Feng, XH; Zu, YQ; Tan, WF; Liu, F
Oxidation of As(Ill) by three types of manganese oxides and
the effects of pH, ion strength and tartaric acid on the oxidation were investigated by means of
chemical analysis, equilibrium redox, X-ray diffraction (XRD) and transmission electron
microscopy (TEM). Three synthesized Mn oxide minerals, bimessite, cryptomelane, and hausmannite,
which widely occur in soil and sediments, could actively oxidize As(III) to As(V). However, their
ability in As(Ill)-oxidation varied greatly depending on their structure, composition and surface
properties. Tunnel structured cryptomelane exhibited the highest ability of As (III) oxidation,
followed by the layer structured birnessite and the lower oxide hausmannite. The maximum amount
of As (V) produced by the oxidation was in the order (mmol/kg) of cryptomelane (824.2) >
birnessite (480.4) > hausmannite (117.9). As pH increased from the very low value(pH 2.5), the
amount of As(Ill) oxidized by the tested Mn oxides was firstly decreased, then negatively peaked
in pH 3.0-6.5, and eventually increased remarkably. Oxidation of As(Ill) by the Mn oxides had a
buffering effects on the pH variation in the solution. It is proposed that the oxidative reaction
processes between As (III) and bimessite(or cryptomelane) are as follows: (1) at lower pH
condition: (MnO2)(x) + H3AsO3 + 0.5H(+) = 0.5H(2)AsO(4)(-) + 0.5HAsO(4)(2-) +Mn2++ (MnO2)(x-1) +
H2O; (2) at higher pH condition: (MnO2)(x) + H3AsO, = 0.5H(2)AsO(4)(-) + 0.5HAsO(4)(2-) + 1.5H(+)
+ (MnO2)(x-1)center dot MnO. With increase of ion strength, the As(HI) oxidized by birnessite and
cryptomelane decreased and was negatively correlated with ion strength. However, ion strength had
little influence on As (III) oxidation by the hausmannite. The presence of tartaric acid promoted
oxidation of As(Ill) by bimessite. As for cryptomelane and hausmannite, the same effect was
observed when the concentration of tartaric acid was below 4 mmol/L, otherwise the oxidized As
(Ill) decreased. These findings are of great significance in improving our understanding of As
geochemical cycling and controlling As contamination.