Iron oxide coated sand (IOCS) is a potential adsorbent for metallic ions, e.g. arsenic, lead, manganese etc., however the effect of process conditions and the mechanism of its adsorption of Mn(II) have not been thoroughly investigated. This study determined the capacity, rate, mechanisms involved and the effect of process conditions on the adsorption of Mn(II) onto IOCS using laboratory scale batch reactors with modeled water. Alkalinity and pH values of 8 and higher markedly affected the solubility of Mn(II) that is governed by manganese carbonate; solubility is very limited (1–2 mg/L or lower) even at low alkalinity (60 ppm).
The IOCS demonstrated an increasing adsorption capacity ('K' values: 4.73–147) of Mn(II) from pH values 6 to 8. For the initial short term, comparable adsorption capacity was found at pH 6 under both oxic and anoxic conditions. This indicates that no significant quantities of adsorbed Mn(II) were oxidized at pH 6 to form extra capacity within that period.
Kinetic studies using the linear driving force, Lagergren and potential driving second order kinetic (PDSOK) models revealed that the rate of manganese(II) adsorption onto aggregate IOCS declines after the initial phase likely due to the saturation of easily accessible adsorption capacities on grain surface and/or pH drop in the pores of the IOCS grains due to Mn(II) adsorption. The changing adsorption rate constants prevented the equilibrium concentration being predicted with the applied models.