Lead immobilization in simulated polluted soil by Douglas fir biochar-supported phosphate

This study compared the lead (Pb²⁺) immobilization efficacy of biochar-supported phosphate to conventional in-situ heavy metal immobilization methods (with lime, neat biochar and phosphate). The biochar-supported phosphate was obtained by treating Douglas fir biochar (BC) with anhydrous calcium chloride and potassium dihydrogen phosphate. The amount of Pb²⁺ immobilized was determined by comparing the concentration of ammonium nitrate extractable Pb²⁺ lead from lead-spiked soil (without amendment) to that of a 30 d incubation with (a) lead-spiked soil plus 5% (wt./wt.) biochar supported-phosphate, (b) lead-spiked soil plus 5% (wt./wt.) untreated Douglas fir biochar, (c) lead-spiked soil plus 5% (w/w) lime and (d) lead-spiked soil plus 5% (wt./wt.) potassium dihydrogen phosphate. The control (lead-spiked soil without amendment) produced the largest quantity (96.08 ± 9.22 mg L^-1) of NH₄NO₃-extractable Pb²⁺, while lead-spiked soil treated with 5% (wt./wt.) biochar-supported phosphate resulted in the lowest quantity of NH₄NO₃ extractable Pb²⁺ (0.3 ± 0.2 mg L^-1). The mechanism for immobilization of Pb²⁺ by BP occurs at pH < 7 through dissolution of hydroxyapatite embedded in BP during modification, followed by precipitation of insoluble Pb₁₀(PO₄)₆(OH)₂. The residual lead fraction in the lead-spiked soil increased by 20.9% following amendment with BP. These results indicate that biochar-supported phosphate is a candidate to reduce lead mobility (bioavailability) in polluted soil. This amendment may lower Pb²⁺ uptake into plants while minimizing the potential for water contamination due to Pb²⁺mobility.