Removal of heavy metals with sequential sludge washing techniques using saponin: optimization conditions, kinetics, removal effectiveness, binding intensity, mobility and mechanism

Testing of sequential sludge washing in triplicate using typical biosurfactant saponin was conducted to remove heavy metals. The aim of the testing was to improve efficiency and reduce mobility of heavy metals. Different washing conditions such as concentration, contact time, liquid/solid ratio and pH were optimized. The heavy metal desorption performances were investigated strictly and the experimental data fitted the pseudo-second order equation very well. A sequential washing step and heavy metal mobilization was investigated, and the results demonstrated that the total content and fractions of the heavy metals reached remarkable extraction efficiencies, after three washings, Cr obtained the highest extraction efficiency (65.00%), and the Pb exchangeable fraction acquired the highest extraction efficiency (85.00%). Also, the heavy metal binding intensity and mobility were investigated, and the results indicated that the heavy metal binding intensity (IR) increased after washing, conversely, the mobility (MF) decreased, due to the weak fractions extracted by saponin. The toxicity characteristic leaching procedure (TCLP) indicated that heavy metals have more stable fractions in the after washing sludge. SEM, FTIR and XRD revealed that mechanism of saponin enhanced heavy metal extraction from the sludge, elucidated that carboxyl and hydroxyl were the main functional groups. This study indicated that the saponin may be a replacement for organic acids, inorganic acids and less environmentally friendly refractory chelators in the extraction of heavy metals from the sludge.