Reduction of arsenic oxoanion toxicity from contaminated water by adsorption onto iron impregnated biochar


 Several ways have systematically been employed to modify carbons for removing heavy metal from contaminated solution. In the present study, Fe impregnated pomegranate peel carbon (Fe-PPC) as a green adsorbent was prepared by addition of Fe3+ions into the pomegranate peel followed carbonization. Stability of iron on PPC was examined by releasing of Fe ions in aqueous solution at the pH range of 2–10. The effect of contact time, variable pH, and initial concentration were studied in the adsorption of arsenite and arsenate anions by Fe-PPC. Maximum removal of arsenite and arsenate anions were observed in the pH range of 5–6 and 9–11 respectively which were 76.3% and 100%. The elemental analysis and characterization of As -FePPC were performed by energy dispersive X-ray analysis (EDX) and BET. The adsorption rate (ν0) value indicated that the initial rates are suitable for arsenic anions adsorption on Fe-PPC in aqueous solution. The more than one mechanism was governed by the adsorption arsenate and was chemisorption, while the physical adsorption had the more contribution to the arsenite adsorption.


Introduction
Arsenic is a toxic metalloid that exists in the + 3 and + 5 oxidation states. It is present in the environment in a variety of minerals as oxyanions. Common environmental aqueous forms are arsenate (AsO 4 3− ) and arsenite (AsO 3 3− ), referred to as As(III) and As(V) anions. Arsenite predominates in reducing anaerobic environments and act as hard acid. Arsenate is stable in the oxygen-rich aerobic environment and behaves like a soft acid (Baig, Sheng et al. 2015). Arsenic compounds entered the environment due to the natural and anthropogenic activities (Smedley andKinniburgh 2005, Chung, Yu et al. 2014). Long-term exposure to arsenic caused skin and circulatory problem, degenerative diseases and eve bladder, kidney, liver, and lungs cancer (Sun, Rathinasabapathi et al. 2014). In order to reduce health risk, World Health Organization (WHO) recommended a maximum contaminant level of total arsenic of 10 µg.l − 1 in drinking water (WHO 2008). Also, it was reported that arsenic is toxic at low levels of 10 − 6 to 10 − 5 M in natural and drinking water; thereby treatment of contaminated water to decrease the concentration of arsenic anions to the standard level (10 − 6.87 M) is necessary. Chemical precipitation is a common method to remove heavy metals from wastewaters. To reach a level below 10 µg.l − 1 , a further additional process is required in the treatment of heavy metals after precipitation.
Recently, different methods have been examined for the treatment of waters contains a low concentration of heavy metal ions. Among them, adsorption is an e cient and effective method for removal of arsenic anions from aquatic environments. For the cost effective adsorption, modi ed carbon agricultural wastes have received great attention. Hence, several researches used cheap and stable iron-base support such as sand (Vaishya and Gupta 2003) cellulose (Guo andChen 2005, Hokkanen, Repo et al. 2015), polymeric materials (Vatutsina, Soldatov et al. 2007), and magnetic biochar prepared from pinewood and natural hematite (Wang, Gao et al. 2015) for removal of arsenic anions. Also, different efforts have been made to impregnate iron into a carbon structure with various concentrations, in order to increase arsenic adsorption capacity; Chang et al. 2010 impregnated Fe(II) chloride in granular activated carbon; Chen et al. 2007 inserted Fe(NO 3 ) 3 in powder activated carbon; Fierro et al. 2009 used Fe(III) chloride for activation of carbon; Muniz et al. 2009 doped Fe(II) in commercial activated carbon but the doping iron into the agricultural wastes and carbonization was not used for removal of arsenic anions from aqueous solutions. In previous work, we found that pomegranate peel (PP) was an ideal support for iron loading because of the high content of polyphenols and the formation of stable complexes with the iron ions and after carbonization used for removal of Cd ions. The results showed 1.5 fold increases in removal e ciency of cadmium (Abedi, Salmani et al. 2016). Hence, the aim of this study is the investigation of iron impregnated to pomegranate peel carbon to adsorb arsenite and arsenate anions from aqueous solution for reducing the toxicity of arsenic oxoanions.

Materials And Methods
Pomegranate peels were procured from pomegranate gardens of Yazd city, Iran. While the lower concentrations of As (<10 ppm) were analyzed by graphite furnace atomic absorption (GFAA) spectrometer.
Nitrogen adsorption-desorption Brunauer-Emmett-Teller (BET) was used to determine the porosity, speci c surface area, pore volume and diameter of Fe-PPC by Belsorp porosimeter. The measurements were performed at 77 ºK and saturated vapor pressure of 87.8 KPa of N 2 gas and the specimen was degassed in 300 ºK. Barrett-Joyner-Halenda (BJH) method was used for the evaluation of the pore size distribution.

Impregnation of pomegranate peel by Fe(III) ions
The separated peels were thoroughly washed with water to remove any impurity on the peel surfaces. The peel samples gently dried at Lab temperature for a week in the absence of sunlight. The samples were ground using electrical mortar, sieved through a 40-60 mesh sieves and then heated in 70°C for 1 h.
These dried granules were used for impregnation of iron and modi cation of bioadsorbent. The modi cation procedure was conducted in two steps for loading iron into the PP to achieve optimum amount and stable impregnation ratio. First, 50 ml of 1-5 % (w/V) FeCl 3 solution was mixed with 50 g PP granules until the peels were completely adsorbed the solution and after one night dried at 105 °C for 3 h.
The solids were rinsed with distillate water 3 times to remove any un-reacted iron and chloride ions and again dried at 105 °C. Second, 10 g of PP impregnated with iron introduced to the crucible, and placed in the programmable furnace at atmosphere of nitrogen. The mixture was heated in the 400 °C and 3 h to be carbonized in the absence of air that this procedure was optimized in our previous work (Abedi, Salmani et al. 2016).
The stability of iron impregnated into the Fe-PPC was evaluated from the measurement of the iron released at different pH conditions. For each test, 0.2 g of Fe-PPC sample was introduced into the 100 ml solution that its pH was initially adjusted at 2, 4, 7, and 10 using suitable amounts of 1.0 or 0.1 M HCl or NaOH. The suspension was shaken at a contact time of 8 h at 25 °C. The concentration of iron released from Fe-PPC was measured by AAS after ltering of suspension with 0.45 μm lter. The percent of releasing iron was evaluated according to the mass of Fe-PPC.

Adsorption experiments
The effect of solution pH on the adsorption of arsenite and arsenate by Fe-PPC was investigated by varying the pH from 2 to 10. For each test, 0.2 g of adsorbent was mixed with 100 ml 20 mg/l of arsenite or arsenate solution in a series of Erlenmeyer, separately. The pH of the suspension was adjusted and the reaction vessels were shaken in an orbital shaker at 150 rpm and 25ºC and optimal contact time of 8 h.
The kinetics experiments were recorded by adding 0.2 g of Fe-PPC to 50 ml of 20 mg/l arsenite or arsenate solution. Adsorption experiments were carried out at pH 4 for arsenite and pH 6 for arsenate.
The asks were agitated on an orbital shaker at 150 rpm and 25 ºC. Each glass bottle was taken off the shaker at 0.25, 0.5, 1.0, 1.5, 2.0, 4.0, 8.0 and 10 h. To carry out adsorption isotherm experiments, the pH was set like the pH of kinetic tests. The same amount (0.2 g) of Fe-PPC was added to 50 ml of arsenite or arsenate in range of 5, 10, 20, 30, and 50 mg/l. The glass bottles were put in the shaker at 150 rpm and 25 ºC. After the end time of each experiment, the suspensions were ltered and the solutions were analyses at the end of the experiments, immediately.

Characterization of adsorbent
The elemental analysis and physiochemical properties of prepared Fe-PPC adsorbent were studied by EDX and BET experiments. The EDX analysis of Fe-PPC adsorbent impregnated with 3% iron before and after adsorption of arsenate has been shown in Fig. 1 (a and b), respectively. Nitrogen adsorption and desorption isotherm for Fe-PPC and its pore size distribution before and after adsorption of arsenate anions has been examined. Surface area, pore volume total and average pore diameter of Fe-PPC before and after adsorption of arsenate are represented in Table 1.   The stability of impregnated iron in Fe-PPC that contained 6.0 mg of iron in unite of mass was examined in four desorption tests at the same pH of adsorption experiments. According to the results in Table 2, the iron content of Fe-PPC nearly 0.56 mg/L was lost in desorption process when pH was 4. At pH 2, the releasing iron percent was 1.6 mg/L.   Table 3. The removal of arsenate appears to be higher than 90% in the low concentration (10 mg/l).  Several models are available to explain the isotherm behavior of adsorption process. Isotherms for arsenic biosorption on Fe-PPC were generally of the Langmuir or Freundlich types that adsorption constants of both models are listed in Table 5. Also, the experimental data were correlated to pseudo-rst order, and pseudo-second order equations. The parameters of K 1 and K 2 and q e were calculated from the slope and intercepts of the lines by plotting log (q e -q t ) and t/q e against t and the parameters of K id and q id were obtained from the plot of q t versus the square root of time (t 1/2 ) that all calculated parameters are listed in Table 7. The pseudo second-order model could best describe the kinetic results with R 2 =0.974 and R 2 =0.992 for arsenite and arsenate, respectively. Fig. 6 shows a linear plot of the linearized forms of the pseudosecond order model for adsorption of both anions on Fe-PPC.  Arsenic species adsorption has been studied by various investigators, using a variety of different adsorbents. In Table 6 the respective adsorption capacities of common adsorbents are presented for comparison with the studied modi ed PPC biosorbent.

Discussion
The EDX analysis of Fe-PPC adsorbent impregnated with 3% iron before and after adsorption of arsenate indicated that the arsenate has been adsorbed by Fe-PPC (Fig. 1b). The speci c surface area and pore volume of the compound were slightly reduced after adsorption of arsenate anions. In fact, arsenate adsorption leads to reduce of cavity space that caused change in the pore volume and speci c surface area of Fe-PPC (Monárrez-Cordero, Amézaga-Madrid et al. 2016). According to the results of Table 1, the surface area was reduced from 314 to 233 m 2 .g -1 and consequently a decrease in pore volume. High surface area of Fe-PPC (314 m 2 .g -1 ) makes it a suitable adsorbent for removal of arsenite and arsenate anions. The high porosity percentage indicates that high surface area of adsorbent is due to the pore surface.
As it seen in Fig 1,  solution for treating of ACs was 0.2 M that is similar to our nding. (Muñiz, Fierro et al. 2009). Iron desorption experiments indicated that treatment of PP with Fe 3+ and them carbonized of the mixture was obtained a good stable composite. Depending on the results presented in Table 2, the iron content of Fe-PPC, less than 0.005 % was lost in the moderate pH solutions and it was very stable when pH was above 4.
For optimization of adsorption parameters, the one factor at the time procedure was conducted. In the present study, the contact time, pH and initial concentration were optimized and physical parameters as the rpm and temperature were constant at all experiments. The fast adsorption at the initial time of 2 h was due to the concentration gradient between the adsorbate in solution and the number of active sites available on the adsorbent surface at beginning of process. The results presented in Table 3, show that with increasing of initial concentration, the adsorption capacities increase while, removal e ciencies decrease linearly for both anions. This can be explained that adsorbent has a limited number of active sites, which become saturated at a certain concentration, while the ratio of the number of adsorbates to the active sites of adsorbent increase with the increasing of the initial concentration.
The effect of solution pH on arsenic anion adsorption was explained based on the species of adsorbate present in solution and the adsorbent surface chemistry in different pH ranges. The arsenite and arsenate species in different pHs have been represented in Table 4. Arsenite species in aqueous solution present mainly as H 3 ASO 3 at pH less than 9 and above that as H 2 ASO 3 and ASO 3 3-. Arsenate species in aqueous solution mainly present as H 3 ASO 4 at pH less than 2.2, H 2 ASO 4 at pH between 2.2-6.98, HAsO 4 2at pH between 6.08-11.5 and ASO 4 3at pH above 11.5 (Jadhav, Bringas et al. 2015). 3-Adsorbent surface chemistry is described based on the zero point charge (pH zpc ). The pH zpc is the pH that adsorbent has a net zero charge on the surface which is a fundamental importance in surface chemistry. According to the previous characteristic of the surface charge, the pH zpc for Fe-PPC was found to be 6.08.
Then, Fe-PPC has positively charged when pH is less than pH zpc (6.08) and able to adsorbed anions from the solution. While, pH is above pH zpc , the surface has negatively charged and can react with cations in solution. According to Table 4, the electrostatic force between Fe-PPC and arsenate can occur at pH between 2.2 -6.1 and changes to repulsion force when the pH increases above 6.1, and for arsenite occurs at pH between 2-8 and changes to repulsion force when the pH increases over 8. The experimental results showed that effective removal of arsenate and arsenite anions occurred in the pH range of 2-6.
The equilibrium adsorption is an important parameter to design of adsorption system. Adsorption isotherm describes the equilibrium between adsorbent in the liquid and adsorbed on the surface of adsorbent at constant temperature. According to Table 5, the correlation coe cients of the Langmuir isotherm were 0.981 and 0.997, and those for Freundlich model were 0.640 and 0.933 for arsenite and arsenate, respectively. This revealed the data were tted better the Langmuir equation than Freundlich equation for both cases. Thus, the adsorption of arsenite and arsenate anions on the Fe-PPC obeyed the Langmuir adsorption isotherm. The value of b was found to be 0.307 and 16.46 for arsenate and arsenite on Fe-PPC biosorbent, respectively. A high value of b also implies strong bonding of arsenic to the modi ed biosorbent. The important characteristic of the Langmuir isotherm is a dimensional constant separation factor, R L , which is de ned as R L =1/(1+bC 0 ). The R L value indicated the isotherm type and predicts whether an adsorption system is favorable (0<R L <1) or unfavorable (R L >1). The R L values were found to be 0.14 and 0.003 for concentration 20 mg L -1 of arsenate and arsenite adsorption on Fe-PPC which the isotherms are favorable for both anions.
The higher removal rate of arsenate (0.576) in contrast of arsenite (0.384) on Fe 3+ -PPC is due to the easy to the complex between arsenate and Fe 3+ -PPC sites. It is evident that the adsorption process under this condition is more suitable described by the pseudo second-order model which was based on the assumption that the rate limiting step may be chemisorption including valence forces through the exchange or sharing of electron between adsorbent and adsorbate (Khan, Chaudhry et al. 2015). The initial adsorption rate is an important constant factor, υ 0 , which is de ned as υ 0 =K 2 q e 2 . The υ 0 values were found to be 5.66 and 19.11 mg.(g.min) -1 for arsenite and arsenate adsorption on Fe-PPC which the initial rate is suitable for both anions.
According to the intraparticle diffusion model, the correlation of adsorption capacity (q) with square root of time (t 1/2 ) should be linear if intraparticle diffusion is embraced to the adsorption mechanism. If the intercept of line is zero (line passes through the origin) then the intraparticle diffusion is the rate controlling step of the adsorption process (Wu, Tseng et al. 2009). As shown in Table 7, the correlation of adsorption capacity (q) with the square root of time (t 1/2 ) was linear and passed through the origin at times of under 2 h. This suggested that until 2h the intraparticle diffusion was the rate limiting on the adsorption system.
The q max is the maximum value of q e that is important and represents the monolayer saturation at equilibrium. The magnitude of q max was found to be equal to 7.71 mg.g -1 for the arsenate and 5.73 mg.g -1 for arsenite on the Fe-PPC, experimentally. These values have a good argument with the amount of q m from the Langmuir model (Table 5). Although a direct comparison between the examined iron modi ed absorbent with those obtained in the literature is di cult, due to the varying experimental conditions employed in those studies, the modi ed biosorbent used in the present study shows reasonably high adsorption capacity and binding energy constant, as compared with other adsorbents.

Mechanism of arsenic oxyanion sorption
Adsorption of arsenite and arsenate anions by Fe-PPC could be achieved by physical and chemical adsorption. Physical adsorption allows small solute particles and hydrophobic molecules to easily enter into the pores in adsorbent. The rate of diffusion into a pore depends on the size of particles and the degree of its hydrophobicity. This process depended on the pore size of Fe-PPC and radius of adsorbents.
According to BET, it seems that carbonized Fe-PPC was a porous material that can well adsorb arsenate and arsenite with a radius of 4.2 and 4.8 °A, respectively. Hence, certain particles of arsenate and arsenite in solution can in ltrate on to the pores and across from one pore to the other.
Chemical adsorption, including reaction to iron oxides and reaction with other groups initially contained on the Fe-PPC surface, depended on the existing forms of the arsenic in solution and functional groups which speci ed by considering ZPC of the adsorbent material. In the pH range 2-7, H 2 AsO 4 was predominantly adsorbed. The adsorbent acquired positive charge in this pH range because the pH zpc of Fe-PPC was 6.3, and adsorption was facilitated by columbic interactions. In the pH range 2-7, speci c interactions occurred, since dissociation of arsenate is expected. The transfer of a proton to the hydroxyl groups of the Fe(III) hydroxide on the Fe-PPC surface is also possible. A surface complexation has also been proposed for arsenate adsorption on Fe-PPC biosorbent. So, arsenic species may be chemically removed from the iron hydroxides groups, which in this pH range are present as a cationic form of (PPC-Fe(OH) 2 + ) on the surface of adsorbent through indicated schematically in the 1, 2, and 3 equations: These routes are better for removal of arsenate than arsenite, since arsenite is generally available as neutral molecules at pH < 9, and a low number of active groups could be formed outer complex with arsenite molecules at pH < 9. This indicates the application of either physicochemical or diffusion for adsorption of anions into the pores, in order to increase the overall e ciency of the adsorption method.
The aforementioned reactions suggest that arsenic species is mainly adsorbed on Fe-PPC through speci c adsorption (chemisorption), where the adsorbing of arsenate ions is bonded directly to the surface functional groups and caused the inner sphere complex formation. In the case of arsenite, where outer sphere complexes are probable to be formed, water molecules are present between the adsorbing anion and the surface functional groups. In study was conducted by Liu et al., they have suggested that arsenite can also form inner sphere complexes on Fe oxides, however, outer sphere complexes may also be probable to occur (Liu, Chuang et al. 2015).
The above simpli ed equations can describe the adsorption of arsenic species on Fe-PPC. In order to obtain a more detailed description of adsorption mechanism, studies are usually performed by macroscopic scale. The macroscopic adsorption phenomena are generally described on the basis of analysis of equilibrium data using adsorption isotherms (Thommes 2010). Isotherm models require a qualitative knowledge of functional groups composition and are dependent on the observed relationships between aqueous and adsorbed phases of the adsorbing species (Koretsky 2000 Hence, the overall sorption of arsenate on the Fe-PPC is complex, involving more than one mechanism, such as ion exchange, surface complexation, electrostatic attraction and pore diffusion. Adsorption of oxyanions by modi ed PPC biosorbents was attributed to anion exchange on surface groups, resulting from the formation of a monolayer (Hiemstra and Van Riemsdijk 1999). The difference in the removal of arsenite and arsenate can be explained by the speciation differences. Arsenite is present as an anion exclusively above the pH value of 9.0 (Table 4), whereas in the pH range 2-9 only a small percentage of H 3 AsO 3 is dissociated. Thus the effective parameter on treatment of arsenite from solution must be diffused into the pores. In addition, due to the fact that the arsenite has a larger radius than arsenate and attributed it to having a small diffusion compared to arsenate, consequence the physical adsorption has a contribution on the adsorption of two particles by Fe-PPC. This phenomenon caused maximum about 50% of both adsorbates removal from solution.

Declarations
Ethic approval and consent to participate Not applicable

Consent for publication
All the authors have given consent for the publication of the manuscript.
Availability of data and material Not applicable

Competing interests
No competing nancial interests exist.   The plot of qe vs Ce for adsorption of arsenite and arsenate Figure 6