Journal of Hazardous Materials Synthesis

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Journal of Hazardous Materials 165 (2009) 893–902

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Synthesis, characterization and performance in arsenic removal of iron-doped activated carbons prepared by impregnation with Fe(III) and Fe(II) ˜ a,d , V. Fierro a,∗ , A. Celzard b , G. Furdin a , G. Gonzalez-Sánchez c , M.L. Ballinas d G. Muniz a

Laboratoire de Chimie du Solide Minéral, Nancy-Université, UMR CNRS 7555, BP 239, 54506 Vandœuvre-lès Nancy, France Laboratoire de Chimie du Solide Minéral, UMR CNRS 7555, Nancy-Université, ENSTIB, 27 rue du Merle Blanc, BP 1041, 88051 Épinal Cedex 9, France c Centro de Investigación en Materiales Avanzados (CIMAV) Miguel de Cervantes 120, Compl. Ind. Chih., 31109 Chihuahua, Mexico d Facultad de Ciencias Químicas, Universidad Autónoma de Chihuahua, Circuito Universitario S/N, Chihuahua, Mexico b

a r t i c l e

i n f o

Article history: Received 9 July 2008 Received in revised form 17 October 2008 Accepted 20 October 2008 Available online 28 October 2008 Keywords: Activated carbon Arsenic Iron Natural water Adsorption

a b s t r a c t Arsenic removal from natural well water from the state of Chihuahua (Mexico) is investigated by adsorption using a commercial activated carbon (AC). The latter is used as such, or after oxidation by several chemicals in aqueous solution: nitric acid, hydrogen peroxide, and ammonium persulphate. Raw and oxidised activated carbons are fully characterised (elementary analysis, surface chemistry, pore texture parameters, pHZC , and TEM observation). Adsorption of As is measured in the aforementioned water, containing ca. 300 ppb of arsenic: removal of As is poor with the raw AC, and only the most oxidised carbons exhibit higher performances. By contrast, iron-doped ACs are much more efficient for that purpose, though their As uptake strongly depends on their preparation conditions: a number of samples were synthesised by impregnation of raw and oxidised ACs with HCl aqueous solutions of either FeCl3 or FeCl2 at various concentrations and various pH. It is shown that iron(II) chloride is better for obtaining high iron contents in the resultant ACs (up to 8.34 wt.%), leading to high As uptake, close to 0.036 mg As/g C. In these conditions, 100% of the As initially present in the natural well water is removed, as soon as the Fe content of the adsorbent is higher than 2 wt.%. © 2009 Published by Elsevier B.V.

1. Introduction The presence of dissolved arsenic in groundwater has provoked an international concern due to its known toxicity [1]. The decrease of the maximum arsenic level in drinking water down to10 ␮g L−1 imposed the modification of more than 4000 water supply systems utilised by 20 millions of people [2]. Therefore, there is a great need for applying efficient methods for arsenic removal from drinking water. So far, a variety of methods have been developed for this purpose. The conventional physico-chemical processes used for arsenic removal can be classified on the basis of the involved separation mechanisms: precipitation, ion exchange, membrane, and adsorption technologies. Precipitation is widely used because of its simplicity and reduced cost; however, in order to remove efficiently arsenic at acceptable levels, large amounts of chemicals are necessary. Another disadvantage is that precipitation also creates a great sludge volume, which is not easy to reprocess or cannot be directly disposed. Moreover, arsenic(III) sulphide and calcium or

∗ Corresponding author. Fax: +33 383684619. E-mail address: [email protected] (V. Fierro). 0304-3894/$ – see front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.jhazmat.2008.10.074

ferric arsenates, which are the most common arsenic precipitates, are unstable under some definite conditions and are therefore not suitable for direct disposal, otherwise As could be released in the environment [3]. Ion exchange is the process by which chlorides or other anions bound at the surface of a resin are exchanged with arsenic-based anions from the solution. This process has the disadvantage of releasing harmful chemicals into the environment when the resin is regenerated [4]. Finally, membrane processes are commonly employed, but this technology is expensive, mainly because of the high energy requirements [5]. The technology of adsorption is based on materials having a high affinity for dissolved arsenic. Adsorption of arsenic by iron compounds has been established by several authors [6–8]. Elementary iron [9–11], granular iron hydroxides, and ferrihydrites [12–15] have been proposed for the removal of arsenic from water. Most of the adsorption processes investigated so far were reported in the excellent review of Mohan and Pittman [16], whereas the other techniques were considered in that of Choong et al. [17]. Activated carbons (ACs) are by far the most widely used adsorbents for water purification. AC adsorption are strongly dependent on the physico-chemical properties of the solution, and hence on arsenic speciation; for example, adsorption capacities are very low at high pH [18,19]. Arsenic removal by AC can be improved by dop-

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ing with metals having a high affinity for arsenate and arsenite as copper [20–22] and zirconium [23,24]. However, Cu and Zr can be released in the solution, thus presenting other toxic effects. The northern part of Mexico has been affected by arsenic pollution since many years. During the eighties, concentrations of arsenic as high as 600 ppb were reported in the waters of Coahuila and Durango states, and especially in the region called Comarca Lagunera. Furthermore, arsenic levels up to 500 ␮g L−1 [25] were measured in the state of Chihuahua, in the region of DeliciasMeoqui. There have been many studies focused on arsenic adsorption by modified ACs [26,30], but the influence of the method of doping them with iron on the removal efficiency of arsenic from water was seldom reported. The present study describes the doping of a commercial AC, either as such or formerly oxidised by various oxidants, using aqueous solutions of either FeIII or FeII . Surface chemistry and pore texture were systematically investigated for all the materials produced. Their performances in As removal were finally tested, using an As-polluted natural water from a well of Chihuahua state (Mexico).

• 5 M nitric acid (HNO3 ) was used at its boiling point, during either 0.5 or 3 h; the corresponding carbons were thus termed 05HNO and 3HNO, respectively. • 1.5 M hydrogen peroxide (H2 O2 ) was used at room temperature by successive additions of 100 mL/h, in order to delay its spontaneous decomposition. After the addition of 400 mL (i.e., 4 h later), 50 mL were added each hour up to a final volume of 700 mL [31]. The carbon oxidised by H2 O2 was named N1-HO. • Addition of 1 M sulphuric acid (150 mL) to H2 O2 allowed decreasing the oxidation time down to 1 h. The reaction was carried out at 50 ◦ C, and the oxidised carbon was named N1-HH. • Finally, 300 mL of a saturated solution of ammonium persulphate (NH4 )2 S2 O8 in acidic medium (1 M H2 SO4 ) was used at room temperature [32,33]. The resultant oxidised carbon was named N1-NH.

After the oxidation treatments, the ACs were filtered and washed in a soxhlet for 4 days. Finally, they were dried in an oven at 105 ◦ C for 24 h, and stored under inert atmosphere (dry nitrogen) at room temperature.

2. Experimental 2.3. Iron doping of activated carbons

2.1. Materials The removal of arsenic from water was carried out using chemically modified activated carbons derived from the commercial material NC-100, supplied by the company PICA (Vierzon, France). The latter was oxidised by different treatments, and the oxidised ACs were doped either with FeIII or FeII . For the sake of comparison, iron doping was also tried using the raw, non-oxidised, material. The composition of the Mexican well water used in the present study is given in Table 1. The total arsenic concentration is close to 300 ppb. Redox and pH conditions control the speciation of arsenic, which mainly exists as arsenate (AsV ) and arsenite (AsIII ) after dissolution. Arsenate is typically present in the monovalent H2 AsO4 − and divalent HAsO4 2− anionic forms in oxygenated waters, while arsenite principally occurs in the neutral form, HAsO2 , in waters of pH lower than ca. 9.0. For typical pH of natural waters (pH 5–8), arsenate exists as an anion, while arsenite remains fully protonated and is thus present as a neutral species. The measured redox potential of the water (using platinum and reference electrodes) was +0.24 V. 2.2. Oxidation of the activated carbons The commercial activated carbon NC-100 was oxidised according to the five different following methods.

2.3.1. Speciation of iron, according to pH, concentration, and oxidation state Fig. 1(a) was plotted using the Medusa software [34], and shows how the speciation of iron depends on the initial concentration of FeIII on one hand, and on that of the HCl solution on the other hand (0.5 and 3 M in this example, and redox potential of 0.24 V, thus matching the real experimental conditions we used). The relative amounts of the corresponding soluble species that might have entered the porosity of the AC can thus be deduced, and depend on the total iron concentration. Neutral and cationic species Fe(OH)2.7 Cl0.3 and FeCl2 + are the main ones for total [FeIII ] of 0.05 and 0.2 M, respectively, as soon as the FeIII concentration is above ca. 0.01 M. The higher is the HCl concentration, the higher is the proportion of chloride complexes, and the higher is the concentration of the neutral and negative species FeCl3 and FeCl4 − , respectively. These features will be useful to explain how the iron loading of the investigated activated carbons depends on the concentrations of iron(III) on one hand, and chloride on the other hand. Especially, it will be shown below that the Fe content of non-oxidised ACs increases with both FeCl3 and HCl concentrations. However, the use of very acid solutions (pH < 1.0) of FeCl3 was claimed to be ineffective, because the strong affinity of protons for the surface sites would hinder the adsorption of FeIII [29]. Nevertheless, it was found here that increasing HCl concentration from 0.5 (pH 0.30) to 3 M (pH < 0) significantly increased the amount of adsorbed iron on the AC (see below).

Table 1 Physico-chemical parameters of a well water of Chihuahua. NS, UC, TDS, and TSS mean: Not specified, units of colors, total dissolved solids, and total suspended solids, respectively. Parameter pH Alkalinity (mg L−1 CaCO3 ) Chlorides (mg L−1 Cl) Color (UC) Conductivity (␮S cm−1 ) Total hardness (mg L−1 CaCO3 ) Ca hardness (mg L−1 ) TDS (mg L−1 ) TSS (mg L−1 ) As (␮g L−1 )

8.03 142 21.8