Distinct effects of Fe3+ on the adsorption of chromate and arsenate: A comparison of iron-bearing ferrihydrite and nano-TiO2 absorbents

Fe3+, a particularly prevalent ion in acid mine drainage (AMD), has a substantial impact on the crystallization of iron-bearing minerals and the immobilization of multiple co-existing oxyanions such as Cr(VI) and As(V). To investigate the varied effects of Fe3+ on Cr(VI) and As(V), ferrihydrite (Fh) and nano-TiO2 were selected due to their superior adsorption effectiveness and distinctive stability. As a result of the dissolution of Fe3+ in Fh and its subsequent promotion to As(V) adsorption, As(V) adsorption capacity on Fh (124.0 mg/g) was twice that of TiO2 (62.76 mg/g). The co-existence of Fe3+ in the As(V) adsorption system greatly enhanced As(V) sorption on TiO2 (18.9–56.5%) and Fh (1.03–15.7%), and an additional Fe-As complex was confirmed to contribute extra As(V) removal on both Fh and TiO2 by the FTIR and EXAFS spectra. On the other hand, TiO2 (18.15 mg/g) and Fh (19.50 mg/g) exhibited nearly identical Cr(VI) adsorption capabilities, and the co-presence of Fe3+ even slightly blocked Cr(VI) sorption on Fh. The spectroscopic and modeling results showed that the monodentate Cr(VI) adsorption complex was blocked by Fe3+, but the outer-sphere and bidentate inner-sphere Cr(VI) surface complexes were unaffected. Additionally, in the ternary As-Cr-Fe adsorbate systems, Cr(VI) had little effect on the development of As-Fe surface complexes or precipitates. With the interference mechanisms, actual AMD wastewater adsorption on Fh and TiO2 was well explained and predicted. The findings would offer fresh perspectives on the bioavailability and fate of hazardous metal compounds in the AMD-polluted locations.