Concurrent manganese reduction and adsorption in aquifer sediments and their role in manganese mobilization: Field evidence and experimental study

摘要

Recharge of effluents to aquifers is commonly considered an efficient tertiary treatment process during which the dissolved organic matter is oxidized and trace metals are adsorbed onto the aquifer sediments. During the past 25 years, the Shafdan Plant, which treats the effluent from the Tel Aviv metropolitan area,recharged about 1,700 million cubic meters of pretreated effluents into the Coastal Aquifer of Israel. About seven years ago, high concentrations of dissolved Mn (100-2000 ppb) appeared in the reclaimed water, even though the Mn concentration in the recharged effluent remained low (30 ppb). We show that consumption of dissolved oxygen by nitrification and oxidation of organic matter just after recharge facilitate suboxic reduction of sedimentary Mn to the mobile Mn2+ species. The reduced Mn is retarded in the aquifer by adsorption onto the aquifer sediments, thereby defining a Mn front. Within the front's inner boundary Mn exchange sites are occupied and in equilibrium with the modified effluents. The front propagates outward much slower than the water, and its speed is controlled by the kinetics of Mn-oxides reduction and the time it takes to equilibrate all Mn exchange sites. The retardation model was verified by laboratory simulations in which aquifer solutions with variable Mn content were pumped through columns packed with clayey sand sediments from the Shafdan area. These experiments enabled estimating the sediments' Mn adsorption capacity and corroborated the prolonged retardation of Mn observed in the field.