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TECHNICAL REPORTS

Waste Management

A Method for Determining the Phosphorus Sorption Capacity and Amorphous Aluminum of Aluminum-Based Drinking Water Treatment Residuals

School of Natural Resources, The Ohio State University, Columbus, OH 43210

^* Corresponding author (Dayton.15{at}osu.edu)

Received for publication June 13, 2004.

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION PHOSPHORUS SORPTION MAXIMUM AMORPHOUS ALUMINUM MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
A high amorphous aluminum or iron oxide content in drinking water treatment residuals (WTRs) can result in a high phosphorus (P) sorption capacity. Therefore, WTR may be used beneficially to adsorb P and reduce P loss to surface or ground water. The strong relationship between acid ammonium oxalate–extractable aluminum (Al_ox) and Langmuir phosphorus adsorption maximum (P_max) in WTR could provide a useful tool for determining P_max without the onus of the multipoint batch equilibrations necessary for the Langmuir model. The objectives of this study were to evaluate and/or modify an acid ammonium oxalate extraction of Al_ox and the experimental conditions used to generate P adsorption isotherms to strengthen the relationship between Al_ox and P_max. The oxalate extraction solution to WTR ratio varied from 40:1, 100:1, and 200:1. Batch equilibration conditions were also varied. The WTR particle size was reduced from <2 mm to <150 µm, and batch equilibration was extended from 17 h to 6 d. Increasing the solution to WTR ratio to 100:1 extracted significantly greater Al_ox at levels of >50 mg Al kg^–1. No additional increase was found at 200:1. Reducing WTR particle size from <2 mm to <150 µm increased P_max 2.46-fold. Extending the equilibration time from 17 h to 6 d increased P_max by a mean of 5.83-fold. The resulting empirical regression equation between the optimized Al_ox and P_max (r² = 0.91, significant at the 0.001 probability level) may provide a tool to estimate the P_max of Al-based WTR simply by measuring Al_ox. The accurate determination of WTR P_max and Al_ox is essential in using WTR effectively to reduce P loss in runoff or to reduce the solubility of P in agricultural soils or organic waste materials (biosolids, manure).

Abbreviations: Al_ox, acid ammonium oxalate–extractable aluminum • Fe_ox, acid ammonium oxalate–extractable iron • P_max, Langmuir phosphorus adsorption maximum • P_ox, acid ammonium oxalate–extractable phosphorus • PSI, phosphorus saturation index • WTR, water treatment residual

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION PHOSPHORUS SORPTION MAXIMUM AMORPHOUS ALUMINUM MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
THE ROLE OF EXCESSIVE agricultural P in the degradation of surface water has been well documented (Correll, 1998; Daniel et al., 1998; Parry, 1998; Sharpley et al., 1994, 2003). Current strategies used to reduce P in agricultural runoff water (i.e., buffer strips, cover crops, terracing), though well suited for reducing sediment in runoff, may not be as effective at reducing dissolved P (Sharpley et al., 1994, 2003).

Because of their high P sorption capacity, many researchers have proposed the use of drinking water treatment residuals to reduce P loading into surface or ground water (Dayton et al., 2003; Gallimore et al., 1999; Novak and Watts, 2004). Drinking water treatment residuals are often rich in amorphous Fe or Al oxides because of the use of Fe or Al salts for coagulation of source water to remove turbidity and taste and to speed sedimentation. The American Society of Civil Engineers (1996) reported that WTRs often contain 50 to 150 g amorphous oxide kg^–1. Several best management practices (BMPs) using WTR to reduce nonpoint source P pollution have been proposed. One is to surface-apply WTR to remove dissolved P from agricultural runoff water (Basta and Storm, 1997; Dayton et al., 2003; Gallimore et al., 1999; Haustein et al., 2000; Peters and Basta, 1996). Another beneficial use of WTR is to incorporate it into soil to reduce P solubility and prevent P leaching (Codling et al., 2000; Elliott et al., 2002a, 2002b; Novak and Watts, 2004; O'Connor et al., 2002; Peters and Basta, 1996). Beneficial use of WTR has also been expanded to reduce the solubility of P in organic soil amendments, such as manure or biosolids, by co-blending with WTR (Codling et al., 2000; Elliott et al., 2002b; Ippolito et al., 1999). Although all of these application methods have been successful in reducing P risk to water quality, a method is needed to determine the application amount of WTR required to achieve target reductions in P runoff or risk. Accurately determining WTR P sorption maximum is essential to effectively use it to reduce soluble P in agricultural runoff water, to reduce the solubility of P in agricultural soils or organic waste materials (biosolids, manure), and to calibrate WTR application.

The P sorption capacity of WTR varies widely due to differences in amorphous Al or Fe oxide content. Each treatment plant uses different source water and different treatment chemicals and processes, producing WTR with different chemical compositions and P sorption capacities. Dayton et al. (2003) examined the WTR components and/or chemical processes (precipitation, adsorption) that were thought to contribute to WTR P sorption. Using batch equilibration, P sorption isotherms were generated for 21 Al-based WTRs, and the linearized P_max was determined. A significant (r² = 0.69, significant at the 0.01 probability level) nonlinear relationship between the P_max and Al_ox content was found. No significant relationship was found between P_max and either soluble Ca (r² = 0.008) or clay content (r² = 0.063). In a simulated rainfall study using a subset of 11 of the Al-based WTRs, a significant relationship between runoff P reduction and Al_ox (r² = 0.44, significant at the 0.05 probability level) and P_max (r² = 0.44, significant at the 0.05 probability level) was also found (Dayton et al., 2003). Though significant, the low correlation coefficient for the relationship between P_max and Al_ox precluded it being a strong predictive tool for estimating P_max. Further, the nonlinearity of the relationship suggested that the amorphous metal oxide content may have been underestimated. Additionally, a subsequent incubation study where 100 g WTR was equilibrated with P (KH₂PO₄) solutions to supply from 0 to 4 g P kg^–1 WTR showed that, based on soluble P, WTR sorbed substantially more P than the calculated P_max predicted (Fig. 1) .

View larger version (27K): [in this window] [in a new window]   Fig. 1. Soluble P in two water treatment residuals (WTRs) spiked with KH2PO4 solutions and incubated up to 42 d. Initial Langmuir phosphorus adsorption maximum (Pmax) values were obtained using batch equilibration of WTR (<2 mm) with standard P solutions ranging from 0 to 100 mg P L–1 for 17 h.

	PHOSPHORUS SORPTION MAXIMUM

TOP NOTES ABSTRACT INTRODUCTION PHOSPHORUS SORPTION MAXIMUM AMORPHOUS ALUMINUM MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Olsen and Watanabe (1957) proposed using the linearized Langmuir model to determine the P_max in soil. Since then, many researchers have reported the use of the Langmuir model to determine P adsorption maxima in soil. However, the experimental protocol used to measure P_max is not standardized. Nair et al. (1984) proposed a standard procedure for generating P sorption isotherms for soil. They recognized that, while no single procedure is correct, sorption data could be compared between studies by standardizing a procedure. Their procedure recommended using 0.5 to 1 g soil and equilibrating (shaking) with 25-mL P solutions (in 0.01 M CaCl₂) ranging from 0 to 10 mg L^–1 for 24 h at room temperature. Soil particle size was not specified (Nair et al., 1984). Using a 7-d equilibration and a <2-mm particle size, Borggaard et al. (1990) found a significant relationship between P_max and oxalate-extractable Al in 14 Danish soils. McLaughlin et al. (1981) also used a 7-d equilibration to examine the P sorption capacity of a variety of Fe- and Al-based materials of varying degrees of crystallinity. O'Connor et al. (2002) found that using a single point sorption isotherm with a 24-h equilibration was inadequate to express the sorption capacity of Fe and Al WTR. They conducted additional studies using multipoint isotherms with an 8-d equilibration and found that the P loading (5000 mg P kg^–1) was insufficient for the Al WTR and that the Fe WTR had a P sorption capacity of 3400 mg P kg^–1 (O'Connor et al., 2002). Syers et al. (1973) used a 72-h equilibration and <2-mm particle size to generate P sorption isotherms. Fox and Kamprath (1970) found that an equilibration time of 6 d was necessary for P sorption stabilization in a highly weathered Hawaiian soil containing a large amount of Al_ox. Pautler and Sims (2000) found that in 41 agricultural soils the relationship between the total P sorption capacity and amorphous Al and Fe content was significant (r = 0.61, significant at the 0.01 probability level). After a 20-h equilibration, Börling et al. (2001) found a highly significant (r = 0.83 , significant at the 0.01 probability level) relationship between P_max and (Al_ox + Fe_ox) (where Fe_ox is acid ammonium oxalate–extractable iron) in 10 Swedish soils.

	AMORPHOUS ALUMINUM

TOP NOTES ABSTRACT INTRODUCTION PHOSPHORUS SORPTION MAXIMUM AMORPHOUS ALUMINUM MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Because of their strong affinity for oxyanions, including phosphate, amorphous metal oxides are often used to predict P retention or mobility in soil. A P saturation index is often used to quantify P risk to surface water from agricultural soils. The phosphorus saturation index (PSI) is expressed as:

[1]

where P_ox, Al_ox, and Fe_ox are oxalate-extractable P, Al, and Fe, respectively in mol kg^–1 (Sims et al., 1998). This index has been widely adopted and is a useful tool to characterize potential P risk.

Pautler and Sims (2000) found that, in 41 agricultural soils in Delaware, P solubility increased significantly (r² = 0.70) as soil P saturation increased. Elliott et al. (2002b) found that the P sorbing ability of different types of WTR (Al-, Fe-, or Ca-based), when added to a sandy soil amended with biosolids, could be predicted based on the (Al_ox + Fe_ox) of the WTR, with Al > Fe > Ca. Many other researchers have observed a relationship between soil P saturation and labile or runoff P (Maguire et al., 2001; Maguire and Sims, 2002; Pote et al., 1996; Sharpley, 1995; Sharpley et al., 2003). Hooda et al. (2000) found that the degree of soil P saturation was significantly related to soil P desorption. Similarly, Lookman et al. (1995) found that after 880 h, soil P desorption for 44 German soils was inversely related to soil (Fe_ox + Al_ox). Lu and O'Connor (2001) found increased P retention in a sandy, low P sorbing Florida soil by the addition of biosolids high in oxalate-extractable (Al + Fe). Decreasing the PSI of an organic waste by co-blending with a WTR high in amorphous oxide is a potentially useful way to decrease the PSI and therefore P solubility in the waste material.

The strong relationship between Al_ox and P_max can be a useful tool in utilizing WTR effectively. Routine methods are available for determining Al_ox in soil. However, amorphous metal oxide is often considerably higher in WTR (>50 g kg^–1) than in soil. Routine soil tests must be evaluated to discern their ability to accurately measure the amorphous Al, Fe, or P in WTR. Standardizing the experimental conditions used to measure amorphous Al, Fe, and P in WTR would allow for comparisons to be made between WTR studies. The aim of this study is to propose a method using Al_ox to predict the sorption capacity of WTR. Specific objectives of this study are to (i) evaluate and/or modify the acid ammonium oxalate extraction method for accurate determination of Al_ox in WTR, (ii) evaluate and/or modify experimental conditions used to generate P adsorption isotherms to accurately measure P sorption maxima in WTR, and (iii) identify experimental conditions that strengthen the relationship between Al_ox and P_max.

	MATERIALS AND METHODS

TOP NOTES ABSTRACT INTRODUCTION PHOSPHORUS SORPTION MAXIMUM AMORPHOUS ALUMINUM MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Eighteen WTRs were collected from utilities in Oklahoma and Pennsylvania. All WTRs were air-dried and crushed to <2.0 mm. All chemical analyses and determination were performed in triplicate. Blanks and check samples were included for all chemical determinations to meet quality assurance and quality control (QA/QC) requirements.

Chemical and Physical Characterization
A summary of WTR chemical and physical characteristics is presented in Table 1. Residual pH was determined in a 1:2 WTR to 0.01 M CaCl₂ solution using a glass electrode (McLean, 1982). Electrical conductivity (EC) was determined in a 1:2 WTR to deionized water solution (Rhoades, 1996). Total nitrogen and carbon were determined by the Dumas method using a Carlo Erba (Milan, Italy) 1500 series dry combustion analyzer (Bremner and Mulvaney, 1982). Particle size analysis was determined using the pipet method (Gee and Bauder, 1986). Extractable Fe (Fe_ox) and P (P_ox) were determined by acid ammonium oxalate extraction (McKeague and Day, 1993) using a 100:1 oxalate solution to WTR ratio. Inductively coupled plasma atomic emission spectroscopy (ICP–AES) was used for subsequent analysis of Fe and P. The PSI was determined as previously shown in Eq. [1] (Sims et al., 1998).

Phosphorus Sorption Capacity
Phosphorus sorption isotherms were generated by batch equilibration with standard phosphorus solutions using reagent-grade KH₂PO₄ in deionized water on an end-on-end shaker (1 g WTR to 10 mL P solution). Standard P solutions ranged from 0 to 3.5 g P L^–1. The P_max was determined using the linearized Langmuir model (Sparks, 1995). The conditions of the batch equilibration were varied for the determination of P_max. Residual particle size was reduced from the initial <2 mm (Dayton et al., 2003) to <150 µm. A separate study using five WTRs was conducted to determine when pseudo-equilibrium between WTR and P equilibration solution was established. In this study, WTRs (1 g, <150 µm) were equilibrated with P (KH₂PO₄) solutions (10 mL P solution) by shaking from 1 to 19 d. All solution P measurements were made using ICP–AES.

Serial Desorption
To evaluate the final calculated P_max, five WTRs were used in a desorption study. The WTR (1 g, <150 µm) was equilibrated with 10 mL of P solution for 6 d to produce a WTR containing slightly larger amounts of sorbed P than the calculated P_max. The P-spiked WTRs were subjected to serial extractions with 25 mL of 0.1 M KCl at 10 min, 2 h, 24 h, and then daily through 5 additional days. The cumulative P desorbed was measured and subtracted from the initial P concentration to calculate final P sorbed to WTR.

	RESULTS AND DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION PHOSPHORUS SORPTION MAXIMUM AMORPHOUS ALUMINUM MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Chemical and Physical Characterization
An overview of the chemical and physical characteristics of the 18 WTRs is presented in Table 1. Residual pH ranged from 5.3 to 7.8 with a mean of 7.1. A neutral pH is to be expected since pH adjustment is routinely done during drinking water treatment. The electrical conductivities (EC) were all well below the 4.0 dS m^–1 associated with saline soil (Brady and Weil, 1996) for salt-sensitive plants. The WTR EC ranged from 0.22 to 2.60 with a mean of 0.79. Total N ranged from 0.05 to 18.4 g kg^–1 with a mean of 7.55 g kg^–1. Total carbon ranged from 17 to 149 g kg^–1 with a mean of 73.4. The high total C levels found in many WTRs may be attributed to carbonate additions for pH adjustment, polymer addition during water treatment or dewatering, or perhaps to additions of activated carbon, which is used to remove taste and odor from source water. The clay content of the 18 WTRs ranged from 1.30 to 40.0% with a mean of 9.79%. The oxalate-extractable Fe_ox ranged from 0.12 to 12.3 g kg^–1 with a mean of 5.17 g kg^–1. By comparison, the mean Al_ox content (Table 2) is higher than the Fe_ox by a factor of 29.3 on a molar basis. The WTR oxalate-extractable P_ox ranged from 0.30 to 6.02 g kg^–1 with a mean of 1.39 g kg^–1. Not surprisingly the WTR P saturation index was low, ranging from 0.25 to 6.34% with a mean of 2.10%.

View larger version (19K): [in this window] [in a new window]   Fig. 2. Relationship between initial Langmuir phosphorus adsorption maximum (initial Pmax, <2 mm, and 17-h equilibration) and acid ammonium oxalate–extractable aluminum (Alox, 40:1 solution to water treatment residual [WTR] ratio) for 18 Al-based WTRs.

View larger version (20K): [in this window] [in a new window]   Fig. 3. Relationship between oxalate-extractable Al (100:1 or 200:1 solution to water treatment residual [WTR] ratio) and oxalate-extractable Al (40:1 solution to WTR ratio) for 18 Al-based WTRs.

A more accurate determination of Al_ox will improve the relationship between P_max vs. Al_ox and may allow an easy and reliable way to predict P_max. Accurate measurement of amorphous oxide is also important in co-blending WTR with an organic waste (biosolids, manure) or mixing WTR into high P soil. Decreasing the PSI of an organic waste or high P soil by blending with WTR high in amorphous oxide is a potentially useful way to decrease P solubility. An accurate Al_ox measurement is essential to predict the PSI of blended Al-based WTR.

Phosphorus Sorption Capacity
Improving the relationship between P_max vs. Al_ox in Al-based WTR is a prerequisite for using Al_ox to predict P_max without the onus of the batch equilibration required to generate P sorption isotherms. The P_max values were determined using the linearized Langmuir model (Sparks, 1995) and using the initial batch equilibration conditions to generate P sorption isotherms (initial P_max, <2-mm WTR and an equilibration time of 17 h) described by Dayton et al. (2003). Under these initial conditions, the initial P_max ranged from 0.66 to 16.5 g P kg^–1 with a mean of 3.93 g kg^–1 (Table 2) for the 18 WTRs in this study. However, in a subsequent incubation study (Fig. 1) of WTR incubated with added P solutions, considerably more P than predicted by the calculated initial P_max was sorbed by the WTR. Some of the variation found by Dayton et al. (2003) in the relationship between the initial P_max and Al_ox (40:1 solution to WTR) (r² = 0.69, significant at the 0.01 probability level) may have been due to observed differences in WTR slaking when exposed to equilibration solutions used to generate P sorption isotherms. Slaking may increase the surface area available to react and increase P sorption. Slaking of WTR occurs naturally due to weathering in the field, but differences in slaking during a short-term batch equilibration may be a source of variability. To mitigate the effect of slaking, WTRs were crushed and sieved to <150 µm. Using the same 17-h equilibration time, the P_max after crushing (crushed P_max) ranged from 1.84 to 29.5, representing a mean increase of 2.46-fold (Table 2) over the initial P_max.

To ensure adequate time is allowed to establish pseudo-equilibrium during the batch equilibration, P sorption was examined on a subset of five WTRs using equilibration times of from 1 to 19 d. Solution P decreased rapidly for the first 4 d of equilibration and then remained relatively constant (Fig. 4) . Figure 4 shows that the 17-h equilibration time used in Dayton et al. (2003) underestimated WTR P sorption capacity. Little change in P sorption occurred after 5 d of equilibration (Fig. 4). Six days were selected as the equilibration time for generating P sorption isotherms to be used to generate P_max (final P_max). With the reduced particle size (<150 µm) and the time for equilibration increased to 6 d, the final P_max ranged from 10.4 to 37.0 g P kg^–1 with a mean of 22.9 g kg^–1 (Table 2), representing a mean increase by a factor of 5.83 over the initial P_max.

View larger version (25K): [in this window] [in a new window]   Fig. 4. Solution phosphorus versus time for five Al-based water treatment residuals (WTRs) equilibrated for 19 d.

View larger version (25K): [in this window] [in a new window]   Fig. 5. Phosphorus sorbed versus time for five Al-based water treatment residuals (WTRs) extracted periodically with 0.1 M KCl over 6 d.

[2]

for this data set may provide a tool to estimate the P_max of Al-based WTR simply by measuring Al_ox. For this data set, the addition of Fe_ox (Al_ox + Fe_ox) did not improve (r² = 0.916, significant at the 0.001 probability level) the relationship (Fig. 6B). Many more WTRs will need to be examined and certainly field-testing will need to be done to validate these proposed procedures.

View larger version (20K): [in this window] [in a new window]   Fig. 6. Relationship between final Langmuir phosphorus adsorption maximum (final Pmax, <150 µm and 6 d equilibration) and (A) acid ammonium oxalate–extractable aluminum (Alox, 100:1 to solution to water treatment residual [WTR] ratio) and (B) acid ammonium oxalate–extractable aluminum + iron (Alox + Feox), 100:1 to solution to WTR ratio for 18 Al-based WTRs.

	CONCLUSIONS

TOP NOTES ABSTRACT INTRODUCTION PHOSPHORUS SORPTION MAXIMUM AMORPHOUS ALUMINUM MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

	NOTES

TOP NOTES ABSTRACT INTRODUCTION PHOSPHORUS SORPTION MAXIMUM AMORPHOUS ALUMINUM MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Salaries and support provided by state and federal funds appropriated to the Ohio Agricultural Research and Development Center, The Ohio State University, Columbus, OH 43210.

	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION PHOSPHORUS SORPTION MAXIMUM AMORPHOUS ALUMINUM MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 

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