The effect of pH on the uptake of zinc, copper and nickel from chloride solutions by an uncontaminated sewage sludge

The uptake of zinc, copper and nickel by an uncontaminated sewage sludge was studied as a function of added metal concentration and of pH. The quantity of metal taken up at constant pH increased with added metal concentration up to an apparent saturation value in some cases, but the data did not fit Langmuir or Freundlich adsorption isotherms closely.

The proportion of total metal in the solution phase at a given pH in these uptake experiments was generally larger than that in the solution phase in comparable experiments involving the release of the metals into solution from metal-contaminated sludges, to which the metals had been added during sewage treatment.

Differences between the cation exchange capacity of uncontaminated and of metal-loaded sludges were used to calculate the amount of metal held on cation exchange sites by complexation in the metal-loaded sludges. The amount of exchangeable metal was determined by displacement with barium. The proportion of total metal held on the sites in exchangeable and complexed forms in metal-loaded sludges is 52% of zinc, 71% of copper and 92% of nickel.