Sorption of arsenite and arsenate on ferrihydrite: Effect of organic and inorganic ligands

Abstract

We studied the sorption of As(III) and As(V) onto ferrihydrite as affected by pH, nature and concentration of organic [oxalic (OX), malic (MAL), tartaric (TAR), and citric (CIT) acid] and inorganic [phosphate (PO₄), sulphate (SO₄), selenate (SeO₄) and selenite (SeO₃)] ligands, and the sequence of anion addition. The sorption capacity of As(III) was greater than that of As(V) in the range of pH 4.0–11.0. The capability of organic and inorganic ligands in preventing As sorption follows the sequence: SeO₄ ≈ SO₄ < OX < MAL ≈ TAR < CIT < SeO₃ ≪ PO₄. The efficiency of most of the competing ligands in preventing As(III) and As(V) sorption increased by decreasing pH, but PO₄ whose efficiency increased by increasing pH. In acidic systems all the competing ligands inhibited the sorption of As(III) more than As(V), but in alkaline environments As(III) and As(V) seem to be retained with the same strength on the Fe-oxide. Finally, the competing anions prevented As(III) and As(V) sorption more when added before than together or after As(III) or As(V).

Introduction

Arsenic (As) is an element ubiquitous in the environment and is extremely toxic for humans, animals and plants [1], [2]. Arsenic contamination in water is a worldwide problem, because in drinking water it is rapidly and directly sorbed by humans [1], [3], [4], [5]. Developing countries are the most severely affected by the arsenic crisis, but the situation is especially alarming in south and southeast Asia [3], [6]. The seriousness of this problem has led the World Health Organization to describe it as the greatest mass poisoning in human history. An examined 35 million people in Bangladesh and 6 million in West Bengal are at risk [2], [5], [7].

In soils high concentrations of As can originate from different sources, as weathering of rocks or minerals with high As contents, biological activities and anthropogenic activities, such as mining smelter, disposal waste and application of As fertilizers and pesticides.

Many inorganic and organo-arsenical forms are present in natural environments, but the most common are arsenite [As(III)] and arsenate [As(V)]. The predominant As speciation is strongly influenced by the redox potential and pH [5], [7], [8]. Arsenite is 25–60 times more toxic and mobile than As(V), which mainly arise from its state as H₃AsO₃ at pH < 9.0, as compared to the charged species which predominate in a wide pH range (H₂AsO₄⁻ between 2.5 and 7, HAsO₄²⁻ between pH 7.0 and 12.0 [4], [5], [7], [8].

The mobility of As compounds in soil–water–plant systems is affected by sorption/desorption on/from soil components or coprecipitation with metal ions [9], [10], [11], [12], [13]. The importance of oxides (mainly Fe-oxides) in controlling the mobility and concentration of As in natural environments is well known [4], [5], [8], [9], [12], [13], [14]. In particular, As(V) demonstrates a high affinity for most metal hydroxides and clay minerals, whereas As(III) has a strong preference for hydroxides of iron. At circumneutral pH values and higher, As(III) usually adsorbs to a greater extent on ferric hydroxides than As(V) [8], [15], [16].

Many studies on the structures of As(V) adsorption complexes on various Fe oxyhydroxides have been carried out using X-ray adsorption spectroscopy, whereas very few have been focused on As(III) [4], [5], [8], [13], [17], [18], [19]. The formation of various inner-sphere complexes has been suggested as the primary mechanism for the sorption of As(V) on iron oxides [13], [17]. However, both inner-sphere complexes and outer-sphere complexes have been found in the sorption of As(III) on different iron oxides [13], [19]. Ona-Nguema et al. [19] found using EXAFS spectroscopy that As(III) forms bidentate mononuclear edge-sharing and bidentate binuclear corner-sharing on ferrihydrite. A redox reaction between Fe(III)-oxide and As(III) did not occur within 72 h, indicating that the kinetics of the redox reaction between As(III) and Fe(III) is relatively slow [20].

As reported by Inskeep et al. [21] on iron oxides, the sorption capacity of As(III) compares or exceeds that of As(V), the former showing an adsorption envelope centered at pH 8.0, while the latter increases continuously with decreasing pH, but caution should be used in drawing conclusions regarding binding strength from the magnitude of retention.

Numerous studies have been carried out on the sorption of As(III) and As(V) onto Fe or Al oxides [4], [8], [9], [22]. The presence of inorganic and organic ligands affects the sorption of As onto soil minerals and soils by competing for available binding sites and/or reducing the surface charge of the sorbents [8], [10], [12], [23], [24], [25], [26]. The competition in sorption is affected by the affinity of the competing anions for the surfaces of the sorbents, the nature and surface properties of the minerals and soils, the surface coverage and the reaction time.

Competition between As(V) and phosphate has been widely studied [4], [7], [8], [10], [11], [12], [27], [28] but scant attention has been devoted to compare the effect of different inorganic and organic ligands on the adsorption of As(V) and As(III) onto soil components [24], [25], [29]. Low molecular mass organic ligands (particularly aliphatic acids, such as acetic, oxalic, tartaric, succinic, malic, and citric acids) are abundant in the rhizosphere, being continuously released by plant roots and microorganisms ([8], [30] and references there in). Their presence may affect the mobility of As at soil–root interface.

In order to have useful information on the factors which may influence the mobility and potential toxicity of arsenic in natural environments, we carried out a study on the sorption of As(IIII) and As(V) onto ferrihydrite as affected by pH, nature and concentration of organic [oxalic (OX), malic (MAL), tartaric (TAR), and citric (CIT) acid] and inorganic [phosphate (PO₄), sulphate (SO₄), selenate (SeO₄) and selenite (SeO₃)] ligands, surface coverage and the sequence of anion addition.