Arsenate and arsenite adsorption onto iron-coated cork granulates.

There is a growing demand for low-cost, effective adsorbents for arsenic removal from water intended for human consumption in affected rural areas. This work presents a novel adsorbent based on the coating of cork granulates with iron (oxy)hydroxides for the removal of As(III) and As(V) from aqueous matrices. A 26-3 fractional factorial design was used to determine the optimal conditions for the iron coating procedure. The optimal adsorbent was produced by coating low-density cork granulates with iron (oxy)hydroxides precipitated from a 0.05 mol L-1 FeCl3 solution at pH 7, 20 °C temperature and 20 g L-1 S/L ratio, in a single coating cycle. Arsenic adsorption was found to be dependent on pH, with inverse trends for As(III) and As(V). The iron leaching from the adsorbent was also taken into account to select the optimum pH, which was pH 9 for As(III) and pH 3 for As(V). Adsorption kinetics were better described by the pseudo-second-order model for As(III) and the Elovich model for As(V). Equilibrium was reached in 16 h for As(III) at pH 9 and 48 h for As(V) at pH 3. The isotherm models indicated different adsorption behaviours for As(III) and As(V), with better fits by Langmuir and Freundlich models, respectively. The Langmuir maximum adsorption capacity of iron-coated cork adsorbent for As(III) at pH 9 was 4.9 ±â€¯0.3 mg g-1. However, at low equilibrium concentrations, As(V) adsorption was higher than As(III) (e.g. 2.1 ±â€¯0.2 mg g-1 in equilibrium with 0.16 ±â€¯0.03 mg L-1). Speciation studies and XPS analyses indicated that no substantial oxidation of As(III) to As(V) occurred during the adsorption process. The study shows that iron coating can enhance both arsenate and arsenite adsorption capacity of cork materials, leading to an innovative natural adsorbent with high resilience and stability, with possible application in arsenic remediation.

Arsenic removal from water using iron-coated seaweeds.

Arsenic is a semi-metal element that can enter in water bodies and drinking water supplies from natural deposits and from mining, industrial and agricultural practices. The aim of the present work was to propose an alternative process for removing As from water, based on adsorption on a brown seaweed (Sargassum muticum), after a simple and inexpensive treatment: coating with iron-oxy (hydroxides). Adsorption equilibrium and kinetics were studied and modeled in terms of As oxidation state (III and V), pH and initial adsorbate concentration. Maximum adsorption capacities of 4.2 mg/g and 7.3 mg/g were obtained at pH 7 and 20 °C for arsenite and arsenate, respectively. When arsenite was used as adsorbate, experimental evidences pointed to the occurrence of redox reactions involving As(III) oxidation to As(V) and Fe(III) reduction to Fe(II), with As(V) uptake by the adsorbent. The proposed adsorption mechanism was then based on the assumption that arsenate was the adsorbed arsenic species. The most relevant drawback found in the present work was the considerable leaching of iron to the solution. Arsenite removal from a mining-influenced water by adsorption plus precipitation was studied and compared to a traditional process of coagulation/flocculation. Both kinds of treatment provided practically 100% of arsenite removal from the contaminated water, leading at best in 12.9 µg/L As after the adsorption and precipitation assays and 14.2 µg/L after the coagulation/flocculation process.