Bonemeal, coir, compost, green waste compost, peat and wood bark all potentially could be used as amendments to remediate heavy metal contamination in soils. Their ability to sorb Pb, Cu and Zn was evaluated in the laboratory, using metal solutions ranging from 0 to 5 mmol/L as contaminants. The effects on sorption of metal concentration, background salt concentration and metal competition were evaluated. Single metal sorption by the six amendments was significantly different at metal concentrations of 1.5–5 mmol/L, with green waste compost, coir, compost and wood bark having the highest capacities to adsorb Pb, Cu and Zn. Langmuir sorption maxima were approximately 87 mg Pb/g (coir and green waste compost), 30 mg Cu/g (compost and green waste compost) and 13 mg Zn/g (compost and green waste compost) (equivalent to approx. 0.5 mmol/g of Pb and Cu, and 0.2 mmol/g Zn), all in a background solution of 0.001 M Ca(NO3)2. A higher background salt concentration and a combination of all three metals led to significant reduction in the amounts of Pb, Cu and Zn sorbed by all the amendments tested. Competing heavy metal cations in solution decreased Pb sorption to about 50–60% of that from a solution containing Pb alone; Cu sorption was reduced to about 30–40%; the effect of competition on Zn sorption was variable. Overall, in both single metal and competitive sorption, the order of strength of binding was Pb>Cu>>Zn.

Green waste compost, peat, coir and wood bark were applied to metal-contaminated mine waste at the rates of 1%, 10% and 20% on dry weight basis, and perennial ryegrass grown over a period of 6 weeks. Addition of amendments led to increased biomass yield in all soils when compared with the non-amended soil. EDTA extractable Pb, Cu and Zn was significantly reduced in amended soil, while leaf and root metal concentrations were also significantly reduced by the application of amendments, especially at applied rates of 10% and 20%. Coir, green waste compost and wood bark stood out as amendments which were consistent in reducing soil extractable and plant tissue Pb, Cu and Zn; while peat rates above 10% enhanced solubility of Cu and Zn because of a lowering of the soil pH.

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Incubation experiment was carried out to evaluate and compare effects of some selected amendments on adsorption of heavy metals in soils and their chemical properties. The experiment was CRD consisting of twelve treatment combinations with three replications. Soil samples were collected from the vicinity of Michael Okpara University of Agriculture and amended with organic amendments at the rate 20 ton/ha and heavy metals at the rate of 60 mg/kg, thoroughly mixed and wetted daily throughout the incubation period of 30 days. 0.05M EDTA solution was used to extract heavy metals. EDTA solution was filtered from soil with Whatman No1 filter paper. The soil was analyzed for chemical properties before and after experiment using standard procedures. Higher amount of heavy metals was extracted from the control soils than the amended soils indicating retention of heavy metals by the amendments. All the amendments were significantly (P<0.05) different in adsorption of copper with poultry manure having significantly (P<0.05) the highest adsorption capacity while the control had the least. Cocoa pod was significantly (P<0.05) higher on adsorption of lead with cow dung and poultry manure not being significantly (P>0.05) different from each other. Cow dung had the highest (P<0.05) on adsorption of zinc. Heavy metals combined with amendments had significant (P<0.05) effect on soil chemical properties. Most chemical properties were significantly (P<0.05) higher in soils treated with both amendments and heavy metals than soils treated with only heavy metals. Poultry manure, cocoa pod and cow dung appeared to have greater potential in removing heavy metals from soils.

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