The effects of orthophosphate on the adsorption of natural organic matter (NOM) on aluminum hydroxide were investigated using three organic compounds as surrogates, including humic acid (HA), phthalic acid, and 2,3-clihydroxybenzoic acid (2,3-DIABA). The adsorption of phthalic acid and 2,3-DHBA was very limited compared to that of HA, whereas their adsorption was reduced Much more significantly than that of HA by phosphate. The efficiency of phosphate in reducing HA adsorption increased with increasing phosphate concentration. Phosphate adsorption was slightly reduced by plithalic acid and 2,3-DHBA but moderately Suppressed by HA. The adjacent carboxylic groups mainly contributed to the adsorption Of humic acid at low pH. while the adjacent phenol groups were responsible for the adsorption Of humic acid at high pH. HPLC-SEC and SUVA analysis revealed that HA molecules with high Molecular Weight were adsorbed preferentially but were easily displaced by the specifically adsorbed phosphate. TM-AFM images revealed that the aggregation of HA molecules and the protonation of carboxylic groups at 41 low pH facilitated the adsorption under acidic conditions. The presence of phosphate increases the coagulant dosage for NOM removal as some sites oil the coagulant precipitates become utilized by phosphate. (c) 2005 Elsevier Inc. All rights reserved.