Phosphorus Immobilization in Micropores of Drinking-Water Treatment Residuals:  Implications for Long-Term Stability

Abstract

Drinking-water treatment residuals (WTRs) can immobilize excess soil phosphorus (P), but little is known about the long-term P retention by WTRs. To evaluate the long-term P sorption characteristics of one Fe- and one Al-based WTR, physicochemical properties pertinent to time-dependency and hysteresis of P sorption were assessed. This study also investigated the P sorption mechanisms that could affect the long-term stability of sorbed P by WTRs. Phosphorus sorption kinetics by the WTRs exhibited a slow phase that followed an initial rapid phase, as typically occurs with metal hydroxides. Phosphorus sorption maxima for both Fe- and Al-based WTRs exceeded 9100 mg of P kg(-1) and required a greater specific surface area (SSA) than would be available based on BET-N2 calculations. Electron microprobe analyses of cross-sectional, P-treated particles showed three-dimensional P sorption by WTRs. Carbon dioxide gas sorption was greater than N2, suggesting steric restriction of N2 diffusion by narrow micropore openings. Phosphorus-treated Co2 SSAs were reduced by P treatment, suggesting P sorption by micropores (5-20 A). Mercury intrusion porosimetry indicated negligible macroporosity (pores > 500 A). Slow P sorption kinetics by WTRs may be explained by intraparticle P diffusion in micropores. Micropore-bound P should be stable and immobilized over long periods.

Inorganic arsenic sorption by drinking-water treatment residual-amended sandy soil: effect of soil solution chemistry

“…The present study demonstrated the effectiveness of WTRs in removing As(V) from a low Fe/Al hydroxide containing sandy soil under a varying set of soil solution chemistry. The Al-WTR-amended soil had a greater As(V) sorption capacity than that of Fe-WTR-amended soil, consistent with previous experiments (Makris et al 2004;Nagar et al 2010). For lowest As load (125 mg kg -1 ), no pH-dependent effect was observed on As(V) sorption by Al-or Fe-WTRamended soil at both application rates (25 and 50 g kg -1 ).…”

“…Initial arsenic load was 125 mg kg -1 . Data are the mean of three replicates ± one standard deviation more pronounced compared to Al-WTR-amended soil due to comparatively smaller external and internal specific surface area of Fe-WTR for anion sorption (Makris et al 2004). Arsenic(V) sorption (at a 25 g kg -1 application rate of Fe WTR) decreased to 97, 84, and 60 % at pH 3 and As:P ratios of 1:1, 1:2, and 1:5, respectively, which further decreased to 76, 57, and 55 % after increasing the pH to 8, compared to the control (without P) treatment (Fig.…”

Selenium adsorption to aluminum-based water treatment residuals

“…Our results suggest that Se(0) may possibly be entrained in WTR microsites to such a degree that it was kinetically difficult to oxidize. Entrainment and long-term stability has been shown to occur for phosphate trapped in WTR micropores [4]. Oxic adsorption of Se(IV) onto WTR followed by purging with compressed air for 28 days also shows no apparent change in Se speciation, Se-O and Se-Al shell bond distance and coordination number, further supporting the contention that Se(IV) microsite adsorption is aiding in retention and oxidation resistance.…”

“…Water treatment residuals (WTRs), a waste product of drinking water treatment facilities, tend to have a mineral form similar to amorphous Al(OH) 3 when alum is utilized. Because of their amorphous nature, WTRs have a large surface area (up to 105 m 2 g À1 ) [4] and are highly reactive. They have the proven ability to adsorb tremendous quantities of P [5,6] and have been shown to adsorb other oxyanions such as As(V), As(III), and ClO À 4 [7,8].…”

Characteristics and mechanisms of phosphate adsorption on dewatered alum sludge

“…7 a), implying the slow P-adsorption process. Makris et al [24] opined that the slow P-adsorption process of drinking water treatment plant sludge might be explained by intraparticle phosphate diffusion in micropores.…”