A novel arsenic immobilization strategy via a two-step process: Arsenic concentration from dilute solution using schwertmannite and immobilization in Ca-Fe-AsO compounds.

Acid mine drainage (AMD) with toxic arsenic (As) is commonly generated from the tailings storage facilities (TSFs) of sulfide mines due to the presence of As-bearing sulfide minerals (e.g., arsenopyrite, realgar, orpiment, etc.). To suppress As contamination to the nearby environments, As immobilization by Ca-Fe-AsO compounds is considered one of the most promising techniques; however, this technique is only applicable when As concentration is high enough (>1 g/L). To immobilize As from wastewater with low As concentration (~10 mg/L), this study investigated a two-step process consisting of concentration of dilute As solution by sorption/desorption using schwertmannite (FeO(OH)(SO); where (1 ≤ x ≤ 1.75)) and formation of Ca-Fe-AsO compounds. Arsenic sorption tests indicated that As(V) was well adsorbed onto schwertmannite at pH 3 (Q = 116.3 mg/g), but its sorption was limited at pH 13 (Q = 16.1 mg/g). A dilute As solution (~11.2 mg/L As) could be concentrated by sorption with large volume of dilute As solution at pH 3 followed by desorption with small volume of eluent of which pH was 13. The formation of Ca-Fe-AsO compounds from As concentrate solution (2 g/L As(V)) was strongly affected by temperature and pH. At low temperature (25-50 °C), amorphous ferric arsenate was formed, while at high temperature (95 °C), yukonite (CaFe(AsO)(OH)·xHO; where x = 2-11) and johnbaumite (Ca(AsO)OH) were formed at pH 8 and 12, respectively. Among the synthesized products, johnbaumite showed strongest As retention ability even under acidic (pH < 2) and alkaline (pH > 9) conditions.