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Soil Mineralogy

Origin of Talc, Iron Phosphates, and Other Minerals in Biosolids

Plant and Soil Science Dep., Texas Tech Univ., Lubbock, TX 79409

^* Corresponding author (william.jaynes{at}ttu.edu)

Biosolids (i.e., sewage sludge) contain as much inorganic as organic materials. The inorganic materials include local soils, sediments, and other materials washed down residential, industrial, and storm drains. In this study, the chemistry and mineralogy of the inorganic materials in New York, NY (NYC) biosolids applied to soil surfaces in western Texas from 1992 to 1999 were examined. Inorganic residues were 44 to 77% of whole biosolid weights after oxidation of the organics with H₂O₂. Inorganic residues consisted largely of silt- and clay-size particles with smaller amounts of fine and very fine sand. Selective dissolution and chemical analyses of the residues indicated >20% Fe phosphates, formed during anaerobic digestion, that contained Al, As, Cr, Cu, Hg, and Zn. X-ray diffraction, selective dissolution, and surface area analyses showed that the Fe phosphates were poorly crystalline. The mostly quartz and feldspar sands also contained glass shards, textile fibers, and zircon grains. New York City area biosolids contained quartz, feldspars, kaolinite, mica, and expandable layer silicates. Electron microscopy and x-ray spectra of the clay fractions revealed aggregates of layer silicates and Fe phosphates containing Al, Ca, and Ti. Pigment TiO₂, commonly used in foods and cosmetics, contributed Ti to the layer silicate aggregates. Talc derived from cosmetic products was identified in silt and clay fractions by x-ray diffraction and electron microscopy. Less talc occurred in the 1999 and 1997 biosolids relative to 1992, 1993, and 1994 biosolids suggesting reduced use of talc products. Nationally, cosmetic talc use decreased during that time, probably due to health concerns.

Abbreviations: CBD, citrate–bicarbonate–dithionite • ICP, inductively coupled plasma spectroscopy • NYC, New York, NY • Ox, acid ammonium oxalate • PSRP, process to significantly reduce pathogens