As(III) removal by modified granular natural siderite (MGNS) was less affected by NOM. Adsorbed NOM inhibited As(V) adsorption more significantly than dissolved NOM. The inhibitory effect of fulvic acid on As(V) removal was stronger than humic acid. As(V) removal was adversely affected by competition of dissolved fulvic acid. As(V) removal was due to adsorption and aggregation of complexes with humic acid.