Co-transport of Cu²⁺, Pb²⁺, Cd²⁺, and Zn²⁺ in the columns of polyaluminium chloride and anionic polyacrylamide water treatment residuals

Knowledge of co-transport behavior of heavy metals in polyaluminium chloride and anionic polyacrylamide water treatment residuals (PAC-APAM WTRs) is critical if PAC-APAM WTRs are to be used as a layered media of stormwater bioretention systems to effectively remove contamination of some heavy metals. Column studies of PAC-APAM WTRs were conducted to investigate the transport behavior of Cu²⁺, Pb²⁺, Cd²⁺, and Zn²⁺ in mono-component, binary, ternary, and quaternary systems, and the impact of the presence of one or more heavy metals on the retention and mobility of the other heavy metal. The analysis of breakthrough curves and the retardation factors (R_f) demonstrated that the selectivity order of heavy metal ions in the quaternary system was primarily governed by electronegativity with a higher electronegativity being more favorable to adsorption. The mobility of the tested heavy metals through PAC-APAM WTRs was in the order of Zn²⁺ (~91% R_f decrease) > Cd²⁺ (~55% R_f decrease) > Cu²⁺ (~75% R_f decrease) > Pb²⁺ (~66% R_f decrease) with predominant retention of Pb²⁺ and Cu²⁺. The competitive adsorption was found to lower the retardation factor and the average maximum adsorption capacity (Cu²⁺: ~71% decrease; Pb²⁺: ~67% decrease; Cd²⁺: ~54% decrease; and Zn²⁺: ~81% decrease) and facilitated the transport in the tested systems. The adsorption of a metal became more reversible due to competitive adsorption. It is inferred that the presence of some heavy metals in the stormwater runoff would increase the mobility of other metals in the PAC-APAM WTRs layer of a bioretention system.