## 1 Introduction
Its operations are affected by equilibrium (isotherm and capacity), kinetic (diffusion and convection coefficients) and hydraulic (liquid holdup, geometric analogies and mal-distribution) factors [28]. In practice, the influence of operative conditions on the overall system performance is not experimentally verified [17; 29; 30], but they are extremely important to large-scale development. Although the FBR system is highly valuable, its analysis is unpredictably multi-faceted [28; 31] and even more so with the presence of numerous interfering ions. Complications due to ion competition and solute-surface interactions [4], as well as the unique affinity sequences and sorbent material selectivity [25], have been reported. The authors have conducted a four-phase research project, consisting of the analysis of: (1) the effects of preliminary parameters and operative conditions (particle size, sorbent-to-sorbate dosage, influent concentration, contact time, set-temperature, heat pre-treatment), (2) heavy metallic ions component system combinations and selectivity order [32], (3) kinetic modelling trends [33], and finally in this current study, (4) the design of a packed fixed-bed, dual-column sorption system. The first three phases are conducted in batch mode, to which reveal a key trend among the HMIs selected as lead (Pb\({}^{2+}\)) >> iron (Fe\({}^{3+}\)) > copper (Cu\({}^{2+}\)) > zinc (Zn\({}^{2+}\)) > nickel (Ni\({}^{2+}\)). The findings of these preliminary phases have established a platform for the design of the sorption system in continuous mode, presented in this paper. Existing column experimental designs involve various limitations, including:  
1. The evaluation of predominantly single- or dual-component HMI system combinations;
2. The implementation of primarily slender column aspect ratios (i.e., bed depth/particle diameter, column height/diameter), causing a challenge to eventual scale-up design;
3. The use of inconsistent and/or vague sorbent compaction techniques, and;
4. The application of simple, idealized flow patterns (i.e., set single and continuous flow rate). The objective of this paper is to develop a novel dual-column sorption system to overcome some of these shortcomings. Important design factors such as the zeolite compaction, column dimensions and aspect ratios, flow control, sampling and analytical procedure, will be taken into consideration. The exclusivity of this prototype is attributed to an automated, variable-flow configuration with a custom sampling technique. In contrast to most previous single-component sorption set-ups, this study evaluates the simultaneous sorption process by natural zeolite of the five most commonly occurring HMIs found in AMD, including Pb\({}^{2+}\), Cu\({}^{2+}\), Fe\({}^{3+}\), Ni\({}^{2+}\) and Zn\({}^{2+}\). This paper will demonstrate the effectiveness and the removal efficiency in a continuous-flow FBR system over a 3-h duration from the dual columns, providing insights on HMIs selectivity and treatment system breakthroughs. It is envisaged that this research will provide much-needed information to the wastewater treatment industry for the design and implementation of innovative sorption technologies.