摘要:

Calcium phosphate is widely used for the remediation of lead-contaminated sites, where calcium/lead phosphate coprecipitates (Ca/Pb CoPs) form. This research investigated such coprecipitation with model low-molecular-weight organics (LMWOs), produced in the rhizosphere with representative functional groups, which were found to regulate both the composition (Ca/Pb and C/Pb ratios) and stability (aggregation and transformation) of Ca/Pb CoPs. The strong complexation ability of –SH in l-cysteine with aqueous Ca2+/Pb2+ ions inhibited coprecipitation to a great extent. Meanwhile, coprecipitates with lysine containing both –NH2 and –COOH had a higher Ca/Pb ratio than those with citrate containing only –COOH, probably due to the elevated local supersaturation caused by both –NH2 and –COOH in lysine that attracted phosphate ions and cations, promoting Ca doping in CoPs. Also, the strong binding of both –NH2 and –COOH with coprecipitates resulted in a higher C/Pb ratio for the CoPs with lysine than citrate. The oriented aggregation of nano-CoPs formed needle-shaped hydroxylpyromorphite crystals without organics. Unexpectedly, lysine/citrate disrupted and l-cysteine promoted such oriented aggregation, resulting in inhibited and promoted crystallinity, respectively. This study provided new mechanistic insights on LMWO effects on Ca/Pb CoP formation and their stability and can help understand Pb speciation and availability in the rhizosphere.

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