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Published online 2 June 2005
Published in Soil Sci Soc Am J 69:1047-1056 (2005)
DOI: 10.2136/sssaj2004.0207
© 2005 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
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Soil Mineralogy

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

W. F. Jaynes* and R. E. Zartman

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

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


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
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


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
IN ADDITION TO organic matter, biosolids (i.e., sewage sludge) contain significant amounts of inorganic materials. Manahan (2000) observed that sludge contains inorganic silt and grit contributed by storm water runoff. In addition to the local soil and sediment materials washed into storm drains, cosmetics and other products washed down residential drains contribute to the mineral content of biosolids. Chemical precipitants, such as Fe and Al salts, are added to remove phosphates at some sewage treatment facilities (Manahan, 2000). Ash content is a measure of the mineral content in biosolids. Ash is inorganic residue that is left after the organic matter in a sample is burned off in a furnace at temperatures >500°C. Sommers et al. (1976) noted that the ash contents of biosolids from medium-sized cities (populations of 5500–76000) ranged from 46 to 57%. Terry et al. (1979) reported ash contents of 60.0 and 49.8% for Lafayette and Muncie, IN anaerobic sewage sludges. The high ash content of biosolids results from effective removal of much of the organics during waste treatment.

A number of other inorganic materials that might be found in biosolids are derived from cosmetics, foods, and other household products. Palygorskite (attapulgite) clay, bentonite clay, diatomaceous earth, iron oxides, SiO2, TiO2, salts of Al, Ca, Cu, Fe, Mg, Mn, and Zn, silicates of Ca, Mg, Na, and Zn, phosphates, and other inorganic materials are used in food processing (NAS-NRC, 1965; Ellinger, 1983; U.S.F.D.A., 1993). Bentonite and other clays are the principal ingredients in flushable kitty litter products. Kaolinite has been used in medical, pharmaceutical, and cosmetic products (Virta, 1999). Titanium dioxide, TiO2, is a white pigment widely used in cosmetics, food products, paint, paper, and plastics (Taylor, 1980; Gambogi, 1994, 2002). Aluminum salts [e.g., Al2(SO4)3] are used as firming and crisping agents (e.g., in pickles) to preserve the texture of vegetable tissues (Taylor, 1980). The hydrolysis of Al salts, such as Al2(SO4)3, would form poorly crystalline Al hydroxides. Hydroxides of Al and Zr are used in deodorants as antiperspirants. Talc, Mg3Si4O10(OH)2, is used as inert filler in drug tablets, as a drying agent in baby powders, and as an anti-caking agent in vitamin supplements (Hollinger, 1990; NAS-NRC, 1965). All of these products can potentially contribute inorganic materials to biosolids.

Ross et al. (1993) examined the health effects of mineral dusts other than asbestos and concluded that heavy exposure to talc dust can cause nonmalignant respiratory disease. Wehner (1994) examined the biological effects of cosmetic talc and reported a weak association between hygienic use and ovarian cancer. Cook et al. (1997), however, concluded in a case-control study that powder from perineal dusting contributes to the development of ovarian cancer. Wehner (1994) stated that exposure of animals to talc aerosols caused lung overload. The worst health effect of talc reported by Wehner (1994) was the 20% mortality rate in infants after accidental inhalation of large quantities of talc. Wehner (1994), however, concluded that there was no convincing evidence that cosmetic talc presents a health hazard when used as intended. The Mine Safety and Health Administration proposed an 8-h average exposure limit of 2.5 mg m–3 of air for talc containing no asbestos (Virta, 1994). Van Oss et al. (1999) examined the impact of different asbestos species and other mineral particles on pulmonary pathogenesis. They reported that the fibrous minerals (i.e., asbestos species) were the most dangerous. Because of particle asymmetry, phagocytic cells are unable to completely engulf fibrous minerals and these particles are prone to stay trapped in the lungs for the life of the patient. The dimensions of symmetrical mineral particles allow more complete engulfment by phagocytic cells and removal from lungs. However, van Oss et al. (1999) reported that of the symmetrical mineral particles (e.g., talc, silica, metal oxides), small talc particles were the most dangerous.

Phosphate minerals precipitate during wastewater treatment. Borgerding (1972) examined decreased flow problems caused by precipitation of phosphates (i.e., struvite, NH4MgPO4 · 6H2O) in the pipes of an anaerobic sewage sludge digestion system in Los Angeles. He found that buildup was greatest in cast iron pipes and least in PVC pipes. Nriagu and Dell (1974) showed that vivianite [Fe3(PO4)2 · 8H2O], reddingite [Mn3(PO4)2 · 3H2O], and anapaite [Ca2Fe(PO4)2 · 4H2O] were the most dominant phosphate minerals forming in fresh water reducing environments, such as in the Great Lakes. Robertson (2000) reported that during anaerobic sludge digestion, the Fe in Fe oxides is reduced and it reacts with dissolved P to form insoluble Fe phosphates, such as vivianite. Subsequent oxidation of the sludge would produce ferric phosphates like strengite (FePO4 · 2H2O). Based on these observations, Robertson (2000) proposed a passive wastewater treatment system that relies on the reductive dissolution of Fe oxides in contact with sewage effluent to precipitate Fe phosphates. Iron oxides derived from Fe rust and local soils washed into drainage pipes might also provide an Fe source to promote Fe phosphate mineral crystallization during anaerobic sewage sludge digestion. Schwertmann and Taylor (1977) found that organic acids are particularly effective in inhibiting transformation of ferrihydrite to more crystalline Fe oxides. Similarly, high concentrations of soluble organic matter in sewage effluent would likely thwart formation of crystalline phosphate minerals.

Selective chemical dissolution methods are often used in soil analysis for the practical classification of different forms of P. Care must be exercised, however, in attributing the results of these methods to actual P phases. Pautler and Sims (2000) examined relationships between different P availability tests on Delaware topsoils and reasonably attributed oxalate-extractable Fe and Al to be a measure of P sorption capacity. However, oxalate-extractable P should not be attributed to P sorption capacity in soils treated with biosolids. Soils amended with biosolids (anaerobic sludge) are likely to contain poorly crystalline Fe and Al phosphates, especially for Fe and Al salt-amended biosolids. Maguire et al. (2000) reported that oxalate-extractable P was much greater in soils treated with biosolids from sewage treatment plants that used Fe salts to precipitate P. Soon and Bates (1982) reported that about 70% of the inorganic P in biosolids treated with Al2(SO4)3 or FeCl3 was extracted by NaOH. Similarly, Jaynes et al. (2003) reported that 66 to 78% of the P in MERCO biosolids was removed in the 0.1 M NaOH extract. This P would be attributed to "nonoccluded Al- and Fe-bound P" in the soil P fractionation method (Olsen and Sommers, 1982; method 24–4.1). However, oxalate-extractable P and NaOH-soluble P in these instances would likely be attributed to Fe and Al phosphates.

MERCO Joint Venture LLC, a State of New York limited liability company, operated a biosolids management and beneficial land application program in Hudspeth County, Texas near the city of Sierra Blanca (MERCO, 2000). The MERCO site has a semiarid climate with 310 mm yr–1 precipitation, a mean annual temperature of 18°C, and an elevation of 1350 m above sea level (Rostagno and Sosebee, 2001). From 1992 to 2001, biosolids from NYC municipal sewage treatment plants were applied to the surface of native range soils at the site. The biosolids from NYC were largely residential in origin with only about 5% from industrial sources (Department of Environmental Protection, 1998c). Most of the biosolids received at the MERCO site from 1992 to 1998 were anaerobically treated to reduce pathogens by a method termed "Process to Significantly Reduce Pathogens" (PSRP). Biosolid materials are very persistent in the dry climate of the MERCO site and could be readily identified and sampled many years after application.

Organic matter, plant nutrients (i.e., N, P, K), and heavy metals contributed by land applications of biosolids have frequently been examined because of the potential effects on plant growth and environmental degradation. However, biosolids contribute about as much inorganic as organic matter. The composition of the mineral fraction partly reflects the soils and sediments that were washed into storm drains contributing to the sewage treatment plants. Other inorganic materials are washed down residential or industrial drains or form at the treatment plants. Heavy metals in biosolids might be associated with particular mineral phases. Hence, the objective of this study is to examine the mineralogy and chemistry of the inorganic fraction of NYC biosolids that were applied to the soil surface at the MERCO site near Sierra Blanca.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Samples of fresh PSRP biosolids and PSRP biosolids applied 2 to 7 yr before sampling were collected at the MERCO site. More details on the MERCO site and biosolids applications were presented in Jaynes et al. (2003). Fresh biosolids were collected from a temporary holding shed at the MERCO site and 2- to 7-yr-old samples were collected from the soil surface at various locations on the site. The fresh biosolid sample was moist when collected in 1999 and was later air-dried. The other samples were dry when collected. The field sampling sites had received only one application of biosolids. At the 1993 site, biosolids were applied to open rangeland, whereas, biosolids at the 1992, 1994, and 1997 sites were applied to enclosed 0.5- to 7-m2 research plots. The equipment used to distribute biosolids on the open rangeland site yielded many large (>100 mm in diameter) readily identifiable aggregates. Biosolids at the research plots were hand-applied. Larger biosolid aggregates were sampled at both the rangeland and research plot sites to minimize contamination by local soil materials. Extraneous soil and other materials were removed from the biosolid sample aggregates (approximately 5–50 mm in diameter) during field collection. The biosolid aggregates were later agitated on a 2-mm sieve to further remove any remaining extraneous material. Biosolid aggregates were subsequently ground to <2 mm using a coffee grinder.

Ashing and Inorganic Analysis of Whole Biosolids
Whole biosolid samples were weighed in duplicate into porcelain crucibles and heated to 700°C for 1 h to remove organic matter. The crucibles were allowed to cool and weighed to determine the ash contents. Duplicate samples (100 mg) of the ash were weighed into Teflon-lined, steel decomposition bombs and 6 mL of HF and 1 mL of aqua regia were added. After 1 h of heating at 110°C, the sealed bombs were allowed to cool, were opened, and the digests were transferred to 100 mL of a 56 g L–1 boric acid solution (Bernas, 1968). After complete dissolution, the solutions were diluted to 250 mL and transferred to plastic bottles. The solutions were later analyzed for total Si, Al, Fe, Ca, Mg, Na, K, P, Mn, Zn, Cu, Cd, As, Cr, Hg, and Pb using a Leeman Labs (Hudson, New Hampshire) inductively coupled plasma spectrometer (ICP).

Organic Matter Removal from Whole Biosolids
Whole biosolids (50 g) were weighed into 1000-mL glass beakers that contained 100 mL of pH 5.5, 1 M sodium acetate buffer and 100 mL of deionized water. Approximately 20 mL of 30% (w/w) H2O2 were added to each beaker. The beakers were set aside and allowed to react overnight at room temperature. The beakers were later placed on a hot plate and the heat gradually increased as the reaction rate decreased. The pH was periodically checked and additional buffer and H2O2 were added as needed. Biosolids reaction with H2O2 ended after approximately 4 d and the beakers were set aside to cool and sediment. The beaker contents were transferred to centrifuge bottles and centrifuged to concentrate the mineral residues. The mineral residues were washed three times with deionized water to remove soluble salts, washed with ethanol once, and dried at 60°C in an oven. After drying, the biosolid mineral residue samples were weighed and lightly ground in a mortar and pestle. Residue weight fractions of the whole biosolids were calculated by dividing the mineral residue weights by the initial biosolid weights. Soil color was determined on the dry mineral residue samples by comparison to a Munsell soil color chart. Weight-loss-on-ignition values were determined on weighed subsamples of the mineral residues after heating in crucibles at 700°C for 1 h.

Particle-Size Fractionation of Mineral Residues
Portions (10 g) of the mineral residue samples were separated into sand, silt, and clay fractions by sieving and gravity sedimentation. Aqueous suspensions of the residue samples were washed through a 0.05-mm sieve to separate the sand fraction. After sand removal, the <2-µm clay fraction was removed from the suspensions by gravity sedimentation. Silt was left in the suspensions after sand and clay were removed. The sand, silt, and clay fractions were dried at 60°C, weighed, and transferred to vials.

Microscopic Examination of Sand Fractions
Thin sections of the sand fractions were prepared for optical microscopy on glass slides. Sand samples were embedded in Canada balsam and ground to a thickness of 30 µm in the Texas Tech Geosciences Department. The embedding media has an index of refraction (1.537) very close to that of {alpha}-quartz (1.544–1.553). The prepared slides were examined and photographed using a petrographic microscope.

X-ray Diffraction Analysis of Sand, Silt, and Clay Fractions
Sand and silt fractions were back-filled into aluminum sample holders and analyzed by powder x-ray diffraction using CuK{alpha} ({lambda} = 1.5418 Å) radiation. A Philips (Eindhoven, The Netherlands) x-ray diffractometer interfaced to a computer was used to collect and store all of the x-ray diffraction patterns as computer disk files. The sands were scanned from 2 to 60°2{theta} and the silts were scanned from 2 to 30°2{theta}. Clay samples (30 mg) before and after oxalate extraction were Mg saturated, dispersed in water, and air-dried to form oriented films on 27 x 46 mm glass slides. The clay slides were scanned from 2 to 30°2{theta}. Dominant mineral phases were identified by comparing peak positions and relative intensities in the diffraction patterns to reference values in the powder diffraction file (Joint Committee on Powder Diffraction, 2001).

Electron Microscopy and Elemental Analysis
Clay samples were examined using a JEOL (Japan Electron Optical Laboratories) model 100cx scanning transmission electron microscope (STEM) and chemically analyzed using energy dispersive x-ray spectra in the Texas Tech Geosciences Department. Samples of the July 1993 biosolid H2O2 residue clay were examined before and after oxalate extraction using the electron microscope. Samples of clays were dispersed in water and small droplets of the suspensions were dried on formvar-coated copper grids. Before electron optical examination, the sample grids were made electrically conductive by coating with C-Pt using a vacuum evaporator. Particle images and x-ray spectra were photographed from the computer screen of the electron microscope. Photographic prints were scanned to produce digital image files.

Oxalate and Dithionite Extractions of Biosolid Mineral Residues
Acid ammonium oxalate and citrate–bicarbonate–dithionite were used to extract non-crystalline and reductant-soluble Fe and P from the whole mineral residue samples and from the silt and clay fractions. The "acid ammonium oxalate in darkness" (Ox-D) and the "citrate–bicarbonate–dithionite" (CBD) methods described by Loeppert and Inskeep (1996) were used in the biosolid residue sample extractions. Iron in the extracts was determined using the colorimetric orthophenanthroline method described by Loeppert and Inskeep (1996). Phosphorus in the CBD extracts was determined using the modified Murphy and Riley (1962) colorimetric method described by Olsen and Sommers (1982)(Method 24–3.4). Portions of the Ox-D extracts of the whole mineral-residue samples were saved for later chemical analysis by ICP. After oxalate extraction, the remaining material from each whole mineral-residue sample was washed with deionized water, dried at 60°C, and saved for later surface area analysis. Oxalate residues from the silts and clays were saved for x-ray diffraction and electron microscopic analysis.

Nitrogen Surface Area Measurements
Nitrogen surface areas were measured by the single-point method using a Micromeritics (Norcross, GA) Flowsorb II model 2300 surface area meter with a carrier gas containing 30% N2 and 70% He. Before surface area measurements, the instrument was calibrated to local atmospheric pressure using aliquots of N2 measured and delivered using a Hamilton gas-tight syringe. Samples were dried in U-tubes at 105°C in a convection oven and degassed at 120°C on the instrument degassing station using the carrier gas. Sample U-tubes were then dipped in liquid N2 to adsorb N and subsequently dipped into water to desorb adsorbed N. Three or more measurements were made for each sample. Amounts of desorbed N2 measured by the detector were used with the sample weights to calculate specific surface areas. For comparison, the surface area of a non-crystalline iron phosphate (FePO4 · 2H2O), obtained from Aldrich Chemical Company (St. Louis, MO), was also measured.

Lithium Metaborate Fusion and ICP Analysis of Mineral Residues
Residues following treatment with H2O2 were sent to the Texas Tech University Geosciences Department for elemental analysis. Mineral residue samples (0.2000 g) were mixed with 1.2000 g of LiBO2 and transferred to graphite crucibles. The crucibles were placed in muffle furnaces at 1000°C and left for 20 min. The crucibles were removed from the furnace and the molten LiBO2 that contained the fused mineral residue samples were poured into beakers containing 50 mL of 0.06 M HCl. After complete dissolution, the sample solutions were transferred to plastic bottles and later analyzed using ICP. National Bureau of Standards standard rock samples were similarly fused with LiBO2 and dissolved in 50 mL of 0.06 M HCl. The NBS solutions were used as standards in ICP analysis of the biosolid samples.

Titanium in Silt and Clay Fractions
Total and oxalate extractable Ti were determined on silt and clay fractions of the biosolid H2O2 residues using the Tiron (disodium-1,2-dihydroxybenzene-3,5-disulfonate) colorimetric method (Shapiro, 1975). For total Ti, 200-mg silt and clay samples were dissolved in Teflon-lined steel decomposition bombs using 6 mL of HF, 1 mL of aqua regia, and 5.6 g of boric acid diluted to 100 mL with deionized water (Bernas, 1968). The Ox-D method was used to extract poorly crystalline Ti (Loeppert and Inskeep, 1996). Fitzpatrick et al. (1978) determined that amorphous TiO2 in soils was completely soluble in acid ammonium oxalate.


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Most of the biosolid H2O2 residues had colors and surface areas that were comparable with the poorly crystalline Fe phosphate sample (Table 1). The August 1992 residue, however, had a redder color suggesting the presence of ferric oxides. The biosolid residues largely consisted of silt- and clay-size particles. Sand (mostly fine sand and very fine sand) was a minor component. This was not surprising because sewage treatment facilities undertake the following procedures to remove coarse materials during treatment: (i) wastewater is first passed through screens during preliminary treatment; (ii) after primary treatment in settling tanks, the sludge is degritted to remove sand and gravel; and (iii) after secondary treatment and anaerobic digestion, the sludge is passed through a grinder before dewatering with centrifuges (Department of Environmental Protection, 1998a, 1998b). Surface areas of the biosolid H2O2 residues decreased from about 20 to about 3 m2 g–1 after Ox-D extraction (Table 1). This suggests that a poorly crystalline material, such as a short-range-order Fe phosphate, might account for much of the surface area. The H2O2 residue weight fractions of the whole biosolids were greater for biosolids that were exposed at the soil surface for a longer time. Similarly, Jaynes et al. (2003) showed that increased ash content with greater exposure age was due to organic matter decomposition.


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  		Table 1. Physical characteristics of biosolid H2O2 residues.

 
Chemistry
Extractable Fe and P data for the biosolid H2O2 residues suggest >20% poorly crystalline Fe phosphates (Table 2). The Ox-D extraction method is a selective dissolution procedure that mostly dissolves poorly crystalline minerals (Loeppert and Inskeep, 1996). The CBD extraction dissolves both crystalline and poorly crystalline Fe oxides and phosphates. The ratio of Ox-D- to CBD-extractable Fe is a measure of the relative amounts of non-crystalline and crystalline Fe phases in soils. This ratio was about 1 for the FePO4 · 2H2O sample and the biosolid H2O2 residues and indicates that most of the Fe was present as a poorly crystalline material. In contrast, Ox-D- to CBD-extractable Fe ratios in the MERCO soil ranged from 0.08 to 0.13. Maguire et al. (2000) reported large amounts of oxalate-extractable Fe, Al, and P in soils treated with biosolids from sewage treatment plants that used Fe or Al salts to precipitate P. The NYC sewage treatment plants add organic polymers to dewater biosolids, but do not add Al or Fe salts (Department of Environmental Protection, 1998a). Whole-biosolid H2O2–residue sample Ox-D-extracts were analyzed using ICP for more complete chemical analyses (Table 3). The extracts contained mostly Fe, Al, and P suggesting a mixed Fe and Al phosphate, such as barrandite (Al,Fe)(PO4) · 2H2O. However, chemical analysis cannot determine whether it is a single mixed phosphate or separate Fe and Al phosphates. Calcium was probably also extracted, but it was not measured. Arsenic, Cr, Cu, Hg, and Zn were extracted (Table 3). Oxalate extraction suggests these heavy metals coprecipated with the Fe and Al phosphates. Yet, the heavy metals might originally have been associated with the organic matter. Slansky (1986) reported mean trace element contents in natural (i.e., Ca) phosphate deposits with 40 mg kg–1 As, 1000 mg kg–1 Cr, 100 mg kg–1 Cu, and 300 mg kg–1 Zn. Manahan (2000) noted that AsO43– substitutes for a small part of the PO43– in natural phosphates. Arsenic was below detection limits in the whole biosolid ash chemical analyses (Table 4). The quantities of these elements in the oxalate extracts equals about 40% of Cr, 78% of Cu, 26% of Hg, and 94% of the Zn contents in the whole biosolids (Table 4). Very little of the Pb measured in the ash (Table 4) was detected in Ox-D extracts (Table 3) of the biosolid H2O2 residues. The Pb might have dissolved during organic matter removal.


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  		Table 2. Citrate-bicarbonate-dithionite (CBD) and oxalate (Ox-D) extractable Fe and P in biosolid H2O2 residues.

 

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  		Table 3. Oxalate (Ox-D) extract composition of biosolid H2O2 residues.{dagger}

 

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  		Table 4. Elemental analysis of ashed whole biosolid samples.{dagger}

 
Elemental analyses of biosolid H2O2 residues were used to characterize some of the more refractory elements (Table 5). The Cr, Nb, Sc, and V concentrations follow similar trends as Fe with higher concentrations in the older biosolids. The origin of Cr, Fe, and V in the biosolids is steel components that are present in industrial wastes. The rare earths, Nb and Sc, probably also had an industrial source. The biosolids from NYC applied at the MERCO site were largely residential in origin; the industrial portion of sewage plant influent was reduced from 30% in 1973 to approximately 5% in 1992 and remained near 5% in 1998 (Department of Environmental Protection, 1998c). The changes in heavy metal contents are consistent with reported decreases in total metal loading from <1800 kg d–1 in 1992 to 1400 kg d–1 in 1998 (Department of Environmental Protection, 1998c).


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  		Table 5. Elemental analysis of LiBO2 fused biosolid H2O2 residues.{dagger}

 
The ratio of Ti to Zr is used in pedology and soil mineralogy studies to distinguish parent material sources (Milnes and Fitzpatrick, 1989; Fitzpatrick and Chittleborough, 2002). The Ti/Zr ratios in the three MERCO site soil A-horizons and throughout the soil profiles (not shown) were nearly constant (Table 5). In contrast, the wide variation in Ti/Zr ratios and Zr contents in the biosolid residues suggest anthropogenic inputs of Zr and/or Ti. Zirconium follows the same trend as Cr, Fe, and V suggesting an industrial origin. All titania pigments used in foods and other products are usually synthetic poorly crystalline anatase and rutile, which are roughly spherical 200- to 300-nm diam. particles that are prepared by the hydrolysis of Ti(SO4)2 or TiCl4 solutions (Solomon and Hawthorne, 1983). Hence, the size and morphology of the crystalline TiO2 and other Ti minerals from soils and sediments will differ from pigment TiO2. Biosolid Ti contents were less variable than Zr, but probably include contributions from pigment TiO2 used in foods, cosmetics, and paints. Much (16–61%) of the Ti in the biosolid residues was extracted by oxalate (Table 6). In contrast, very little of the Ti in the MERCO soil was extracted by oxalate. This suggests that biosolids contain poorly crystalline forms of Ti, such as pigment TiO2, in addition to some crystalline Ti forms contributed by pigment TiO2 and by local soils and sediments that wash into the sewage treatment plants.


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  		Table 6. Total and oxalate (Ox-D) extractable Ti in biosolid H2O2 residues{dagger}.

 
Mineralogy
The center of the micrograph (Fig. 1a) reveals a textile fiber about 0.5 mm long and matrix consisting largely of quartz and feldspar grains. The quartz and feldspar grains have low relief because the refractive index is similar to the embedding media. The darker grains in Fig. 1 consist of opaque minerals and H2O2 oxidation-resistant aggregates of organic matter. The textile fiber in Fig. 1a was brown, but fibers of various colors were also identified in the biosolid residue sands. The fibers are composed of a synthetic fiber, such as nylon, that is more resistant to H2O2 oxidation than cotton. The glass shard in the center of Fig. 1b was isotropic and colorless, but glass shards of many colors were also identified. A green pyroxene grain with perpendicular cleavage traces is on the right side of Fig. 1b. Many zircon grains were also identified in the biosolid residue sands. In contrast, very fine sand from a soil at the MERCO site (Pit 4, A-horizon, Casby-Horton, 1997) contained only colorless quartz and feldspar grains. No fibers, glass shards, zircons, or opaque minerals were evident. X-ray diffraction of the biosolid residue sand fractions and the very fine sand from the MERCO site soil A-horizon revealed only quartz and feldspar peaks.



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  		Fig. 1. Petrographic micrographs of sand (mostly very fine sand) separated from biosolid H2O2 residues: (a) brown isotropic fiber in June 1999 biosolid residue, much of background is quartz grains; (b) colorless, isotropic glass shard and green pyroxene grain in July 1994 biosolid residue.

 
X-ray diffraction of the biosolid H2O2 residue silt fractions and the MERCO site soil A-horizon silt fraction indicated mostly quartz and feldspars (Fig. 2) . Sommers (1977) identified quartz, calcite, dolomite, feldspars, and layer silicates in anaerobically digested sewage sludges from Indiana. The MERCO biosolid H2O2 residue silt fractions, however, also contained mica, kaolinite, and talc. Mineralogy data (USDA NRCS pedon ID:69CT009004, Soil Survey Staff, 2005) for the Enfield series, which is mapped in the NYC area (Soil Conservation Service, 1987), indicates mostly quartz, feldspar, and mica with small amounts of amphibole, weatherable minerals, and resistant minerals (e.g., zircon). The talc was clearly not derived from local sediments and must have originated from baby powder and other products washed down residential drains.



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  		Fig. 2. X-ray diffraction powder-mount patterns of silt fractions of biosolid H2O2 residues compared with silt fraction of MERCO site soil from Casby-Horton (1997): (a) June 1999; (b) July 1994; (c) August 1992; (d) Pit 4, A horizon silt.

 
X-ray diffraction of the MERCO site soil A-horizon clay fraction indicates smectite, mica, and kaolinite (Fig. 3) . In contrast, apart from quartz, mineral peaks are nearly absent in the biosolid residue clay fractions. After oxalate extraction, however, the biosolid residue clays (Fig. 4) had strong peaks for expandable minerals, such as vermiculite and smectite, as well as mica, talc, kaolin, and possibly chlorite. The strong absorption of CuK{alpha} radiation by Fe phosphates evidently prevented identification of these minerals in Fig. 3. However, after oxalate dissolution of the Fe phosphates, the presence of these minerals was clearly indicated. Clay mineralogy data on the Enfield soil series indicates vermiculite, mica, and kaolinite as the dominant mineral phases (USDA NRCS pedon ID:69CT009004, Soil Survey Staff, 2005). However, the biosolid samples also contained talc, which was likely contributed by residential drains. The talc peaks in x-ray diffraction patterns of the biosolid silt and clay fractions suggest approximately 5% talc because identification of less abundant phases is generally not possible. The smaller peaks suggest less talc in the 1999 biosolid clay fraction than in the older samples. The February 1997 biosolid clay fraction (not shown) also had smaller talc peaks than the older samples. Less talc in the more recent biosolids could reflect a decreased use of products containing this mineral because of health concerns.



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  		Fig. 3. X-ray diffraction patterns of oriented, Mg 25°C-treated clay fractions of biosolid H2O2 residues. The patterns are compared to soil clay from the MERCO site from Casby-Horton (1997): (a) June 1999; (b) July 1994; (c) August 1992; (d) Pit 4, A-horizon clay.

 


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  		Fig. 4. X-ray diffraction patterns of oriented, Mg-25°C-treated clay fractions of biosolid H2O2 residues after Ox-D extraction: (a) June 1999, (b) July 1994, and (c) August 1992.

 
Virta (1992)(2002) reported a decrease in cosmetic talc use in the USA from 38000 (5%) metric tons in 1992 to 16000 (2.5%) metric tons in 2002. Most talc (approximately 74%) consumed in the USA is used in ceramics, paint, paper, and roofing. Virta (2002) noted that large U.S. talc deposits of metamorphic origin consistently have fibrous amphibole species, such as anthophyllite or tremolite. He observed that the stresses imposed by regional or contact metamorphism apparently encourage fibrous amphibole growth. A study of fibrous talc deposits from Montana, New York, and Texas identified fibrous talc and fibers composed of amphibole and talc intergrowths (Virta, 2002). However, the National Toxicology Program of the U.S. Department of Health and Human Services voted not to include asbestiform talc as reasonably anticipated to be identified as a human carcinogen in the 10th report to Congress (Virta, 2000).

Electron microscopy and elemental analysis of the July 1993 biosolid residue clay particles revealed common aggregates of layer silicates and Fe phosphates (Fig. 5a) . Elemental analysis of the aggregates indicates an Fe phosphate with significant amounts of Al and Ca (Fig. 5b). The Ti might consist of small TiO2 particles occluded within the Fe phosphates. The Cl in Fig. 5b might be associated with pigment TiO2. Fitzpatrick et al. (1978) noted that traces of Cl in amorphous Ti oxides hinder crystallization into anatase. After Ox-D extraction, particles of silicate clays including talc (Fig. 6) and clay mica (not shown) were identified. The phyllosilicate flake in Fig. 6a was identified as talc based on the Mg and Si composition. The Cu in Fig. 5b and Fig. 6b is attributed to the Cu support grids.



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  		Fig. 5. Scanning transmission electron micrograph of July 1993 biosolid H2O2 residue clay fraction: (a) common aggregate of clay particles with Fe phosphates; (b) chemical analysis of clay aggregate.

 


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  		Fig. 6. Scanning transmission electron micrograph of July 1993 biosolid H2O2 residue clay fraction after Ox-D extraction: (a) uncommon talc particle; (b) chemical analysis of talc particle.

 

   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Biosolids contain a variety of inorganic materials. Local soil, sediments, broken glass, and other inorganic materials washed into drains end up in biosolids. The inorganic materials in the MERCO biosolids consisted largely of silt- and clay-size particles because coarser materials were removed before or during sewage treatment. Quartz, feldspars, kaolinite, mica, and expandable clays were contributed to the biosolids by NYC soils and sediments. Poorly crystalline Fe and Al phosphates (>20%) in the biosolids probably formed during sewage treatment by reductive dissolution of Fe oxides derived from rust and soil materials. Food residues and other consumer products washed down residential drains might have contributed Al. Arsenic, Ca, Cr, Cu, Hg, and Zn might have coprecipitated with the Fe and Al phosphates. Highly variable Ti/Zr ratios in the biosolids suggest that, in addition to local soil and sediments, Ti and Zr were contributed by both residential and industrial sources. Talc (approximately 5%) was identified in silt and clay fractions of the biosolids. Baby powder and other cosmetic products were the likely talc source. Less talc in the 1997 and 1999 biosolids than in the 1992, 1993, and 1994 biosolids suggests a decreased use of talc-containing products, probably due to health concerns. Inorganic components in biosolids record local mineralogy as well as other materials contributed by residential and industrial wastewater.


   ACKNOWLEDGMENTS
 
We gratefully acknowledge the assistance of Dr. B.L. Allen in the field collection of biosolid samples and in the petrographic microscopic examination of sand fractions. The authors also appreciated the help provided by Dr. Necip Güven of Geosciences in the electron microscopy and X-ray diffraction work.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
College of Agricultural Sciences and Natural Resources Publication# T-4-554.

Received for publication June 22, 2004.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 





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