Fig. 6. Relationship between pH and removal efficiency of heavy metals at different sludge solids concentrations: (a) Zn, (b) Cu, (c) Cr, (d) Pb, (e) Ni, (f) Al. Symbols: •, 20 g L^-1; , 50 g L^-1; , 90 g L^-1; , 130 g L^-1.

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