Co-adsorption behaviors and mechanisms of Cd(Ⅱ), Pb(Ⅱ), and Cr(Ⅵ) on sodium dodecyl sulfate modified attapulgite clay-supported nano zero-valent iron: Competitive or synergistic effect?

Cadmium(Ⅱ), plumbum(Ⅱ), and chromium(Ⅵ) universally co-exist in the environment, greatly aggravating their environmental risks and elevating the difficulty of remediation. Herein, a novel sodium dodecyl sulfate modified attapulgite clay-supported nano zero-valent iron (SDS@ATP/nZVI) was exploited as the synchronous adsorbent of Cd(Ⅱ), Pb(Ⅱ), and Cr(Ⅵ) composite pollution. With the aid of the carrier SDS@ATP, nZVI was highly dispersed and became the primary active ingredient. The adsorption kinetics results showed that there existed a strong competitive effect between Cd(Ⅱ) and Pb(Ⅱ), while the co-existence of Cr(Ⅵ) accelerated the removal of Cd(Ⅱ) on SDS@ATP/nZVI. The adsorption capacity (qmax) of Cd(Ⅱ) in their binary system was only 71.3 mg/g and 0.34 times that of single Cd(Ⅱ) adsorption under pH = 6.0 and 298 K, whereas that of Pb(Ⅱ) (qmax = 558.7 mg/g) was almost equivalent, supporting that Pb(Ⅱ) adsorption had the priority and quickly reduced to Pb0 by Fe0. Meanwhile, residue Pb(Ⅱ) and Cd(Ⅱ) mightily competed for active sites of surface chemical adsorption of iron hydroxide. Similarly, the qmax of Cd(Ⅱ) and Cr(Ⅵ) in their binary system were 238.7 and 70.9 mg/g, which was 1.12 and 0.94 times to those of individual Cd(Ⅱ) and Cr(Ⅵ) adsorption, implying a synergistic effect of Cr(Ⅵ). Cr(Ⅵ) species firstly was reduced and simultaneously facilitated the generation of iron hydroxide, of which both free Cd(Ⅱ) and newly reduced Cr(Ⅲ) with iron hydroxide co-precipitated to constitute the amorphous Fe-Cr(Ⅲ)-Cd(Ⅱ) sediment. These findings also provide exhaustive inspiration and strategy for the remediation of multiple composite pollution.