Flow-through passive sampler for zinc in freshwaters free from flow pattern, water cationic composition and temperature effects

Abstract

Providing reliable passive sampling results for the time-weighted average concentrations of pollutants in an aquatic system requires the effects of its flow pattern, temperature and composition to be either eliminated or taken into account. This study focused on the elimination of the water cationic composition effect (i.e., cations that compete with Zn2+ in terms of extraction, such as Na+, K+, Ca2+, and Mg2+) and taking into account the temperature effect on the performance of a recently developed Zn2+ flow-through passive sampler (FTPS) that acted independently of the flow pattern. The reliability issues associated with all three environmental variables mentioned above were thus successfully addressed. The FTPS was composed of 3 glass vessels (accommodating an acidic receiving solution) each attached to a flow-through compartment. The source solution was pumped through it at a constant rate by a piezoelectric micro pump and its temperature was monitored continuously using a thermistor to allow temperature correction. A polymer inclusion membrane (PIM) separated the flowing source solution from the receiving solution of each glass vessel. PIMs containing as extractants either dinonylnaphthalene sulfonic acid (DNNS) or di(2-ethylhexyl) phosphoric acid were tested to assess the effect of the water cationic composition. The Zn2+ accumulation in the receiving solution of both PIM-based FTPSs decreased as the complexity of the source solution (i.e., concentration of the competing cations) increased, although the accumulation was not significantly different when the DNNS-based PIM was exposed to synthetic source solutions representing typical soft, hard and very hard waters with electrical conductivities in the range of 400 – 1400 µS cm−1. This allowed a single laboratory calibration of the FTPS using synthetic environmental water to be used for a wide range of environmental waters. A portable FTPS, powered by a battery and a solar panel, was deployed together with conventional dip-in passive samplers in a freshwater wetland. The FTPS results were in agreement with the average Zn2+ concentration acquired by concurrent spot sampling unlike the conventional passive samplers’ results.