Abstract
The capacity and mechanism of the adsorption of aqueous barium (Ba), cobalt (Co), strontium (Sr), and zinc (Zn) by Ecuadorian (NatAllo) and synthetic (SynAllo-1 and SynAllo-2) allophanes were studied as a function of contact time, pH, and metal ion concentration using kinetic and equilibrium experiments. The mineralogy, nano-structure, and chemical composition of the allophanes were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, transmission electron microscopy, and specific surface area analyses. The evolution of adsorption fitted to a pseudo-first-order reaction kinetics, where equilibrium between aqueous metal ions and allophane was reached within <10 min. The metal ion removal efficiencies varied from 0.7 to 99.7% at pH 4.0 to 8.5. At equilibrium, the adsorption behavior is better described by the Langmuir model than by the Dubinin–Radushkevich model, yielding sorption capacities of 10.6, 17.2, and 38.6 mg/g for Ba 2 + , 12.4, 19.3, and 29.0 mg/g for HCoO 2 − ; 7.2, 15.9, and 34.4 mg/g for Sr 2 + ; and 20.9, 26.9, and 36.9 mg/g for Zn 2 + , by NatAllo, SynAllo-2, and SynAllo-1, respectively. The uptake mechanism is based on a physical adsorption process rather than chemical ion exchange. Allophane holds great potential to effectively remove aqueous metal ions over a wide pH range and could be used instead of other commercially available sorbent materials such as zeolites, montmorillonite, carbonates, and phosphates for special wastewater treatment applications.
Highlights
Pollution of groundwater and drinking water by heavy metals is increasingly becoming a major environmental issue worldwide
Heavy metals are typically released by the weathering of rocks and minerals and from a wide range of human activities such as agriculture, energy production, manufacturing, transportation, mining, and waste disposal [1]
There is a high demand for the development of new techniques and materials for wastewater treatment
Summary
Pollution of groundwater and drinking water by heavy metals is increasingly becoming a major environmental issue worldwide. Heavy metals are typically released by the weathering of rocks and minerals and from a wide range of human activities such as agriculture, energy production, manufacturing, transportation, mining, and waste disposal [1]. They often cause deleterious effects on aquatic life and to the terrestrial environment [2,3]. Contamination of surface water and groundwater with toxic and/or cancerogenic heavy metals and its subsequent use as a source of drinking water. Different techniques for the removal of aqueous metal ions have been developed and are routinely used in practice such as adsorption, precipitation, ion exchange, flocculation, sedimentation, membrane filtration, and electrochemical methods [8,9]
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