Transport Behavior of Cu2+ Under Binary and Multi-Component Systems in the Columns of Polyaluminium Chloride and Anionic Polyacrylamide Water Treatment Residuals: Implication for Reuse in Stormwater Bioretention Systems

The lack of information from column studies of Cu2+ transport in polyaluminium chloride and anionic polyacrylamide water treatment residuals (PAC-APAM WTRs) has been hampering their reuse as a media in stormwater bioretention systems. The transport behavior of inactive tracer Br? and Cu2+ in various systems with and without the presence of Pb2+, Cd2+, and Zn2+ was investigated in the columns of PAC-APAM WTRs under saturated steady flow conditions. Modeling Br? breakthrough curves (BTCs) using CXTFIT revealed that the transport of a solute in the columns was an equilibrium process almost without stagnant water, with all active sites on PAC-APAM WTRs being instantaneously available for the adsorption reaction during transport (?p?=?0.99 and ?L?=?100). The results of modeling Cu2+ BTCs with HYDRUS-1D showed that the chemical non-equilibrium two-site model (R2 0.8911–0.9999; RMSE 0.00–0.05) described the experimental data of Cu2+ better than the equilibrium convection–dispersion model using the linear isotherm (R2 0.4877–0.9901; RMSE 0.02–0.12) and the Langmuir isotherm (R2 0.7083–0.9938; RMSE 0.01–0.10). The fraction of instantaneous adsorption sites at the equilibrium (fe) of Cu2+ decreased with the increase of co-existing heavy metal ions from 0.27 (monocomponent system) to 0.06 (quaternary system) due to competitive adsorption with the Cu2+, Cd2+, and Zn2+ systems as an exception. The first kinetic rate (?) of Cu2+ increased with the increase of competitive heavy metal ions from 0.0076 (monocomponent system) to 0.0410 (quaternary system). This research could contribute to the understanding of Cu2+ transport and potential leaching in stormwater bioretention systems.

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