Adsorption of ng L−1-level arsenic by ZIF-8 nanoparticles: application to the monitoring of environmental water

Abstract

The provisional contamination level of arsenic in drinking water is 10 μg L⁻¹. For decreasing this value to a safer level, a more precise method for analyzing dissolved arsenic is required. With this aim, we synthesized zeolitic imidazolate framework-8 (ZIF-8) in the aqueous phase and characterized its potential application for monitoring the trace arsenic in fresh water. In this regard, we report following three notable outcomes. First, we demonstrate the excellent performance of ZIF-8 nanoparticles (nZIF-8) for the adsorption of ng L⁻¹ levels of AsO₄³⁻. nZIF-8 is able to adsorb over 99% of arsenic from as low as 10 ng L⁻¹ AsO₄³⁻ solutions. This performance was maintained even in the presence of commonly coexisting anions, for example, >90% adsorption from a 0.1 μg L⁻¹ arsenic solution was observed in the presence of 10 mg L⁻¹ of Cl⁻, NO₃⁻, CO₃²⁻, or SO₄²⁻, or 1 mg L⁻¹ of PO₄³⁻. Second, we clarified that the mechanism of arsenic adsorption by ZIF-8 is simply a ligand exchange process, in which the As(V) oxide anion replaces the imidazolate unit in the framework. Third, we propose a handy scheme for the analysis of ng L⁻¹ levels of arsenic in drinking water, in which nZIF-8 is used for the concentration of trace level AsO₄³⁻. By doing this, as low as 100 ng L⁻¹ arsenate in drinking water can be quantified by colorimetric analysis, the detection limit of which is 5 μg L⁻¹ in pure water. The application of this scheme is expected to highly enhance AsO₄³⁻ detection first by concentrating it to an easily detectable range, and second by excluding the majority of interfering ions present in the system. Therefore, a reduction in the minimum quantifying limit of arsenic in fresh water to as low as 1 ng L⁻¹ can be expected if the method is coupled with ICP-MS.