Phosphate stabilization contaminated soil

The use of phosphate has been widely evaluated and subjected to field trials for Pb-contaminated soils. Most treatment systems involve excavation, pug milling of the soil with the stabilization agent, and either replacement or landfill disposal. Occasionally, for larger sites and deeper contamination, in situ mixing with large augers is used. 

Phosphate is widely used as a chemical stabilization agent for MSW combustion residues in Japan and North America and is under consideration for use in parts of Europe. The application of this technology to MSW ashes generally parallels its application to contaminated soils. Metal phosphates (notably Cd, Cu, Pb and Zn) frequently have wide pH distribution, pH-pE predominance, and redox stability within complex ash pore water systems. Stabilization mechanisms identified in other contaminated systems (e.g., soils), involving a combination of sorption, heterogeneous nucleation, and surface precipitation, or solution-phase precipitation are generally observed in ash systems. 

Xenidis, A., Stouraiti, C. Paspaliaris, I. 1999. Stabilization of oxidic tailings and contaminated soils by monocalcium phosphate monohydrate addition the case of Montevecchio (Sardinia, Italy). Journal of Soil Contamination, 8, 681-697. 

A. Wagh, S. Jeong, D. Singh, R. Strain, H. No, and J. Wescott, Stabilization of contaminated soil and wastewater with chemically bonded phosphate ceramics, Proceedings of the Waste Management Annual Meeting, WM 97, eds. R. Post and M. Wacks, Tucson, AZ, 1997. 

Lead immobilization by phosphates arises from the low solubility of pyromorphite (Pb5(P04)3Cl). As stated, lead members of the apatite group minerals are much less soluble than their calcium congeners. Nriagu [39], as early as 1974, proposed the removal of lead from wastewaters and the stabilization of lead in contaminated soils and sediments by reaction with phosphate ions to precipitate pyromorphite. 

An additional benefit of the dissolution-reprecipitation process is that, typically, the entrapped metal becomes nonbioavailable. For example, bioavailability of Cd, Pb, Cu, Zn, and As to vegetation in contaminated soils was observed to be significantly reduced by addition of apatite (Laperche et al., 1996). Presumably, the cations substitute for Ca sites, whereas As substitutes as an oxyanion in to the anion sites. Not only did apatite addition reduce the Pb content in sudax shoot tissue from 170 to 3 mg kg the accumulated Pb in the plant root was stabilized by formation of pyromorphite (Pb5(P04)3Cl) (Laperche et al., 1996). Consequently, phosphate soil amendmenls have received considerable attention for contaminant immobi-... 

The Metals Treatment Technology (MTT ) is a chemical fixation process that stabilizes heavy metals in soils, slndges, and sediments. The process nses bnffered phosphate componnds to convert heavy metals into insolnble metallic salts. The process chemicals may be applied to contaminated media in situ or ex situ. 

Apatite, a natural calcium fluoride phosphate, can adsorb low to moderate levels of dissolved metals from soils, groundwater, and waste streams. Metals naturally chemically bind to the apatite, forming extremely stable phosphate phases of metal-substituted apatite minerals. This natural process is used by UFA Ventures, Inc., and is called phosphate-induced metals stabilization (PIMS). The PIMS material can by used in a packed bed, mixed with the contaminated media, or used as a permeable barrier. The material may be left in place, disposed of, or reused. It requires no further treatment or stabilization. Research is currently being conducted on using apatite to remediate soil and groundwater contaminated with heavy metals, and the technology may also be applicable to radionuclides. The technology is not yet commercially available. 

Phosphate-induced metals stabilization can be used for the remediation of metals in mixed waste streams concurrently with other remediation technologies such as vapor stripping or bioremediation of organics. Using apatite to treat soils contaminated with lead, cadmium, and/or zinc can significantly reduce the amount of metals leached from the soil. The amount of apatite needed for treatment is less than 1% by weight. The reaction between metals and apatite is immediate, and the apatite can be heavily loaded with metals. 

Raicevic, S. 2001. Remediation of uranium contaminated water and soil using phosphate-induced metal stabilization (PIMS). Hemijska Industrija, 55, 277-280. 

These examples illustrate that biomolecules may act as catalysts in soils to alter the structure of organic contaminants. The exact nature of the reaction may be modified by interaction of the biocatalyst with soil colloids. It is also possible that the catalytic reaction requires a specific mineral-biomolecule combination. Mortland (1984) demonstrated that py ridoxal-5 -phosphate (PLP) catalyzes glutamic acid deamination at 20 °C in the presence of copper-substituted smectite. The proposed pathway for deamination involved formation ofa Schiff base between PLP and glutamic acid, followed by complexation with Cu2+ on the clay surface. Substituted Cu2+ stabilized the Schiff base by chelation of the carboxylate, imine nitrogen, and the phenolic oxygen. In this case, catalysis required combination of the biomolecule with a specific metal-substituted clay. 

The examples given above and the work done in the last 10 years on phosphate washing demonstrate that phosphates are very powerful stabilizers of inorganic hazardous contaminants. Phosphate washing is very economical, and once the treated waste is disposed, because phosphates are common fertilizer components, the soil becomes enriched with phosphates. Hence, the entire disposal process is ecologically sound. 

Soils contaminated with Pb may also be remediated by iron-based nanoparticles. Iron phosphate (vivianite) nanoparticles stabilized with CMC have been reported to reduce the toxicity-characteristic leaching procedure (TCLP) and physiologically based extraction test (PBET) bioaccessibility in calcareous, neutral, and acidic soils (84). While phosphate addition has been known since at least 1993 to immobilize Pb(II) in soils, phosphate addition can cause its own problems in that it easily leaches into surface and groundwaters, where it causes problems related to excessive nutrient input. CMC-vivianite nanoparticles release 50% less phosphate into the environment than more traditional phosphate soil amendments, partly because of the insolubility of vivianite. Unlike the PdNPs/S mixture, the Pb-sequestering reactions take place at ambient temperatures. At acidic pH values, the reaction sequence is as shown in Equations (20.5a) and (20.5b). 

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