The widely used p-lactam antibiotics are susceptible to oxidative and/or hydrolytic degradation promoted by some metal ions (e.g., Cu(II)). Ferric ions (Fe(III)) are among the most common metal ions, but their role in the environmental transformation and fate of beta-lactam antibiotics is still unknown. This study elucidates that Fe(III) can promote degradation of p-lactam antibiotics under environmental aquatic conditions. Degradation rate constants of ampicillin (AMP) linearly increased with increasing Fe(III) concentration, but were independent of AMP concentration when AMP was higher than Fe(III) concentration. Neutral pH was most favorable for Fe(III)-promoted degradation of AMP, and the promoted degradation was also significant in real surface water and wastewater matrix. Among the various beta-lactam antibiotics, Fe(III)-promoted degradation of penicillins was faster than that of cephalosporins. Product analysis indicated that only two isomers of hydrolysis products were observed without detection of oxidation products. The Fe(III)-promoted degradation likely occurred via complexation of beta-lactam antibiotics with carboxyl group and tertiary nitrogen, and then enhancing the hydrolytic cleavage of beta-lactam ring. This study is among the first to identify the role of Fe(III) in the degradation of beta-lactam antibiotics and elucidate the mechanism. The new findings indicate iron species are among the factors affecting the environmental fate of beta-lactam antibiotics. (C) 2017 Elsevier B.V. All rights reserved.