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Published online 1 March 2007
Published in J Environ Qual 36:549-556 (2007)
DOI: 10.2134/jeq2006.0155
© 2007 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
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TECHNICAL REPORTS

Waste Management

Solid Phosphorus Phase in Aluminum- and Iron-Treated Biosolids

Xiao-Lan Huanga,b, Yona Chena and Moshe Shenkera,*

a Dep. of Soil and Water Sciences, Faculty of Agricultural, Food and Environmental Quality Sciences, The Hebrew Univ. of Jerusalem, Rehovot 76-100, Israel
b Cooperative Institute of Marine and Atmospheric Studies, Rosenstiel School of Marine and Atmospheric Science, Univ. of Miami, Miami, FL 33149

* Corresponding author (shenker{at}agri.huji.ac.il)

Received for publication April 17, 2006.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Stabilization of phosphorus (P) in sewage sludge (biosolids) to reduce water-soluble P concentrations is essential for minimizing P loss from amended soils and maximizing the capacity of the soil to safely serve as an outlet for this waste material. The chemical form at which P is retained in biosolids stabilized by Al2(SO4)3·18H2O (alum) or FeSO4·7H2O (FeSul) was investigated by scanning electron microscopy (SEM) equipped with energy-dispersive X-ray elemental spectrometry (EDXS) and by X-ray diffraction (XRD). Both treatments resulted in the formation of a Ca-P phase, probably brushite. Phosphorus was further retained in the alum-treated biosolids by precipitation of an Al-P phase with an Al/P molar ratio of about 1:1, while in the FeSul-treated biosolids, P was retained by both precipitation with Fe/P molar ratios of 1:1 or 1.5:1, and by adsorption onto newly formed Fe hydroxides exhibiting an Fe/P molar ratio of up to 11:1. All of these mechanisms efficiently reduced P solubility and are crucial in biosolids environmentally safe agronomic beneficial use for this waste product; however, each P phase formed may react differently in the amended soil, depending on soil properties. Thus, the proper P stabilization method would depend on the target soil.

Abbreviations: alum, Al2(SO4)3·18H2O • FeSul, FeSO4·7H2O • EDXS, energy-dispersive X-ray elemental spectrometry • FBS, fresh dewatered anaerobically digested biosolids • SEM, scanning electron microscopy • WSP, water-soluble P • XRD, X-ray diffraction


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
SEWAGE sludge (hereafter referred to as biosolids) is an inevitable byproduct of wastewater treatment. Its handling and disposal are the most costly phases of sewage treatment. Agricultural application provides a cost-effective alternative for biosolids disposal but it is essential that the sludge be stabilized before its recycling on agricultural land, to minimize potential environmental problems (Switzenbaum et al., 1997). Only environmentally safe practices of biosolids stabilization for agronomic use can provide a long-term outlet for this troublesome waste stream.

Biosolids generally contains appreciable amounts of N and P and is of significant inorganic-fertilizer replacement value for these major plant nutrients (Sommers, 1977; Kirkham, 1982; Hue, 1995). Hence, biosolids agricultural use is considered to be a sustainable practice. Biosolids P content usually ranges from 8 to 62 g kg–1 on a dry weight basis, and most of it (70 to 95%) is in an inorganic form (Sommers, 1977; Commission on Geosciences, Environment, and Resources, 1996; Brobst, 1999). While sewage N may escape as gaseous compounds during water and biosolids treatment, the removed P is entirely retained in the biosolids. Consequently, the P/N ratio in the biosolids is significantly higher than that required by plants (Hue, 1995; Commission on Geosciences, Environment, and Resources, 1996; Brobst, 1999). Thus, the practice of applying biosolids in quantities that are based on the N requirements of crops may result in a P supply that is in excess of crop needs. Many studies have shown that prolonged addition of biosolids to soil results in increased total and available soil P and may increase P losses (Chang et al., 1983; Clapp et al., 1994; Frossard et al., 1994), especially in cases where biosolids applications are based on N requirements (Bossche et al., 2000; Withers et al., 2001). Water-soluble P (WSP) is the most available and prone-to-leaching P fraction in biosolids and has been reported with a wide range of content in biosolids (Elliott et al., 2002). Previous studies have shown that WSP, although only a minor fraction of the total P in soils (usually <5%), is much more prone to leaching than particulate P, especially in low-discharge P-loss events occurring a short time after P application as fertilizer, manure, or biosolids; thus, the soluble P percentage of the total P is much higher in runoff than in the soil of origin (Kitaka et al., 2002; Preedy et al., 2001; Smith et al., 2001; Heathwaite et al., 2003). Further, the particulate P in runoff may sink quite rapidly and be buried in the sediments of watercourses and lakes, and depending on its mineral form, burial rate, and the chemical and physical conditions in the receiving water, it may be excluded from the biotic cycle (Reynolds and Davies, 2001; Selig et al., 2002; Dougherty et al., 2004). Therefore, this particulate P may become partially unavailable for algal growth. On the other hand, the soluble P, and especially its inorganic species, is well documented to be the most available P form for algal growth and the most potent agents for P-related eutrophication of lakes, rivers, and some coastal waters (Shan et al., 1994; Reynolds and Davies, 2001; Selig et al., 2002; Dougherty et al., 2004). Other studies have shown that WSP losses from pastures and agricultural soils are positively correlated to WSP in soils and in applied agronomic amendments (Pote et al., 1999; Moore et al., 2000; Peter et al., 2002; Ekholm and Krogerus, 2003). In accordance, Huang and Shenker (2004) had suggested that WSP in biosolids can be stabilized to maximize the agronomically beneficial use and the environmentally safe application rates of biosolids. By chemically treating biosolids with different amounts of FeSO4·7H2O (FeSul), Al2(SO4)3·18H2O (alum), or CaO they found that P solubility can significantly decrease, to as low as 2 to 3% of its solubility in the untreated biosolids and they expected that the loss of dissolved P from biosolids-amended soils and its environmental implications will decrease with these biosolids treatments.

In this study we hypothesized that different stabilization methods will result in different mechanisms of P retention and will lead to formation of different P solid phases. Each phase may react differently in the environment, thus, the aim of this work was to explore the mechanisms of P stabilization and the transformations of P species in biosolids treated with alum or FeSul after the dewatering process. The results are expected to provide data that are essential to our understanding of P chemistry and leachability in the biosolids treated by these two chemicals, and in soils amended by the stabilized biosolids. Based on this information, the appropriate stabilization treatment might be assigned in regard to the target soil to maximize the agronomically beneficial use and the environmentally safe application rates of biosolids.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Fresh dewatered anaerobically digested biosolids (FBS) was obtained from the Netanya wastewater treatment plant, located in the central coastal plain of Israel. The plant serves a population of ~200000 people, and is a modern activated sludge treatment facility. It was selected for this study to represent midsize plants which are becoming common in Israel as well as in the USA and Europe, whether for water reuse or to allow safe discharge to surface waters. The water treatment includes an aerobic organic matter degradation process of 24 to 30 h employing a conventional activated sludge recycling approach along with mechanically aerated ponds. Excess sludge is transferred to a 10-d mesophilic anaerobic fermentation tank, followed by a dewatering process using a belt press which increases the dry weight percentage from 0.5 to 0.7% to 19 to 20%. The dewatered biosolids were sampled during a 6-h period to form a composite 300 kg (fresh weight) representative sample. The raw material, referred to as FBS, had a pH (in water, 1:60 weight ratio) of 7.54 and contained 73% water, 630 g kg–1 total organic matter, 29.8 g kg–1 total P, and a total metal content (in g kg–1) of: 72.1 Ca; 7.9 Fe; 7.2 Al; 6.1 Mg; 3.7 K; 3.1 Zn; 0.37 Cu; 0.105 Mn; 0.061 Cr; and 0.046 Ni. These concentrations are based on biosolids dry weight (105°C) (Huang and Shenker, 2004).

Chemically pure (CP) grade Al2(SO4)3·18H2O and FeSO4·7H2O were separately used as chemical stabilizers. In a previous study (Huang and Shenker, 2004) the efficiency of six rates of each of these chemicals were tested to depress WSP in the treated biosolids. The lowest rate of each chemical examined in that study was employed in this study, since these rates were shown to effectively decrease the biosolids P solubility while reducing the pH (in water, 1:60 weight ratio) to a similar value of 6.8. Higher rates employed in the previous study had a larger effect on the pH, with values of down to 3.0–3.6, and were not included in this study. Alum was added at a rate of 49.0 g kg–1 FBS (dry weight base) that formed an Al/P molar ratio of 1.04 in the treated biosolids and FeSul was added at a rate of 30.7 g kg–1 FBS (dry weight base) to form an Fe/P molar ratio of 0.58. After completely mixing the biosolids with the designated amount of FeSul or alum, the treated biosolids was incubated at room temperature in loosely covered containers to allow aeration and minimize evaporation. After 1 mo, the covers were removed to allow air-drying until a constant weight was obtained (by a weekly weighing). The entire process took 4 mo. The air-dried, chemically stabilized biosolids were milled to pass 0.4-mm sieve and stored dry in closed dark bottles for chemical and mineralogical analyses. The FBS was used fresh for the chemical analyses and freeze-dried for the mineralogical tests.

The chemical analyses included determination of pH, WSP (both in water, 1:60 weight ratio, 16-h shaking), and total content of organic matter (weight loss in dry combustion, 400°C) and Fe, Ca, Al, and P (HNO3–HClO4 digestion), as described by Huang and Shenker (2004).

The biosolids samples were examined using SEM (scanning electron microscope; JEOL JSM-5410LV, Tokyo, Japan) equipped with an EDXS (energy-dispersive X-ray elemental analysis spectrometer; Oxford Instruments, ISIS, Chilton, UK). To allow near-edge elemental analysis of the particles, samples were not polished but dry mounted and fixed by a double adhesive tab on plastic mounts. Samples were scanned by beam voltage of 20 kV under low vacuum (27 Pa) with no coating. The working distance between the sample and the EDXS detector was 20 mm. Concentrations of P, Ca, S, Fe, Al, O, C, Mg, Si, Cu, Zn, and K were located in the stabilized biosolids by dot mapping. At least 50 particles in each sample were further analyzed by the electron microprobe for elemental content as calculated by the ZAF method, which includes corrections for the effects of atomic number (Z), X-ray absorption (A), and fluorescence of secondary X-ray signals (F). Since samples were not polished, particle topography and slopes within the scanned area allowed only semiquantitative determination of the mentioned elements; however, the relative attenuation for the analyzed element was considered equal, thus allowing a relative elemental analysis (given as percentage of the total). X-ray diffraction (XRD) analyses were conducted using a Philips (Eindhoven, The Netherlands) diffractometer with Co K{alpha} radiation (PW 1720) on powdered samples from 2{theta} of 4 to 54°.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
In a previous study, Huang and Shenker (2004) found that biosolids WSP was significantly reduced after the addition of the two chemicals (Huang and Shenker, 2004). The total WSP concentration in the FBS was 3520 ± 138 mg kg–1 (average ± standard deviation of three replicates), 11.8% of the total biosolids P; 96% of it was in inorganic forms. As found in that study, WSP in the alum- and FeSul-treated biosolids were reduced after stabilization to 496 ± 30 and 922 ± 37 mg kg–1, respectively. Most of the P stabilization was ascribed to the inorganic fraction, which decreased from 11.8% in the FBS to 1.8 and 3.0% of the total P in the alum- and FeSul-treated biosolids, respectively.

Energy-dispersive X-ray elemental spectrometry dot maps of elements in particles and electron microprobe analyses of biosolids particles were used to evaluate the solid phase. While clear overlaps of P, S, and O patterns of distribution were evident in the untreated FBS (Fig. 1, the O dot map is not shown), no clear overlaps of P distribution with Ca, Fe, and Al were evident and P concentration in the solid phase was low; the greatest P content in particles tested by EDXS as calculated by the ZAF method was 2.93%, close to the average P concentration in these biosolids (2.98%). On the contrary, in the alum- and FeSul-treated biosolids, P became relatively concentrated (above 9% in some of the analyzed particles), and dot maps of P, Ca, and Al or Fe clearly showed the association of P with the corresponding cations (as shown in Fig. 2 for alum-stabilized biosolids). Point analyses at locations with Mg, Si, K, Cu, and Zn in the FBS and the two stabilized biosolids did not show any clear relationships between these elements and P (data not shown). It is generally assumed that the main process taking place after treating biosolids with Al or Fe salts is an interaction between the introduced metal ion and the biosolids-borne P, forming insoluble precipitates of Al-P or Fe-P (Jenkins et al., 1971; Wiechers, 1987; Brett et al., 1997; De Hass et al., 2000). Nevertheless, we found formation of Ca-P phases in both treated biosolids materials.


Figure 1
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  		Fig. 1. Scanning electron microscope (SEM) image (upper left) and elemental (Al, Ca, Fe, S, and P) dot maps of untreated biosolids. The color intensity (white to black, or yellow to red, pink, blue, and black in the colored figures published online) reflects elemental concentration from high to low.

 

Figure 2
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  		Fig. 2. Scanning electron microscope (SEM) image (upper left) and elemental (P, S, Ca, Fe, and Al) dot maps of alum-stabilized biosolids. The color intensity (white to black, or yellow to red, pink, blue, and black in the colored figures published online) reflects elemental concentration from high to low. The particles of (a) Ca-P; (b) Al-P; (c) gypsum; and (d) a probable residual CaCO3 are shown on the SEM image.

 
The stoichiometries of the Fe-P and Al-P molar ratios of the precipitates are expected to be about 1:1 for strengite (FePO4·2H2O) and variscite (AlPO4·2H2O), and 1.5:1 for vivianite (Fe3(PO4)2) (Wiechers, 1987, De Hass et al., 2000). However, our XRD study did not detect any crystallized Al-P or Fe-P minerals in these treated biosolids, probably because of noncrystalline precipitation.

Based on the elemental dot maps (Fig. 2), two different P phases seem to exist in the alum-treated biosolids: Ca-P (e.g., particle "a" in Fig. 2) and Al-P (Fig. 2, particle "b"). No relationship was found between Fe and P. Phases of Ca-S (Fig. 2, particle "c") and a probable residual CaCO3 (Fig. 2, particle "d") were also identified. These phases were attributed to interactions between biosolids-borne P, alum-originated SO4 and Al, and Ca that was released by biosolids Ca carbonate acidification and dissolution (Huang and Shenker, 2004). The quantitative ratios of Ca to P or S were compared by point analyses of EDXS. In the dominant Ca region (including particles "a," "c," and "d" in Fig. 2), many points were located close to the 1:1 P/Ca molar ratio line, indicating the presence of brushite (CaHPO4·2H2O) (Fig. 3A). The other points, located below the 1:1 P/Ca line, are plotted in Fig. 3B and shown to fall close to the 1:1 molar ratio lines of S to Ca or P+S to Ca, indicating gypsum (CaSO4·2H2O) formation or a mixture of brushite and gypsum, respectively (Fig. 3B). Only 4 out of 39 points were distanced from the line, indicating probably the presence of residual CaCO3 mineral. The greatest P concentration in the Ca-P phase was 9.14%, accompanied by Ca (12.57%), S (2.14%), and Al (1.71%). The Ca/P molar ratio was 1.06 and that of S/Al was 1.05, probably indicating precipitation of brushite on residual alum.


Figure 3
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  		Fig. 3. Chemical composition relationships in different types of particles in alum-stabilized biosolids. (A) Relations between the contents of Ca and P in Ca-rich particles; (B) relations between Ca, S, and P in the points enclosed by the triangle in A; (C) relations between Al and P contents in Al-rich particles.

 
Similar to the alum-treated biosolids, Ca-P, Ca-S, and interim-type particles were found in the FeSul-stabilized biosolids, as shown in Fig. 4, which depicts typical X-ray spectra of particles in the FeSul-stabilized biosolids, but no relationships were found between P and Al. Relatively large (ca. 10-µm diam.) particles were found to be dominated by gypsum (Fig. 4A) with 8.54% Ca, 6.86% S, 1.19% P, 1.07% Fe, and a Ca/S molar ratio of 1.00. However, most particles belonged to the interim type (ca. 83% brushite and 17% gypsum). A typical EDXS spectrum is shown in Fig. 4B. The composition of these particles was 8.54% P, 13.27% Ca, 1.82% S, and 2.56% Fe, with a (P+S)/Ca molar ratio of 1.00. Residual CaCO3 in the FeSul-stabilized biosolids was also detected (typical EDXS spectrum in Fig. 4C) with 23.4% Ca, 1.42% P, 1.90% Fe, and 1.40%. Interim-type brushite-gypsum and Ca carbonate minerals were also found (EDXS spectrum, Fig. 4D) with 8.68% P, 2.11% S, 2.98% Fe, and 17.17% Ca. The relationship between Ca, P, and S contents in the Ca-dominated particles from FeSul biosolids is presented in Fig. 5A and B, which divides the Ca-rich particles into brushite-, gypsum-, and brushite-gypsum-type interim phases, and a residual biosolids originated calcite phase. These results were supported by XRD analyses. No crystal phosphate minerals were found by XRD in the FBS. However, after alum or FeSul were introduced, the diffraction lines at 7.57, 4.24, 3.05, 2.93, and 2.6 Å, which are characteristic of brushite and gypsum (Rinaudo et al., 1996; Hina et al., 2001), were detected.


Figure 4
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  		Fig. 4. Typical X-ray spectra (EDXS) of Ca-rich particles of FeSul-stabilized biosolids. (A) Ca-S, gypsum; (B) Ca-P-S dominant phase, brushite & gypsum; (C) probable residual CaCO3; and (D) probable residual CaCO3, gypsum & brushite.

 

Figure 5
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  		Fig. 5. Chemical composition relationships in different types of particles in FeSul-stabilized biosolids. (A) Relations between the contents of Ca and P in Ca-dominant particles; (B) relations between Ca, S and P in the points enclosed by the triangle in A; (C) relations between the contents of Fe and P in Fe-dominant particles.

 
To verify the presence of brushite in the alum-stabilized biosolids, an additional large (about 240-µm diam.) Ca-P particle was tested (Fig. 6). Almost no Al was precipitated on the particle surface, which exhibited the presence of only Ca, P, and S (Table 1), as well as O, C, Mg, and K (not shown). The point analyses by EDXS also supported our suggestion of brushite formation, with some inclusions of gypsum, as the (P+S)/Ca molar ratio was close to 1, even though the absolute P, Ca, and S contents of the particles were variable. Many small, needle-shaped particles (length approximately 40 µm) differing in their P concentrations were observed (not shown in Fig. 6). The P concentration at the needle's edge was high and the Ca/P molar ratio was close to 1, but the P concentration near the middle was significantly lower and concurrent with an increase in S concentration. This precipitate resembled that of brushite-crystal as described by Rinaudo et al. (1994) in a mixed phosphor-sulfate solution, but in our case the needle sizes were much smaller (ca. 10% of those described by Rinaudo et al.). This discrepancy probably resulted from the presence of natural dissolved organic matter which has been shown to be sorbed on the surface of newly precipitated brushite (Grossl and Inskeep, 1991) and to inhibit the crystallization process of Ca-P minerals (Alvarez et al., 2004). This implies that ligand exchange between humate and phosphate may have occurred when the conditions favored the precipitation of calcium phosphate; the dissolved organic carbon (DOC) ligand (carboxylate group) was probably in competition with the phosphate ligand for binding to calcium ions. When competition is strong, binding of calcium ions to oxygen atoms originating from both ligands can take place. Complexation and ligand exchange of DOC on surfaces of Al hydroxide (Parfitt et al., 1977; Kaiser et al., 1997; Kaiser and Zech, 1999) may selectively remove this component from the solution and promote brushite formation. Similar to alum, the same specific sorption sites for H2PO4 and DOC have been reported at the surface of Fe hydroxide (Gu et al., 1994). The ligand exchange between P and the carboxyl/hydroxyl functional groups of DOC to the surface of Fe hydroxide could also have contributed to the formation of calcium phosphate in the FeSul treatment.


Figure 6
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  		Fig. 6. Scanning electron microscope (SEM) image (upper left) and elemental dot maps of alum-treated biosolids, showing a big (~240-µm) Ca-P particle and the location of P, S, Ca, Fe, and Al. The color intensity (white to black, or yellow to red, pink, blue, and black in the colored figures published online) reflects elemental concentration from high to low. The chemical constituents of points 1 to 6 on the SEM image (left) are shown in Table 1.

 

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  		Table 1. Main chemical components of Ca-P dominant phase in alum-stabilized sewage biosolids.

 
Gypsum and brushite have been reported to commonly coexist and transform from one to the other in Ca-SO4–PO4 solutions. Rinaudo et al. (1994) found gypsum to form in solutions with S/P ratios above 0.43, while at lower S/P ratios (0.11 to 1.0), they found brushite crystallization. Rinaudo et al. (1996) further demonstrated that the occurrence of pure brushite is extremely high in a brushite-water-gypsum system (S/P = 1.5–9), and that brushite always formed first, even if the solution contained up to 50 mol % sulfate with respect to P in the pH range of 4.7 to 5.7. Our results are consistent with laboratory experiments by these authors, although the S/P molar ratio in the stabilized biosolids solution in our case was greater than under their experimental conditions.

The relationship between Al and P content in Al-rich particles in the alum-stabilized biosolids (e.g., Fig. 2, particle "b") is presented in Fig. 3C with most data points falling along the line representing a P/Al molar ratio of 1.0. The greatest Al content in this group was 3.84%, accompanied by 3.84% P, 3.14% Ca, and 2.33% S. These results strongly support the suggestion that a defined chemical constituent with a P/Al molar ratio of 1:1 is formed by Al-P precipitation. It should be noted, though, that the XRD analysis did not detect any crystal Al-P mineral.

In contrast to the Al/P molar ratios in the alum-stabilized biosolids, the Fe/P molar ratios varied significantly among the Fe- and P-rich particles in the FeSul-treated biosolids. This ratio is presented in Fig. 5C. The uppermost points are distributed along the lines representing a 1:1 to 1.5:1 molar ratio of Fe to P, which may indicate amorphous strengite or vivianite precursors which were not detected by the XRD analyses. Most of the points, however, were far below the 1.5:1 line, indicating an Fe/P molar ratio of up to 11, which indicates P sorption onto an Fe mineral surface. Thus, it is shown that the applied Fe reacted with biosolids-borne P via two mechanisms simultaneously: (i) direct interaction and precipitation of a Fe-P mineral phase; and (ii) adsorption onto a newly formed Fe hydroxide surface. Typical EDXS spectra of four Fe-P-rich particles in the FeSul-treated biosolids are presented in Fig. 7A-D. The corresponding chemical compositions of these particles (as calculated by the ZAF method) were: 7.67% P, 1.52% S, 2.48% Ca, 50.81% O, and 13.80% Fe (Fe/P = 1.00); 6.46% P, 1.20% S, 2.44% Ca, 31.42% O, and 18.46% Fe (Fe/P = 1.58); 3.54% P, 1.18% S, 3.56% Ca, 21.4% O, and 19.6% Fe (Fe/P = 3.06); and 1.25% P, 0.95% S, 2.70% Ca, 23.5% O, and 24.93% Fe (Fe/P = 11.04), respectively. We assume that the newly formed Fe hydroxide in the FeSul-treated biosolids played an important role in P stabilization, whereas this mechanism did not occur in the alum-treated biosolids.


Figure 7
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  		Fig. 7. Typical X-ray spectra (EDXS) of Fe-rich particles of FeSul-stabilized biosolids with decreased Fe/P molar ratios: (A) 1.00; (B) 1.58; (C) 3.06; and (D) 11.04.

 

   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Recently Shober and Sims (2003) investigated U.S. regulations governing biosolids. Twenty four states had regulations or guidelines that could be imposed to restrict P-based land applications of biosolids. Of those, 13 states had established upper limits for allowable soil P concentrations. However, most regulations were based on soil properties alone and did not consider the P status in the applied material (Kyle and McClintock, 1995; Maguire et al., 2000; Elliott et al., 2002; Penn and Sims, 2002).

Our previous study (Huang and Shenker, 2004) demonstrated that biosolids P solubility can be easily reduced with a simple chemical pretreatment. However, the reactions and mechanisms taking place in the chemical biosolids stabilization process are complicated and related to the chemicals used. By detailed EDXS point analyses we could show in this study that while brushite formed in both alum- and FeSul-stabilized biosolids, Al-P precipitation in the alum-stabilized biosolids, and Fe-P precipitation and P adsorption to Fe hydroxides in the FeSul-stabilized biosolids played an important role in reducing the solubility of P in the biosolids. Each of these processes can effectively reduce the amount of water-soluble and available P, but each leads to the formation of solid-phase P with different properties and is expected to induce different environmental reactivities. Thus, Ca-P phases can dissolve and release P on application to soils of lower pH, or by acidification in the rhizosphere; Fe-P and Al-P minerals may release P by dissolution, depending on soil pH and the stability of each mineral; and Fe-P, but not Al-P or Ca-P phases may release P on reduction in the environment. Higher Fe/P ratios in the solutions, attributed to release of adsorbed P by reductive dissolution of ferric hydroxides, in contrast to lower Fe/P ratios released by reductive dissolution of ferric Fe-P minerals, were shown in earlier studies to pose lower implications on the environment (Zak et al., 2004). We believe that the properties of the P phases and the fate of the stabilized biosolids once it is incorporated into different soils should be taken into account in the regulation of land application. The proper stabilization method and the proper ratio of chemical added to stabilize P in the biosolids would depend on the target soil and deserve further study. We believe that the stabilized biosolids materials may become important products that meet agricultural needs, and implementation of the results of this study will assist to maintain land capacity to safely store disposed biosolids and achieve a sustainable and long-term outlet for this troublesome waste stream.


   ACKNOWLEDGMENTS
 
This research was supported by the Chief Scientist Office, the Israeli Ministry of the Environment. The statements, findings, conclusions, and recommendations are those of the authors and do not necessarily reflect the views of the Cooperative Institute of Marine and Atmospheric Studies (CIMAS), a joint institute of the University of Miami and the National Oceanic and Atmospheric Administration.


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The SCI Journals Agronomy Journal Crop Science
Journal of Natural Resources
and Life Sciences Education		 Vadose Zone Journal
Soil Science Society of America Journal Journal of Plant Registrations The Plant Genome