Dual functional sites strategies toward enhanced heavy metal remediation: Interlayer expanded Mg-Al layered double hydroxide by intercalation with L-cysteine

The discharge of toxic heavy metals poses a serious threat to human health and environment. The existing water purification systems are lack of promising materials for rapid, efficient, and cost-efficient remediation of numerous toxic heavy metals. Herein, we report on the development of L-cysteine (Cys) intercalated Mg-Al layered double hydroxide (MgAl-LDH/Cys) with a loose lamellar porous architecture as an efficient and economically viable adsorbent for Pb(II) and Cd(II) removal. The intercalation with Cys creates dual functionality, i.e., the interlayer expansion accelerates the diffusion of heavy metals, while Cys acts as additional capture sites for heavy metals. Therefore, remarkable high maximum sorption capacities of 279.58 and 135.68 mg g-1 for Pb(II) and Cd(II) were obtained for MgAl-LDH/Cys compared to those for pristine MgAl-LDH (30.15 and 36.77 mg g-1). MgAl-LDH/Cys exhibits also much faster sorption kinetics in comparison with MgAl-LDH. Such enhancements are attributed to the intercalation of the chelating agent Cys in the MgAl-LDH interlayer channels. Moreover, it is proposed that the adsorption mechanisms involve the isomorphous replacement of Mg sites by Cd(II) forming CdAl-LDH, the precipitation of PbS and CdS, and the chelation of sulfhydryl, carboxyl and amine groups toward Cd(II). Altogether, its facile and environmentally friendly fabrication, ultrahigh sorption efficiencies, and rapid kinetics demonstrate that MgAl-LDH/Cys has potential for practical applications in heavy metal remediation.