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 Pb-10(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 Pb(2+)mobility.

Automated summary

This study compared the effectiveness of different methods in immobilizing lead in soil and found that biochar-supported phosphate showed the best results in reducing the mobility of lead in contaminated soil. The concentration of ammonium nitrate extractable lead in the amended soil was significantly lower, indicating that this method can reduce lead uptake by plants and minimize the potential for water contamination.