The development of electrochemical methods for determining nanoparticles in the environment. Part II. Chronoamperometric study of FeS in sodium chloride solutions

Abstract

Environmental contextIn anoxic environments FeS is both an important mediator in the Fe and S biogeochemical cycles and plays a vital role in controlling the scavenging and availability of many trace metals. Electrochemical detection of colloidal and particulate FeS in natural waters can be done by voltammetric measurements. The recorded anodic waves, however, are rather qualitative and lack information on the FeS concentration and size distribution. AbstractThe interactions of FeS nanoparticles (NPs) with a hanging mercury drop electrode in NaCl solutions were monitored by chronoamperometric measurements. Collisions of FeS NPs with the mercury surface were studied over a wide range of electrode potentials (between 0 and –1.9V v. Ag/AgCl). Faradaic impact transients were recorded only at the negative potentials (between –1.5 and –1.9V). It was shown that the mercury electrode surface modified with a FeS adlayer catalyses sodium reduction by shifting the potentials of this process to more positive values. This catalytic process together with possible hydrogen evolution is assumed to be the physicochemical basis for the determination of FeS NPs. Chronoamperometric measurements at the electrode potential of –1.9V showed that the reduction processes of sodium and hydrogen on FeS NPs upon collision are the main cause of sharp reduction current transients. At sufficiently positive electrode potentials (~–1.5V) the colliding FeS NPs would not be immediately repelled; instead they remained adhered to the mercury surface, causing ‘staircase-like’ chronoamperometric signals. It appears that recorded reduction current transients are carrying FeS NPs’ size information, which is consistent with parallel dynamic light scattering (DLS) measurements.