Adsorption-Desorption Behavior of Arsenate Using Single and Binary Iron-Modified Biochars: Thermodynamics and Redox Transformation.

Arsenic (As) is a dangerous contaminant in drinking water which displays cogent health risks to humans. Effective clean-up approaches must be developed. However, the knowledge of adsorption-desorption behavior of As on modified biochars is limited. In this study, the adsorption-desorption behavior of arsenate (As(V)) by single iron (Fe) and binary zirconium-iron (Zr-Fe)-modified biosolid biochars (BSBC) was investigated. For this purpose, BSBC was modified using Fe-chips (FeBSBC), Fe-salt (FeCl(3)BSBC), and Zr-Fe-salt (Zr-FeCl(3)BSBC) to determine the adsorption-desorption behavior of As(V) using a range of techniques. X-ray photoelectron spectroscopy results revealed the partial reduction of pentavalent As(V) to the more toxic trivalent As(III) form by FeCl(3)BSBC and Zr-FeCl(3)BSBC, which was not observed with FeBSBC. The Langmuir maximum As(V) adsorption capacities were achieved as 27.4, 29.77, and 67.28 mg/g when treated with FeBSBC (at pH 5), FeCl(3)BSBC (at pH 5), and Zr-FeCl(3)BSBC (at pH 6), respectively, using 2 g/L biochar density and 22 ± 0.5 °C. Co-existing anions reduced the As(V) removal efficiency in the order PO(4) (3-) > CO(3) (2-) > SO(4) (2-) > Cl(-) > NO(3) (-), although no significant inhibitory effects were observed with cations like Na(+), K(+), Mg(2+), Ca(2+), and Al(3+). The positive correlation of As(V) adsorption capacity with temperature demonstrated that the endothermic process and the negative value of Gibbs free energy increased (-14.95 to -12.47 kJ/mol) with increasing temperature (277 to 313 K), indicating spontaneous reactions. Desorption and regeneration showed that recycled Fe-chips, Fe-salt, and Zr-Fe-salt-coated biochars can be utilized for the effective removal of As(V) up to six-repeated cycles.