Arsenic desorption from ferric and manganese binary oxide by competitive anions: significance of pH

Ferric and manganese binary oxide (FMBO) has been used to remediate an arsenic (As)-polluted river in China, but insufficient data was available to (1) evaluate its effects on the environment and (2) propose a feasible strategy of addressing the arsenic-bearing FMBO. The desorption behavior of arsenic in the presence of four competitive anions (i.e., phosphate, silicate, sulfate, and bicarbonate) at different concentrations was investigated with pH ranging from 3 to 11. The presence of these anions promoted the desorption of arsenic from arsenic-bearing FMBO and followed the sequence of phosphate > silicate > sulfate approximately equal to bicarbonate across a wide pH range. Desorption of arsenate (As[V]) was more significant than that of arsenite (As[III]). Sequence dissolution of arsenic-bearing FMBO particles by NH4-oxalate/oxalic acid and hydrochloric acid were performed. The laboratory results indicated that As(III) was primarily occluded in the crystalline parts of the FMBO. The desorption behavior of arsenic could be described by kinetic models using the Elovich and power function equations under different pH conditions and was related to the adsorption of phosphate and silicate. pH played an important role in the desorption of arsenic, because of its effects on the species distribution of anions, surface charge of the arsenic-bearing FMBO, and subsequent electrostatic forces between anions and FMBO.