Treatment of arsenic-containing solutions using chitosan derivatives: uptake mechanism and sorption performances
Introduction
Arsenic, resulting from geochemical reactions, industrial and mining waste discharges, or from the agriculture use of arsenical pesticides, is found in many surface and ground waters [1]. It has become a critical problem for water supplies in several countries such as Bangladesh, India and Chile [1], [2], [3]. Due to the carcinogenic effect of arsenic, a number of processes have been investigated and developed for its extraction. In aqueous media, it is found in its trivalent and/or its pentavalent forms, depending on redox conditions [1]. Many precipitation processes have been developed for arsenic removal from wastewater, using sulfide precipitation, co-precipitation with iron and metal hydroxides [4], [5], [6]. Hering et al. [7] obtained very efficient recovery of arsenic (III) by chemical oxidation followed by As(V) precipitation on ferric and aluminum hydroxides: the drinking water level was reached. More recently, Hug et al. [8] tested a simplified oxidation process coupled with a precipitation procedure for the removal of As(III) at near-neutral pH using citrate (or alternatively lemon juice) and Fe(II). Emett and Khoe [9] used a photochemical process for the oxidation of As(V) in the presence of oxygen and iron for its further precipitation. These processes usually also result in the co-precipitation of other metals, it is difficult to concentrate arsenic in small waste volumes. Coagulation and electro coagulation processes have been also successfully tested for the volumetric reduction of toxic sludges [4], [10]. In order to improve selective separation of arsenic from such dilute media, adsorption processes have been developed using a large diversity of sorbents [11] and ion exchange systems [12], [13]. Activated carbon [14], activated mineral surfaces (silica, bauxite, alumina) [15], [16], fly ash [17], industrial waste [18] and coral limestone [19] have been also used. Recently, the arsenic sorption properties of materials of biological origin have been investigated: chitosan [20], [21], amine modified coconut coir [22]. However, the sorption capacities for these sorbents are usually very low and the maximum sorption capacity rarely exceeds 0.1–0.2 mmol g−1. In the case of chitosan, arsenic sorption capacity is significantly lower than the levels reached with other metal ions [23], [24], [25]. Several processes have been developed to increase the efficiency of sorbents for metal ion uptake especially by ligand grafting. Denizli et al. [26] incorporated dye-ligand onto synthetic polymers to enhance their metal ion sorption properties. Similar modifications have been performed on activated carbon: the impregnation with metal ions significantly enhances arsenic sorption on activated carbon [27], [28], [29]. Min and Hering [30], [31] used the impregnation procedure with ferric and cupric ions to develop alginate derivatives for arsenate and selenate uptake. Ion exchange/precipitation mechanisms are involved in the sequestration of arsenate onto these impregnated sorbents. Usually the recycling is made difficult by low desorption levels or by the non-selective desorption of arsenic: the metal ions used for sorbent impregnation are simultaneously released from the sorbent during the desorption step [32]. A similar procedure was also recently experimented with to improve arsenic sorption using chitosan impregnated with molybdate, in the form of “molybdate impregnated chitosan beads” (MICB) [33]. Indeed, chitosan has a very strong affinity for molybdate (sorption capacity can reach about 1 g of molybdate per g of chitosan under selected experimental conditions) [34]; and molybdate ions are known for their ability to form complexes with arsenate ions. This sorbent is selective over other metal ions or co-ions usually found in industrial effluents (Fe3+, Cu2+, Pb2+, Zn2+, Mn2+, sulfate, chloride), with the exception of phosphate anions, and arsenic can be desorbed using phosphoric acid as the eluent. Molybdate ions sorbed on the biopolymer can be partially released in solution. Recently, a new procedure has been developed for the production of molybdate-impregnated chitosan sorbent [35]: the impregnation was performed by direct coagulation of chitosan beads in a molybdate coagulating bath—to produce “molybdate coagulated chitosan beads” (MCCB)—instead of the sorption procedure which was used in earlier studies. Since arsenic sorption on this chitosan derivative is only efficient in acidic solutions, and due to the release of molybdate the process appears to be more adequate for the treatment of industrial solutions rather than for drinking water treatment.
The present work is focused on the comparison of extraction performance, sorbent stability between the two fabrication procedures and the interpretation of the sorption mechanism using standard procedures, including pH optimization, sorption isotherms and kinetics. X-ray photoelectron spectrometry (XPS) analysis was used for the study of the sorbent and the identification of molybdenum speciation on the sorbent. Desorption studies were also carried out to elucidate the interactions between As and the sorbent. Particular attention was paid to the influence of the oxidation state of As(As(III) and As(V)).
Section snippets
Materials
Chitosan was provided by ABER-Technologies (Brest-France) (Lot No A17G28). Its characteristics were pKa=6.2, number average molecular mass, MWn=125,000 g mol−1, weight average molecular mass, MWw=191,000 g mol−1, and deacetylation percentage=87%. Chitosan gel beads were manufactured using alkaline coagulation/precipitation [34]: the chitosan was dissolved in an acetic acid solution, the viscous solution was added dropwise through a thin nozzle into an alkaline solution (NaOH, 2 M). After 16 h of
Stability of the sorbent
The stability of the sorbent was studied against several kinds of chemical reagents including acids and oxidants, using the percentage of molybdate released in the solution as the experimental parameter. The release of molybdate was also examined when the sorbent was contacted with arsenic solutions. Similar problems of sorbent stability were observed by Rajakovic and Mitrovic [38] on copper-impregnated chemisorption filters (activated carbon, resins): in column systems they correlated the
Conclusions
The sorption of molybdate, which forms complexes with arsenic(V) in solution, on chitosan brings to this biopolymer interesting arsenic sorption properties. Another process for the preparation of this specially tailored sorbent consists in the direct coagulation of acetic acid solutions of chitosan in a molybdate solution. This treatment increases the stability of the sorbent and improves its efficiency for arsenic removal. A simple treatment with phosphoric acid, which allows the labile
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