Enhancement of active thin-layer capping with natural zeolite to simultaneously inhibit nutrient and heavy metal release from sediments

This study strove to enhance the efficiency of sediment capping using active thin-layer capping (ATC) with natural zeolite to simultaneously prevent the release of nutrients (PO43- and NH4+) and heavy metals (Pb2+, Cd2+, Mn2+, and Zn2+) from polluted reservoir sediment. A series of lab-scale column experiments were performed in different conditions by varying capping thickness, ionic strength of Na+ and Ca2+, and the redox environment in overlying water. Results showed that the release of nutrients and heavy metals could be prevented effectively using ATC with zeolite. There was no significant difference in the efficiency of nutrient (except PO43-) and heavy metals inhibition among three capping thicknesses. The greatest efficiency was obtained with 2 cm thickness, which the inhibition rates of NH4+, PO43-, Cd2+, Pb2+, Mn2+, and Zn2+ were 91.9%, 57.7%, 35.7%, 85.7%, 65.6%, and 57.8%, respectively. And, the capping thickness of 1 cm was sufficient to simultaneously prevent the release of nutrients and heavy metals from sediment. The presence of high Ca2+ concentration in the overlying water promoted the release of nutrients and heavy metals. However, aerobic environment could enhance the efficiency of inhibition of nutrient and heavy metal (except Cd2+) release from sediment. Additionally, fluxes of NH4+, PO43-, and Pb2+ in aerobic conditions with capping were significantly lower than those in anaerobic conditions without capping in an in situ capping simulated experiment. Therefore, the efficiency of inhibiting the release of nutrients and heavy metals from sediments could be enhanced by ATC with natural zeolite through a series of measures, including increasing suitable capping thickness, creating an aerobic environment, and reducing the content of Ca2+ in overlying water.