## 1 Introduction  
Acid mine drainage (AMD) generated by industrial mines contains highly acidic wastewater and toxic heavy metallic ions (HMIs). These HMIs are a serious threat to human health and the environment [1; 2; 3; 4; 5], with their high solubility [6], as well as non-biodegradable and bio-accumulative properties [7; 8]. If mines are abandoned, there is a risk of AMD contamination to both surface and groundwater, causing catastrophic damage to the ecosystem [4; 9]. Therefore, the HMIs must be removed by advanced treatment methods prior to discharge [10; 11; 12], to abide by the effluent maximum allowable limits [7; 8]. Due to the high site-to-site variability present in mines, mitigation feasibility can become a challenge [3], and a simple, resilient and cost-effective strategy must be developed [1]. Various industrial wastewater treatment methods include precipitation, electro-chemical remediation, oxidation and hydrolysis, neutralization, ion-exchange and solvent extraction, ion-exchange and precipitation, titration, bio-sorption, adsorption, reverse osmosis [4; 9], and ultrafiltration [13]. Among these, sorption is a simple but promising treatment method [5; 14], based on demonstrated industrial viability and effectiveness, cost efficiency and environmental sustainability [15; 16]. The removal of HMIs is attributed to the mechanisms of both adsorption (on the surface of the sorbents' micropores) and ion-exchange (through the sorbents' framework pores and channels) [17]; and culminate in the unified process of sorption [18; 19]. Zeolites hold great potential as naturally occurring minerals [1], and are recognized as effective materials for the removal of HMIs from industrial wastewater by sorption [20]. They have attracted a lot of researchers' interests [11], due to its coexisting molecular sieve action, ion-exchange and catalytic properties [14; 21]. The mineral zeolite is a hydro-aluminosilicate with a crystalline structure comprised of SiO\({}_{4}\) and AlO\({}_{4}\) tetrahedras linked by oxygen atoms, which form an open, homogeneous microporous three-dimensional framework creating voids and channels [21]. Clinoptiolite, a globally abundant and well-documented form of zeolite [13; 20; 22; 23], is used in this research. Two of the most significant properties of zeolite are its high cation exchange capacity [23], and its selectivity of certain metals [24]. In addition to wide deposit distribution and low exploitation costs [7; 25], zeolite is considered as a strong candidate for the removal of wastewater contaminants [26], especially those containing high HMIs from mine waste. Currently, there is limited comprehension of the sorption by zeolite for waste with multi-component HMIs [24; 27], in order to fully benefit from using it in tertiary treatment processes [1]. In particular, the composition of AMD waste contains numerous contaminants, which include HMIs and other pollutants, and the presence of these in solution affect the overall removal potential [3; 18]. Research is needed on the simultaneous sorption of the multi-metallic components that are prevalent in industrial effluent, and to quantify uptake interference of these HMIs in combination [16]. [PERSON] [18] (p. 201) points out that for multi-component systems, the exchange rate may vary for the various counter-ions of HMIs in solution, with the possibility that the concentrations of certain species in either the sorbent or solution may fluctuate prior to attaining its balanced state. The performance of columns or fixed-bed reactors (FBR) is convenient for industrial scale applications [7], which requires less investment and operational costs, and is more economically feasible than its discontinuous batch mode counterparts [8]. FBR columns have demonstrated performance efficiency in treating large volumes, and are frequently implemented in sorption studies. However, the removal of multi-component solutions of HMIs is complex due to ion competition, different affinity sequences, and zeolite selectivity.