Metal removal effectiveness, fractions, and binding intensity in the sludgeduring the multiple washing steps using the combined rhamnolipid and saponin

PurposeThis study aims to investigate the combined rhamnolipid and saponin enhancing metals removal from the sludge.Materials and methodsThe combined rhamnolipid and saponin was used to enhance metals removal from sludge in the multiple washing steps.Results and discussionHigh content of metals in the sludge led to the obstacle of sludge land application and nutrients recycling. The combined rhamnolipid and saponin enhancing metal removal from sludge was investigated in this study. Results indicated that metals removal efficiencies increased with mass ratio raising. After the multiple washing steps, Cu, Zn, Cr, Pb, Ni, and Mn removal efficiencies were 62%, 74%, 60%, 15%, 68%, and 64%, respectively. Meanwhile, metals fractions variations indicated that Pb exchangeable fraction, Mn reducible and oxidizable fractions, and Cu residual fraction obtained the highest removal efficiencies. Metal binding intensity increased, and metal mobility order was Zn>Cr=Cu>Ni=Pb>Mn after the multiple washing steps. Preservation of fertilizing characteristics demonstrated that sludge would still be suitable for beneficial use in agriculture.ConclusionsCombined rhamnolipid and saponin could be a replacement for inorganic, organic acids and less environmentally friendly refractory chelators. Copper, Zn, Cr, Ni, and Mn obtained better removal efficiencies than removal efficiency of Pb (15%), which were 62%, 74%, 60%, 68%, and 64%, respectively.