Surface Complexation Modeling of Phosphate Adsorption by Water Treatment Residual

Use of water treatment plant residuals (WTR), as a soil amendment is a promising alternative to landfill disposal. Unfortunately, WTR has a propensity to bind with phosphate, which is an important plant nutrient. Phosphate may be added to WTR prior to soil application. This type of pretreatment may convert WTR from a phosphate consumer to a phosphate supplier. The binding of phosphate to WTR is typically attributed to surface complexation with metal oxides. However, attenuated total reflectance Fourier transform infrared (ATRFTIR) data and phosphate-WTR adsorption equilibrium data indicate that phosphate also binds to a cationic polyelectrolyte that is added during water treatment processes. Using the FITEQL optimization program, equilibrium constants and total number of surface sites were determined for the polymer. Results from the FITEQL optimization were used to model binding of phosphate by cationic polymer. Binding of phosphate by hydrous ferric oxide was modeled using a diffuse double layer model, which included surface precipitation (MICROQL). The model was validated through the use of phosphate equilibrium partitioning data at pH values of 6 and 8. The model predicted that a significant fraction of phosphate adsorbed onto WTR is associated with the cationic polymer. T HE water treatment industry generates large quantities of solid residual annually. In the past, landfilling of WTR has been the preferred method of disposal. The current emphasis on pollution prevention and limitations in landfill space have encouraged water treatment plant operators to seek beneficial reuse alternatives for WTR. Use of WTR, as a soil amendment is one such alternative. This option is desirable because little or no residue treatment is required prior to application. The primary constituents of WTR residuals, iron and aluminum, are found in relatively large concentrations in the lithosphere, are less toxic than most other metals, and are not considered hazardous by the U.S. Environmental Protection Agency (USEPA). Over the past two decades, research has been conducted to determine the environmental impact of land-applied aluminum and iron residues (Elliot et al., 1990; Novak et al., 1995). Benefits associated with use of WTR as a soil amendment include improved soil structure (E1-Swaify and Emerson, 1975), increased moisture-holding capacity (Bugbee and Frink, 1985, p. 1-7) and increased availability of nutrients for various plants (Heil and Barbarick, 1989). Soil properties govern the significance of each of these benefits. For example, Heil and Barbarick (1989) reported that addition of ferric hydroxide resiM.A. Butkus, Dep. of Geography and Environmental Engineering, The United States Military Academy, West Point, NY 10996-1695; D. Grasso and C.P. Schulthess, Environmental Engineering Program; and C.P. Schulthess and H. Wijnja, Dep. of Plant Science, Univ. of Connecticut, Storrs, CT 06269-2037. Received 3 Sept. 1997. *Corresponding author ([email protected]). Published in J. Environ. Qual. 27:1055-1063 (1998). due, at low application rates to an iron deficient soil, could improve plant growth until available phosphate becomes limited. The most significant shortcoming associated with using WTR, as a soil amendment is the reaction that occurs with phosphate (Elliot et al., 1990). A strong reaction between oxides, which make up a significant fraction of WTR, and phosphate (Novak et al., 1995) results in decrease in the quantity of plant available phosphate (Parfitt, 1979; Heil and Barbarick, 1989). However, these reports have not identified and quantified the various components of WTR, which have an impact on WTR-phosphate complexation. The chemistry of WTR generated at water treatment plants that treat surface waters, is a function of (i) the type and purity of metal salts added, (ii) the quantity and type of organic polyelectrolytes added, (iii) disinfection agents added, (iv) sand from drying lagoons (which may be combined with WTR during dewatering operations), and (v) raw water characteristics. These characteristics may significantly impact the complexation of phosphate. The liability associated with WTR-phosphate complexation may be overcome by amending WTR with phosphate prior to land application. Moreover, if sufficient phosphate is added, the WTR may be converted from a phosphate consumer to a phosphate supplier (Butkus, 1997). A surface complexation model, which quantifies the speciation of phosphate in WTR, may be of significant utility to both practitioners and regulators (Butkus, 1997). Models of WTR-phosphate complexation in the literature that allow for such quantification are unavailable. Although WTR typically contains a large fraction of an amorphous metal hydroxide precipitate (aluminum or iron), it may also contain a significant quantity of organic polymers that are often added during water treatment processes (Butkus, 1997). We hypothesize that phosphate may also bind to such ancillary WTR components. These forms of bound phosphate may be more labile (and possibly available to plants) than phosphate that is bound to oxides and may govern surface reactions in different pH regions. The purpose of this work is to quantify the effects of two WTR components, ferric hydroxide and a sorbed quaternary polyamine added during water treatment operations, on the binding of phosphate as a function of pH. An accepted surface complexation model that can predict the speciation of phosphate in a multicomponent WTR system was used. Quantification of the distribution of phosphate in this system may allow for accurate prediction of available phosphate in land application scenarios. Abbreviations: WTR, water treatment plant residuals; ATR, attenuated total reflectance; TOTFe, total iron; ATR-FTIR, attenuated total reflectance Fourier transform infrared; TOC, total organic carbon.