Leaching behavior of major and trace elements from sludge deposits of a French vertical flow constructed wetland

Surface sludge deposits were collected from a French Vertical Flow Constructed Wetland (French VFCW) sewage treatment plant. The objectives were to characterize the retention of major elements and trace metals within the sludge deposits particles under regular operating conditions, and the influence of extreme pH conditions on their potential release which may occur in situations when the plant malfunctions or after land application of the dredged sludge. A sequential extraction protocol was first used to assess the distribution of the elements within the sludge deposits. Results showed that most of Cu and Pb were associated to organic matter within the oxidizable fraction. Zn, Ni and Cd were distributed in several fractions, notably bound to Fe-Mn oxides and associated to organic matter. Cr was analyzed mostly in the residual fraction. Aliquot fractions of sludge deposits were also submitted to Acid and Base Neutralization Capacity tests (ANC-BNC) where the samples were suspended into acidic or alkaline aqueous solutions, and the solutions analyzed after 48 h contact time. Results showed a pH-dependent leaching profile for all monitored elements. The role of organic matter was observed for almost all metals. It was particularly dominant for Cu which was leached more extensively under alkaline than acidic conditions. Since Cu is not an amphoteric element, this leaching pattern was attributed to the leaching of organic matter which followed a similar pH-dependent profile than Cu. Spectrometric indices were used to characterize soluble organic compounds. Results showed that complex and humified dissolved organic compounds were mostly released under alkaline conditions.