The use of organic materials as amendments in the remediation of soils contaminated by lead, copper and zinc
The effectiveness of using amendments in metal stabilization was determined, and their potential for remediation of contaminated soil was evaluated. A combined approach of evaluating soil and plant metal availability as well as microbial respiration was used as an indication of effectiveness of metal immobilization. This was done by sorption studies, incubation experiments in contaminated media, and greenhouse pot experiments. Batch sorption studies were conducted to evaluate the ability of bone meal, composts, peat, coir and wood bark to sorb Pb, Cu and Zn. Single sorption carried out over a metal concentration range of 0.1 mmol 1-1 using 0.001M and 0.1M Ca(NO3)2 as background electrolytes showed very high sorption of Pb, Cu and Zn by the amendments, but high background salt led to a reduction in the amount of metal sorbed by all amendments. Of the amendments tested, coir, compost and wood bark were most effective. Sorption was evaluated by applying the Langmuir equation, and maximum sorption values were calculated for all amendments. Theoretical maximum sorption of Pb was 87 mg g-1 (0.42 mmol g-1) by coir and green waste compost; Cu was 30 mg g-1 (0.47 mmol g-1) by green waste and general compost, while maximum Zn sorption was 14 mg g-1 (0.21 mmol g-1) in composts, closely followed by 13 mg g-1 (0.19 mmol g-1) in coir. Sorption mixed metal solutions of Pb, Cu and Zn was evaluated in a background salt of 0.001M Ca(NO3)2 only, either at equimolar concentrations of 0.1 mmol 1-1, equimolar concentrations of 1 mmol 1-1, or combinations of metals at either concentration. Metal sorption was reduced in the presence of other metals when compared with sorption in single metal solution. Pb sorption in equimolar solution of 0.1 mmol 1-1 was approximately 50% of that in single solution, Cu was 35%, while Zn was 40% if wood bark was not considered (wood bark sorption of Zn in low equimolar metal was not different from that in single Zn solution). The effect of metals on microbial respiration was evaluated in metal spiked amendments over a ten week period using metal solutions of Pb, Cu and Zn as contaminants. Pb, Cu and Zn toxicity led to an inhibition in CO2 evolved in all amendments, as addition of any amount of Pb, Cu or Zn led to a decrease I amount of CO2 evolved when compared with the non-contaminated amendments. The effect of increasing metal toxicity on CO2 evolution was reflected best in coir, where inhibition increased with an increase I metal concentration. In other contaminated amendments however, the inhibition was highly pronounced once there was metal contamination regardless of the metal concentration.