Sorption behavior of heavy metals on poorly crystalline manganese oxides: roles of water conditions and light

The objective of this study was to determine the effects of solution properties and light on the metal uptake and release in a nanosized, poorly crystalline manganese oxide (delta-MnO2) system. The results from synthetic water matrices revealed that the aggregation state was strongly affected by ionic strength, Ca2+, and humic acid, and the particle aggregation subsequently changed the ability of delta-MnO2 to adsorb and sequester heavy metal ions (Cu(II)). The extent of Cu(II) uptake onto delta-MnO2 exhibited a negative correlation with the attachment efficiency value, which suggested that a lower sorption capacity could be achieved under aggregation-inducing conditions. Upon exposure to light, the adsorbed Cu(II) was released from the delta-MnO2 surface via photoinduced dissolution of MnO2. The concentration of Cu(II) desorbed was substantially higher when the humic acid was present together with Ca2+. The present investigation enables us to better understand the adsorption-desorption processes of heavy metals occurring at the MnO2-solution interface in response to common environmental stimuli.