The efficacy of passive treatment systems for remediation of acid rock drainage (ARD) can be limited by the seasonal flux of discharge volume and metal concentrations that may not have been considered during treatment design. Drainage and metal variability can lead to surface passivation, precipitation/clogging, and flow bypass, thereby reducing the efficacy and life of a treatment system. Passive treatment systems are often implemented in sites that are difficult to access, where a low maintenance option is preferred and cost effective. Such sites tend to be in upper watersheds that experience intense periods of precipitation and(or) snowmelt. Current passive treatment systems commonly are designed and constructed to an average or limited range of flow and concentration ranges assessed from a small sampling of data; and such systems are difficult and expensive to alter following construction. In cases of highly variable ARD conditions, complimentary treatment systems may assist in reducing metal content and acidity to lessen the variable range of hydrochemical conditions for downstream primary treatment systems. For this study, the treatment efficacy of Fe removal from acidic solutions was compared for synthetic and natural silicate substrates—bare silica glass fiber, functionalized silica glass fiber, and zeolite [(Na,K,Ca)_2-3Al₃(Al,Si)₂Si₁₃O₃₆·12H₂O]. Each substrate was immersed in acidic (pH 3) solutions containing aqueous Fe(II) from the dissolution of Fe(II)SO₄·7H₂O to simulate acidic, metal laden rock drainage. For static batch sorption testing substrates were placed in nylon strainers, suspended in acidic Fe solutions, and mixed using a shaker table while periodically monitoring equilibrium Fe(II) concentrations. Results of the batch sorption experiments indicate a sorption efficacy order of zeolite > functionalized silica fiber > bare silica fiber. Following batch sorption experiments, Fe(II) sorption efficacy of the substrates were evaluated under dynamic flow conditions through the use of a peristaltic pump to input acidic Fe(II) solution through permeable samples of the substrates, contained in 5-cm diameter PVC pipe. Zeolite consistently removed greater quantities of Fe(II) from solution than other substrates and was selected for development of a new passive and modular ARD treatment system. This new treatment system design consists of a series of interconnected PVC pipes and removable, zeolite-filled cartridges that can be assembled in the field. The goal of the new system is as a complimentary system for installation upstream of a primary treatment system to reduce the impact of variation in ARD conditions. The modular design allows for the easy expansion of the system through addition or refreshing of treatment cartridges to ensure sufficient contact time for effective treatment under possible variations in flow volume and metal concentrations.