Mobility of arsenic in saturated, laboratory test sediments under varying pH conditions

The interactions between arsenic and the various reactive sites within a soil matrix are poorly understood and experimental studies of arsenic movement in sediments are extremely limited. A series of laboratory experiments studying the mobility of arsenate (As5+), the oxic arsenic oxyanion, in a saturated laboratory test sediment under varying pH conditions are presented here.A selective sequential extraction showed that very little arsenic is retained in the weakly sorbed or carbonate fractions of the sediment. Approximately one-quarter of the arsenic is retained in the strongly sorbed fraction. This is interpreted as non-specifically sorbed arsenic. 61% of the arsenic is retained by manganese oxides, amorphous iron oxides and clay minerals. This is interpreted as ligand exchange of the arsenate oxyanion on the sediment surfaces. The remainder of the arsenic is associated with the residual fraction, indicating strong bonding in the sediment mineral lattice.The leaching column tests showed that arsenic is most stable under neutral conditions (pH 7). Under acidic (pH 4) or alkaline (pH 10) conditions, the arsenic becomes mobile with greater mobility observed under acidic conditions. It is concluded that acidic and alkaline conditions can result in a change in anionic charge which causes release of electrostatically bound arsenic.