorbate (mg/L), (mg 1-1/n L 1/n /g) and are the Freundlich constants. Large values of reveals high affinity of the adsorbate. For a favorable adsorption, the value of the Freundlich constant (n) should be in the range of 1 to 10.

f K n / 1 f K
The Temkin isotherm is also available for heterogeneous surface adsorption. The non-linear and linear forms of the Temkin model are given by Eq. ( 7) and Eq. ( 8):

) ln( ) ( 1 e t e C k b RT q  (7) ) ln( ) ln( 1 1 e t e C B k B q   (8)
Where B 1 =RT/b 1 , b 1 is the adsorption heat (kJ/mol) and k t is the equilibrium binding constant (L/g) corresponding to the maximum binding energy. A high value of b 1 shows a fast sorption of adsorbate at initial stage. Similarly, a low value of k t is related to weak bonding of adsorbate onto the medium. By plotting q e vs. ln(C e ) one can deduce b 1 and k t from the slope and the intercept of this curve, respectively.

Kinetic modeling

Kinetic sorption of calcium and magnesium onto natural and alkali-modified pumice adsorbents were investigated by adding 6 g/L of natural and modified adsorbent into 250 mL solution containing 50, 100 and 150 mg/L of the considered cation. The pH of the solution was adjusted at 6.0 and shaken at 200 rpm until reaching equilibrium time. The kinetic of sorption was analyzed by means of the pseudo-second order model, which is expressed as follows:

 2 2 t e t q q k dt dq    (9)
Where k 2 is the rate constant (g/mg min). Integration of Eq. ( 9) at the boundary, q t =0 at t=0 and q t =q t at t=t and then rearrangement to a linear form gives (Eq. 10):

t q q k q t e e t 1 1 2 2   (10)
The value of k 2 and q e can be determined from the slope and the intercept of the plot t/q t vs. t, respectively.

Determination of the zero point charge

The zero point charge was determined using 0.01 M solution of NaCl as an electrolyte and by adding 0.1 M solutions of NaOH or HCl. For this purpose, the pH of eight beakers containing 50 mL of electrolyte was set to the desirable values in the range of 2 to12. Then 0.5 gram of adsorbent was added into each beaker and shaken for 48 hours. After completion of the reaction, the adsorbent was filtered and the final pH of each beaker was measured. By plotting the initial pH versus the pH after 48 hours of agitation, the zero point charges of the adsorbents were determined, which were found to be 6.3 and 6.5 for natural and modified pumices, respectively.

Binary System Test

Calcium and magnesium co-exist in actual water streams. Series of experiments were conducted to investigate the effects of one another regarding the adsorption behavior. 6 g/L of adsorbent was added to 250 mL solution containing calcium and magnesium (100 mg/L each). The solution was then shaken at 200 rpm at room temperature for 4 hours (as equilibrium time). Samples were taken at predetermined time intervals, filtered and calcium and magnesium concentrations were measured.

Regeneration of the spent adsorbents

Regeneration tests for saturated natural and modified pumices were carried out by adding 6 g/L of spent adsorbent in 2 M solution of NaOH. For adsorbent saturation, 6 g/L of adsorbent were let in contact with 250 mL solution containing 100 mg/L calcium or magnesium and stirred at 200 rpm until equilibrium time was reached (240 minutes). The spent adsorbent was filtered, washed and dried at 55 o C for 24 h. The dried spent adsorbent was let in contact of 2 N NaOH for 500 min; then filtered, washed several times with deionized water and dried at 55 0 C for 24 h. The regenerated adsorbent was then tested for the adsorption of calcium and magnesium and the regeneration percentage was calculated based on the comparison of the removal efficiencies of fresh and regenerated adsorbents. The specific surface area of the adsorbents was measured using the BET technique. It showed a significantly higher specific surface area for the modified pumice if compared to the natural one, 36.30 and 2.34 m 2 /g respectively. The remarkable improvement of the surface area can most likely be attributed to the removal of components occupying the pores of the pumice resulting in more accessible pores and consequently larger surface area.

Results and Discussion

The XRD patterns of the natural and modified pumice adsorbents are illustrated in Fig. 1. While the existence of crystalline phases in the natural and modified pumice samples can be ascribed to the peaks at 2θ=12.0 0 , 26.0 0 , 28.0 0 , 32.5 0 , 33.0 0 , 33.5 0 , 34.5 0 and 28.0 o ; the observed dome between 2θ= 20 to 40 can be considered as an evidence for the presence of some amorphous phase in both samples. However, the decrease of the dome's height (i.e.

area of the dome) in the modified sample can be attributed to a decrease of the amorphous phase in the modified pumice.

SEM micrographs showing surface morphology of the natural and modified pumice adsorbents at magnification of 500X are displayed in Fig. 2. While the surface of the natural adsorbent showed irregular texture with larger grains and sharper edges, the alkali treated pumice showed a deeply re-structured texture with finer particles size and smoother surface.

Fig. 3 shows the Fourier transform infrared spectroscopy (FTIR) of the natural (a) and modified (b) pumices at wavelengths in the range 400-4000 cm _1 . The absorption band at ~1043 cm -1 was the characteristic peak of (SiO 4 ) 2-groups due to the symmetric stretching vibration of Si-O-Si. The peaks at ~784 and ~464cm -1 can be assigned to bending vibrations of Si-O-Si bond. The small peak at ~615 cm -1 can be assigned to the regional vibration of reductive carbon in crystal lattice. Other peaks can be assigned to adsorbed water molecules including the broadening peak at ~3449 cm -1 that belongs to the asymmetric stretching vibration of H-O bond and the peak at ~1640 cm -1 can be assigned to the bending vibration of H-O-H bond. Overall, a high similarity appeared between both IR spectra, in agreement with the available literature [START_REF] Li | Investigation on Microstructure, Composition, and Cytocompatibility of Natural Pumice for Potential Biomedical Application[END_REF].

Effect of the electrical conductivity (EC) and the turbidity

Partial dissolution of an adsorbent in the surrounding solution can influence the adsorption process impacting metal uptake capacity. EC and turbidity tests were conducted and showed an increase with time for both natural and modified pumices samples (Fig. 4). EC for natural and modified pumices increased from 6 to 20 µs/cm and 11 to 36 µs/cm, respectively. This increase can be attributed to the presence of some soluble constituents from the adsorbents. Furthermore, the higher increase observed for the modified pumice may be attributed to the release of Na + ions loaded during the modification process. Turbidity increased from 0.2 to 1.3 NTU and 0.3 to 1.2 NTU for the natural and modified pumice samples respectively, namely close to the standard values and hence may not influence the disinfection process.

Effect of the contact time and the initial hardness agents concentration

The effects of the initial calcium and magnesium concentrations and the contact time on the removal efficiency of pumice are illustrated in Figure 5. As it can be seen, adsorption efficiency was improved by increasing the contact time and the initial ions concentration.

While for natural pumice sample, the equilibrium time was not reached even after 240 minutes (Figs. 5a andb), equilibrium was reached after 150 and 180 min for calcium and magnesium adsorption on the alkali-modified pumice (Figs. 5c andd). Accordingly, for an initial metal concentration of 150 mg/L, 83% and 94% of calcium and 48% and 73% of magnesium were adsorbed by natural and modified pumices respectively, showing for both adsorbents a higher affinity for calcium over magnesium. Furthermore, alkali modification of pumice improved the adsorption capacity toward both tested cations.

Surface adsorption and ion exchange can be considered as the driving forces of ion removal. While bonding of metal ions to the surface can be considered as the main mechanism responsible for metal uptake by natural pumice, in addition to surface adsorption, ion exchange can also be involved in the case of the alkali-modified pumice leading to a remarkable enhancement of the adsorption capacity [START_REF] Panuccio | Cadmium adsorption on vermiculite, zeolite and pumice: Batch experimental studies[END_REF]. Furthermore, alkali modification can contribute to the removal of impurities, which can unblock some pores improving accessibility to the active adsorption and ion exchange sites of the modified sample. Removal efficiency also showed an increase with the initial metal concentration, which can be attributed to an increase of the concentration gradient increasing the driving force [START_REF] Gode | Column study on the adsorption of Cr(III) and Cr(VI) using Pumice, Yarıkkaya brown coal, Chelex-100 and Lewatit MP 62[END_REF].

Equilibrium data were analyzed by pseudo-first order, pseudo-second order and intraparticle diffusion kinetic models. Fig. 6 shows the fitting of equilibrium data onto pseudosecond order kinetic model and the related parameters are listed in Table 2. Pseudo-first order and intra-particle diffusion kinetic models showed lower correlation coefficient (R 2 ), and hence the corresponding data are not shown. Equilibrium data fitting confirmed that increasing the initial cations concentration increased sorption capacity of raw and modified adsorbents. As can be seen from Table 2, the experimental q e values were close to those calculated. In addition and for both adsorbents, the rate constants (k 2 ) for calcium sorption were higher than those observed for magnesium, confirming the higher affinity of the studied adsorbents for calcium over magnesium.

Effect of the pH and the adsorbent mass

The pH of the solution should be considered as an important factor affecting metal adsorption process due to its impact on the degree of ionization of metal specie and the surface charge of the adsorbent. The effect of pH of the reaction mixture on the adsorption efficiency at various adsorbent doses was examined in order to optimize the adsorbent dosage and the pH. According to the results summarized in Fig. 7, the highest adsorption capacity towards Ca +2 and Mg +2 ions was achieved at pH=6 for both natural and modified pumices and for all tested dosages. This optimal pH was in accordance with the zero point charge values (6.3 and 6.5 for natural and modified pumices respectively, see 2.7). As it is shown in andd) were removed by 10 g/L of the natural and modified pumice adsorbents, respectively. The lower removal efficiencies observed in acidic medium (i.e. pH in the range 2-5) can be attributed to the protonation of adsorbent functional groups or competition of H + with metal ions to bind and occupy the active sites of the adsorbents. On the other hand, in alkaline environment (i.e. pH in the range 8-10), the formation of metal hydroxide can be considered as the main reason for decreasing metal uptake [START_REF] Guzel | Determination of kinetic and equilibrium parameters of the batch adsorption of Mn(II), Co(II), Ni(II) and Cu(II) from aqueous solution by black carrot (Daucus carota L.) residues[END_REF]. Similar results were reported for the removal of magnesium by manganese oxide coated zeolite [START_REF] Taffarel | Removal of Mn 2+ from aqueous solution by manganese oxide coated zeolite[END_REF] and for the removal of calcium using raw sugar cane bagasse modified with tartaric acid and citric acid [START_REF] Soliman | Removal of calcium ions from aqueous solutions by sugar cane bagasse modified with carboxylic acids using microwave-assisted solvent-free synthesis[END_REF]. The highest removal capacity at pH=6, namely close to the zero point charges of natural and modified pumices, confirms that the studied pumice adsorbents can be considered as promising low cost adsorbents suitable for the removal of hardness ions from drinking water.

The linear increase of the adsorption capacity for increasing adsorbent dosage indicated the accessibility of a larger number of sorption sites at higher dosage to adsorb calcium and magnesium ions. Contrarily, in the removal of calcium using raw and modified sugar cane bagasse, adsorption capacity remained constant above 100 mg adsorbent [START_REF] Soliman | Removal of calcium ions from aqueous solutions by sugar cane bagasse modified with carboxylic acids using microwave-assisted solvent-free synthesis[END_REF].

Effect of the temperature

The temperature effect in the range of 10 o C to 60 o C was studied and thermodynamic parameters were calculated and summarized in Table 3. As shown in Fig. 8, while the effect of temperature on magnesium adsorption seemed to be negligible, a low maximum for the adsorption of calcium was observed at 20 o C.

The Arrhenius equation was used to evaluate the nature of the adsorption (either physical (5-40 kJ/mol) or chemical (40-800 kJ/mol) : [START_REF] Apell | Combined ion exchange treatment for removal of dissolved organic matter and hardness[END_REF] Where A o is the temperature independent factor called "frequency factor," E a the activation energy (kJ/mol), R is the gas law constant (8.314 J/mol.K), and T is the absolute temperature (K). A plot of ln k versus 1/T yields to a straight line, from which the E a and A o can be obtained from the slope and the intercept, respectively. In the present work, the values of E a and A o were observed to be 175.32 kJ/mol and 144.45 kJ/mol for calcium and 135.82 kJ/mol and 122.53 kJ/mol for magnesium adsorption on the natural and modified pumice adsorbents respectively, indicating chemical adsorption rather than physisorption.

Thermodynamic parameters were determined for temperatures ranging from 20 to 60°C using the equilibrium constant k d (q e /C e ). The change in free energy (∆G o ) was determined as follows (Eq. 12):

d o K RT G ln    ( 12
)
Where, ΔG o is the standard free energy (kJ/mol). The parameters of enthalpy ∆H o (kJ/mol) and entropy ∆S o (kJ/mol) related to the adsorption process were calculated from the following equation (Eq. 13):

RT H R S K o o d     ln ( 13
)
The parameters of enthalpy (∆H o ) and entropy (∆S o ) can be calculated from the slope and the intercept of the linear plot of ln k d versus 1/T. Fig. 9 shows the thermodynamic plots and the related parameters are collected in Table 3. The values of ∆H o were positive for all systems;

showing that the sorption reaction was endothermic in nature. The negative ∆S o value characterized a decrease in randomness at the solid/liquid interface during the sorption process. In addition, the values of standard free energy (∆G o ) were positive indicating that the sorption of metals was not thermodynamically spontaneous.

Adsorption Isotherms

Langmuir, Freundlich and Temkin models were used to evaluate the results of adsorption tests. The different adsorption isotherms are illustrated in Fig. 10 and the corresponding calculated data are collected in Table 4. The Fitting of equilibrium data onto isotherm models shows that the Freundlich isotherm gave the highest regression coefficients for calcium and magnesium adsorption compared to Temkin and Langmuir isotherms.

However, the closeness of the regression values indicated the relevance of the three isotherm models. The surface of the adsorbent contained most likely heterogeneous moieties which were uniformly distributed on the surface, accounting for Langmuir, Freundlich and Temkin isotherms [START_REF] Low | A Novel Pretreatment Method of Lignocellulosic Material as Adsorbent and Kinetic Study of Dye Waste Adsorption[END_REF]. The values of the separation factor (R L ) were calculated for various initial metal concentrations for raw and modified pumice adsorbents and were found between 0 and 1 (0.0176 to 0.0030 and 0.0292 to 0.0050 for calcium adsorption, 0.0400 to 0.0069 and 0.0320 to 0.0055 for magnesium adsorption on raw and modified pumice adsorbents, respectively), indicating a favorable adsorption process, as confirmed from the values of the Freundlich factor (n) which were in the range 1-10 (Table 4).

Maximum sorption capacities according to the Langmuir constant (q max ) for calcium and magnesium adsorption using various natural adsorbents are listed in Table 5 to allow comparison with the results from the present work. Maximum sorption capacities by raw and modified pumices were 62.34 and 57.27 mg/g for calcium and 44.53 and 56.11 mg/g for magnesium respectively, namely higher than those reported in Table 5, except for the adsorption of magnesium on pecan nutshell.

Binary System

In order to study the interfering effect of Ca +2 and Mg +2 ions one another, adsorption tests were conducted in the optimal conditions using a mixture of Ca +2 and Mg +2 cations (100 mg/L each) (Fig. 11). Cation uptake was 63% and 74% for calcium and 27% and 59% for magnesium on the natural and modified pumices, respectively. It can be noticed that equilibrium time and adsorption efficiency were lower compared to the values observed in single ion adsorption tests, which can be attributed to a competition between the two cations for occupying the active sites leading to a faster saturation of the adsorbent in the simultaneous presence of calcium and magnesium. Nevertheless and similarly to the single ion adsorption tests, calcium showed higher adsorption compared to magnesium indicating a higher selectivity of the adsorbents for calcium. These results also showed the efficiency of natural and modified pumices for the removal of calcium and magnesium from water streams.

Effect of other co-existing anions

Actual water samples contain a wide range of anionic species that may influence on the adsorption of calcium and magnesium ions. In order to study the performance of the pumice adsorbents in actual situation, a synthetic water sample containing chloride (450 mg/L), nitrate (25 mg/L) and sulfate (325 mg/L), as well as 100 mg/L of calcium and 100 mg/L of magnesium, was prepared. This sample simulated the actual water of the Karaj Water Supply system. The total dissolved solid (TDS) of the water sample was 1150 mg/L and the conductivity was adjusted by sodium chloride to 1850 µs/cm (according to the Karaj water supply system). The adsorption tests were conducted using 6 g/L of either natural or modified pumice adsorbents at pH 6 for 240 minute at room temperature. According to the results, counter ions (i.e. anions) decreased the adsorption capacity of the adsorbents for Ca 2+ and Mg 2+ cations, since removal efficiencies decreased to 53% and 61% for calcium and to 24% and 51% for magnesium for the natural and modified adsorbents, respectively. From this, the total remaining hardness in the treated water sample by using the natural and modified pumice adsorbents should be 429 and 297.5 mg/L as CaCO 3 respectively, namely close to the advocated value, since the total hardness for drinking water should be less than 300 mg/L as CaCO 3 (AWWA, 2005); hence the alkali modified pumice can be considered as a promising adsorbent for water softening. In addition, nitrate, chloride and sulfate concentrations were also measured after treatment. Removal efficiencies were 5.6 and 33%

for nitrate, 33 and 63% for chloride and 51 and 59% for sulfate for natural and modified pumices, respectively.

Furthermore, the conductivity of the water sample treated with the natural and modified adsorbents increased to 1975 and 1936 µs/cm, respectively. This should be related to the increase of the total dissolved solid, which were 1234 and 1221 mg/L after water treatment with the natural and modified pumices respectively, most likely due to partial dissolution of the adsorbents. Overall, although the co-existing ions can interfere in the sorption of calcium and magnesium, final total hardness can meet the required standard level when the alkali modified pumice was used as an adsorbent.

Regeneration of the saturated adsorbents

Regeneration experiments were conducted to study the reusability of the spent adsorbents, which is a very important parameter in terms of economical feasibility of the developed process. Regeneration using sodium hydroxide solution was carried out on the spent natural and modified pumice samples. Regeneration percentage of the spent adsorbent versus reaction time is illustrated in Fig. 12. Maximum recovery of the adsorbed calcium and magnesium was achieved in less than 300 minutes leading to 99 and 92% desorption of the adsorbed calcium and 100 and 89% of the adsorbed magnesium from the natural and modified pumices, respectively. Furthermore, maximum cation desorption for saturated natural pumice was observed after 510 min of regeneration, while only 300 min of regeneration was needed to achieve maximum cation desorption of the saturated modified pumice adsorbent. It should be noticed that even if the modified pumice stone showed higher sorption capacity for magnesium rather than natural pumice, its regeneration potential was lower than that of the natural pumice.

Conclusion

Softening of hard water by removing Ca 2+ and Mg 2+ cations was studied using natural and alkali-modified pumices as adsorbents. Increasing the mass of adsorbent, the contact time or the initial ions concentration led to an increase of cations removal. The magnesium. For both adsorbents, an increase of EC and turbidity was shown, which can be attributed to a partial dissolution of the adsorbent; this increase remained however limited.

After treating synthetic water solution simulating an actual water stream with the alkali modified pumice, total hardness of the treated sample met the required standard for drinking water. Reference Black carrot residues -------3.871 [START_REF] Guzel | Determination of kinetic and equilibrium parameters of the batch adsorption of Mn(II), Co(II), Ni(II) and Cu(II) from aqueous solution by black carrot (Daucus carota L.) residues[END_REF] Activated Chilean zeolites ------0.774 [START_REF] Taffarel | Removal of Mn 2+ from aqueous solution by manganese oxide coated zeolite[END_REF] Natural zeolite -------0.259 [START_REF] Taffarel | Removal of Mn 2+ from aqueous solution by manganese oxide coated zeolite[END_REF] Manganese oxide coated zeolite -------1.123 [START_REF] Taffarel | Removal of Mn 2+ from aqueous solution by manganese oxide coated zeolite[END_REF] Sugar cane bagasse modified with citric acid 26.52 ------- [START_REF] Soliman | Removal of calcium ions from aqueous solutions by sugar cane bagasse modified with carboxylic acids using microwave-assisted solvent-free synthesis[END_REF] Sugar cane bagasse modified with tartaric acid 14.72 ----- [START_REF] Soliman | Removal of calcium ions from aqueous solutions by sugar cane bagasse modified with carboxylic acids using microwave-assisted solvent-free synthesis[END_REF] Green tomato husk ------6.76 [START_REF] Garcia-Mendieta | Biosorption properties of green tomato husk (Physalis philadelphica Lam) for iron, manganese and iron-manganese from aqueous systems[END_REF] Chemically modified cellulose 15.6 13.5 [START_REF] Jinior | Removal of Ca(II) and Mg(II) from aqueous single metal solutions by mercerized cellulose and mercerized sugarcane bagasse grafted with EDTA dianhydride (EDTAD)[END_REF] Sugar cane bagasse 46.1 23.50 [START_REF] Jinior | Removal of Ca(II) and Mg(II) from aqueous single metal solutions by mercerized cellulose and mercerized sugarcane bagasse grafted with EDTA dianhydride (EDTAD)[END_REF] Rice husk -------3.87 [START_REF] Krishnani | Biosorption mechanism of nine different heavy metals onto biomatrix from rice husk[END_REF] Pecan nutshell ------103.8 [START_REF] Vaghetti | Pecan nutshell as biosorbent to remove Cu(II), Mn(II) and Pb(II) from aqueous solutions[END_REF] Kaolinite -------0.446 [START_REF] Yavuz | Removal of copper, nickel, cobalt and manganese from aqueous solution by kaolinite[END_REF] Raw pumice 57.

Figure Captions

Ln C e qe

Raw calcium M odified calcium Raw magnesium M odified magnesium

Fig 7 ,

 7 Fig 7, at this pH 79 and 96% of calcium (Figs.7a and b) and 51 and 93% of magnesium

Fig. 1 .Fig. 2 .Fig. 3 .Fig. 4 .Fig. 5 .

 12345 Fig.1. XRD patterns of natural pumice (a) and alkali modified pumice (b) Fig.2. SEM images of natural pumice (a) and alkali modified pumice (b) Fig.3. FTIR spectra of natural pumice (a) and alkali modified pumice (b) Fig.4. Effect of natural and modified pumices on conductivity and turbidity (6 g/L adsorbent, pH 6.5, room temperature, agitation speed 200 rpm, 5 µs/cm initial EC and approximately zero initial turbidity) Fig.5. Effect of initial cation concentrations and contact time on the removal efficiency, (a) Ca 2+ on natural pumice, (b) Ca 2+ on modified pumice, (c) Mg 2+ on natural pumice, (d) Mg 2+ on modified pumice (experimental conditions: pH=6.5, adsorbent dose 6 g/L, room temperature, 200 rpm agitation speed).

Fig. 6 .Fig. 7 .

 67 Fig.6. Fitting of equilibrium data onto pseudo-second order kinetic model, (a) Ca 2+ on natural pumice, (b) Ca 2+ on modified pumice, (c) Mg 2+ on natural pumice, (d) Mg 2+ on modified pumice Fig.7. Effect of pH and adsorbent dosage on calcium and magnesium removals, (a) Ca 2+ on natural pumice, (b) Ca 2+ on modified pumice, (c) Mg 2+ on natural pumice, (d) Mg 2+ on

Fig. 8 .Fig. 9 .Fig. 10 .Fig. 11 .Fig. 12 .

 89101112 Fig.8. Effect of the temperature on the removal efficiency (ions concentration 100 mg/L, adsorbent 6 g/L, pH 6.5, contact time 210 min) Fig.9. Thermodynamic illustration for calcium and magnesium adsorption on natural and modified pumices Fig.10. Fitting of equilibrium data onto isotherm models, (a) Freundlich, (b) Langmuir, (c) Temkin Fig.11. Sorption of calcium and magnesium in binary system (adsorbent 6 g/L, pH 6.5, ions concentration 100 mg/L, room temperature, 200 rpm agitation speed) Fig.12. Regeneration of the saturated adsorbents (spent adsorbent: 6 g/L, initial calcium and magnesium concentration: 100 mg/L each and 2 N NaOH used for regeneration).

Figure 1 Figure 2 Figure 3 Figure 4

 1234 Figure 1

Table 1

 1 Chemical compaction of raw used adsorbent (w/w)

	Component	%	Component	%
	SiO 2	63.45	SrO	0.09
	Al 2 O 3	17.24	MgO	1.03
	TiO 2	0.37	K 2 O	2.16
	P 2 O 5	0.21	SO 3	0.16
	CaO	3.22	Na 2 O	2.00
	Fe 2 O 3	2.86	Cl -	0.30

Table 2

 2 Pseudo second order kinetic parameters for the present work

		Calcium on raw adsorbent	Calcium on modified adsorbent
		k 2	q e, calc q e, exp R 2	k 2	q e, calc	q e, exp R 2
	50 mg/L	0.63	5.59	5.25	0.98	0.71	6.30	5.91	0.97
	100 mg/L	0.67	8.99	8.83	0.99	0.82	17.82	15.33 0.98
	150 mg/L	0.73	16.05 15.75 0.98	0.89	25.70	23.50 0.99
		Magnesium on raw adsorbent	Magnesium on modified adsorbent
		k 2	q e, calc q e, exp R 2	k 2	q e, calc	q e, exp	R 2
	50 mg/L	0.57	2.61	2.33	0.99	0.60	5.77	4.25	0.97
	100 mg/L	0.59	3.61	3.16	0.98	0.61	13.26	10.83 0.98
	150 mg/L	0.61	14.49 12.33 0.97	0.65	21.83	18.25 0.97

Table 3

 3 Parameters collected from the thermodynamic model

		ΔH 0	ΔS 0				ΔG 0
	T (K)			283	293	303	313	323	333
	Raw-Ca +2	0.493	-10.13	3063	3068	3596	3929	4054	4292
	Modified-Ca +2	0.257	-6.41	1993	1816	2258	2332	2536	2614
	Raw-Mg +2	0.418	-22.51	6555	6787	7148	7521	8056	8305
	Modified-Mg +2	0.088	-11.73	3359	3478	3596	3822	3944	4066

Table 4

 4 Parameters collected from isotherm models

	Isotherm	parameter	Raw-Ca +2	Modified-Ca +2	Raw-Mg +2	Modified-Mg +2
	Model
		r 2	0.991	0.996	0.989	0.989
	Freundlich	K f	7.23	9.24	7.9	7.35
		n	0.263	0.357	0.384	0.294
		r 2	0.960	0.911	0.965	0.949
	Langmuir	q m	62.34	57.27	44.53	56.11
		b	2.23	1.33	0.96	1.21
		r 2	0.905	0.937	0.906	0.911
	Temkin	b 1	68.31	82.44	109.03	79.02
		k t	0.8	1.69	0.53	0.66

Table 5

 5 Maximum sorption capacity of some adsorbents for calcium and magnesium

	q max (mg/g)

Acknowledgment

This work was supported by Alborz University of Medical Sciences. The authors are grateful to Alborz University of Medical Sciences for financial support of this work.