Phosphates for Pb Immobilization in Soils: A Review

Abstract

In its soluble ionic forms, lead ( $${\rm Pb}$$ ) is a toxic element occurring in water and soil mainly as the result of human activities. The bioavailability of lead ions can be decreased by complexation with various materials to decrease their toxicity. $${\rm Pb}$$ chemical immobilization using phosphate addition is a widely accepted technique for immobilizing $${\rm Pb}$$ from aqueous solutions and contaminated soils. The application of different P amendments causes $${\rm Pb}$$ in soils to shift from forms with high availability to the most strongly bound $${\rm Pb}$$ fractions. The increase of $${\rm Pb}$$ in the residual or insoluble fraction results from formation of pyromorphite $${\rm Pb}_5 ({\rm PO}_4)_3{\rm X}$$ , where $${\rm X} = {\rm F}$$ , $${\rm Cl}$$ , $${\rm Br}$$ , $${\rm OH}$$ , and the most stable environmental $${\rm Pb}$$ compounds under a wide range of $${\rm pH}$$ and $${\rm Eh}$$ natural conditions. Accidental pyromorphite ingestion does not yield bioavailable lead, because pyromorphite is insoluble in the intestinal tract. Numerous natural and synthetic phosphate materials have been used to immobilze $${\rm Pb}$$ : apatite and hydroxyapatite, biological apatite, rock phosphate, soluble phosphate fertilizers such as monoammonium phosphate (MAP), diammonium phosphate (DAP), phosphoric acid, biosolids rich in P, phosphatic clay, and mixtures. The identification of pyromorphite in phosphate-amended soils has been carried out by different nondestructive techniques such as x-ray diffraction (XRD), scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM-EDX), x-ray absorption fine structure (XSFS), transmission electron microscopy (TEM), and electron microprobe analysis (EMPA). The effectiveness of in situ $${\rm Pb}$$ immobilization has also been evaluated by sequential extraction (SE), by the toxicity leaching procedure (TCLP), and by a physiologically-based extraction procedure simulating metal ingestion and gastrointestinal bioavailability to humans (PBET). Efficient $${\rm Pb}$$ immobilization using P amendments requires increasing the solubility of the phosphate phase, and of the $${\rm Pb}$$ species phase, by inducing acid conditions. Although phosphorus addition seems to be highly effective, excess P in soil and its potential effect on the eutrophication of surface water, and the possibility of arsenic enhanced leaching remains a concern. The use of mixed treatments may be a useful strategy to improve their effectiveness in reducing lead phyto- and bioavailability.