Water Research 37 (2003) 2301–2310 Biosorption of humic and fulvic acids to live activated sludge biomass Mario Esparza-Soto*, Paul Westerhoff Department of Civil and Environmental Engineering, Arizona State University, Engineering Center Room, ECG 252, Tempe, AZ 85287-5306, USA Received 22 March 2002; accepted 5 December 2002 Abstract Biosorption of high molecular weight humic substances (HS) to activated sludge (AS) biomass may be considered as a preliminary step previous to enzymatic hydrolysis breakdown and biological uptake. Two standard HS, Suwannee River humic and fulvic acids, were biosorbed onto live AS biomass collected from full-scale wastewater treatment plants. Biosorption isotherms were corrected for interference from organic matter desorbed from AS biomass. The effect of pH, calcium and ionic strength on biosorption was tested. HS biosorption to live AS biomass obeyed the Freundlich isotherm equation. Biosorption increased with decreasing pH, increasing calcium and ionic strength concentration. Higher biosorption at low pH may be attributed to hydrophobic interactions between HS and AS biomass extracellular polymers (EPS). Hydrophobic and cationic bridging effects between HS and AS EPS were the mechanisms responsible for biosorption under the presence of divalent cations; however, the former was most significant at low pH, whereas the latter was predominant near neutral pH. The effect of ionic strength on HS biosorption followed the colloidal chemistry theory as the electric double layer became compressed when the ionic strengthincreased,resultingincloserapproachofHSandASbiomass.ThehumicacidfractionofSuwanneeRiverwas removed more efficiently than its fulvic acid fraction because the humic acid was more hydrophobic. These results showed that pH, divalent cation concentration and ionic strength play an important role in the fate and removal of influent wastewater HS in full-scale treatment plants. r 2003 Elsevier Science Ltd. All rights reserved. Keywords: Activated sludge biomass; Biosorption; Cation bridging; Extracellular polymers (EPS); Humic substances; Hydrophobic effect 1. Introduction Biosorption is described as the physical–chemical process of compound removal from solution by attach- ment on microbial surfaces. Many organic contaminants have been successfully eliminated from solution by biosorption under different conditions. These studies were performed to understand the interactions between bacteria and pollutants in the environment and treat- ment systems [1–3]. Most biosorption studies have used dried biological sorbants to study the removal of dissolved organic carbon (DOC) from drinking water as an alternative sorbant to powdered activated carbon [4–9]. Biosorption of humic acids onto bacterial surfaces has been also extensively studied to determine the fate and transport of contaminants in the subsurface [1,10,11].Biosorptionstudiesregardingbiologicalwaste- watertreatmenthavefocusedonboththebiosorptionof hazardous organic pollutants and DOC onto aerobic and anaerobic biomass [12–17]. This paper contributes to the understanding of DOC removal on live/hydrated *Corresponding author. Tel.: +1-480-727-6259; fax: +1- 480-965-0557. E-mail addresses: esparza1@imap2.asu.edu (M. Esparza-Soto), p.westerhoff@asu.edu (P. Westerhoff). 0043-1354/03/$-see front matter r 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0043-1354(02)00630-9