The lack of knowledge regarding competitive adsorption of heavy metal ions onto water treatment residuals has been hindering their reuse as a medium in stormwater bioretention systems. Competitive adsorption of copper(II), lead(II), cadmium(II), and zinc(II) onto polyaluminium chloride and anionic polyacrylamide water treatment residuals (PAC-APAM WTRs) was evaluated with different pH, temperature, initial concentration, and time. The competitive adsorption removal increased with the increase of pH and temperature. The analysis of the ratios of maximum adsorption capacity of a heavy metal ionic species in a multi-component system to that in a mono-component system (Qmix/Qmono) demonstrated that the coexisting ion had a negative effect on the adsorption of a metal ionic species. The Langmuir model provided a better fit, indicating that the adsorption could be a monolayer adsorption process. The modified Langmuir isotherm studies showed that the affinity order in the multi-component systems was Cu2+>Pb2+>Cd2+>Zn2+. The pseudo-second-order model better described the adsorption kinetics implying that the competitive adsorption behavior could be interpreted by diffusion-based mechanisms. This study contributed to a better understanding the mobility of those frequently occurring heavy metal ions in stormwater runoff in the PAC-APAM WTRs media layer of stormwater bioretention systems.