Chemical and organic immobilization treatments for reducing phytoavailability of heavy metals in copper-mine tailings

This study was conducted to evaluate the efficiency of diammonium phosphate (DAP), agricultural limestone (lime), and green-waste compost mixed with 30% treated sewage sludge (GCS) applied alone or in combination as chemical immobilization treatment using tomato as a test crop. Mine waste was collected from an abandoned copper-mine tailing site at Mynydd Parys, Anglesey (UK). Lime was applied at the rate of CaCO₃ equivalent (CCE, pH = 7), DAP at the rate of 23 g P per kg substrate, and 10% by weight, GCS as sole application. Half rate of each amendment was also tested as a combined treatment and an untreated substrate (control). Plant-available metals (Cd, Cu, Fe, Pb, and Zn) were measured in substrate with conventional diethylenetriaminepentaacetic acid (DTPA) and sequential Ca(NO₃)₂ extraction. Plant-dry biomass yield was significantly (p < 0.001) increased by the combined application of all the three amendments while sole application of DAP reduced yield by 4-fold compared to unamended soil probably due to P toxicity. Addition of lime reduced the DTPA-extractable Cu, Fe, and Zn by 75%, 81%, and 85%, respectively, while Pb availability was reduced by 88% in combined DAP + lime + GCS treatment compared to control. The extraction capacity of DTPA was higher than that of Ca(NO₃)₂ by 3-fold for Cu and Fe, 8-fold for Pb, and 2-fold for Zn. The leaf-tissue concentrations of Cu and Fe were reduced by 77% and 83% in the lime + GCS amendment, respectively, while both Pb and Zn were reduced by 89% and 33%, respectively, in substrate treated with the combined application of all three amendments. These results suggest that alkaline amendments (both lime and GCS) were effective in reducing the phytoavailability of Cu, Fe, and Zn while DAP mixed with either GCS or lime was effective in reducing Pb availability.