Goethite nanoparticle aggregation: effects of buffers, metal ions, and 4-chloronitrobenzene reduction

Abstract

Iron mineral systems are effective at transforming highly oxidized contaminants in natural and engineered systems. The rate at which the contaminant degrades may be influenced by the amount of available mineral surface area. This study used dynamic light scattering and cryogenic transmission electron microscopy to monitor changes in the aggregation state of goethite nanoparticles before and after reaction with 4-chloronitrobenzene (4-ClNB). The effects of buffer identity, buffer concentration, and adsorbing metal identity on the goethite nanoparticle suspension characteristics and reactivity were monitored. Results demonstrate that buffers, which serve to hold pH nearly constant over the course of a reaction, are not benign additives in batch reactors. In fact, the identity and concentration of the buffer used strongly influences the rate of 4-ClNB degradation by surface-associated ferrous ion. Increasing buffer concentration resulted in more compact goethite nanoparticle aggregates, and slower 4-ClNB degradation was observed. In addition, the rate of degradation changed dramatically with changing buffer identity, with rates of reaction changing by an order of magnitude when switching from 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) to tris(2-hydroxyethyl)amine (TEA). Finally, of the three metal ions selected (Ca(II), Fe(II), and Co(II)), the addition and adsorption of the two transition metals resulted in a dramatic decrease in the average nanoparticle aggregate size. Furthermore, oxidation of the adsorbed Fe(II), via O₂ or 4-ClNB reduction, yielded decreases in zeta potential and increases in aggregate size. This work demonstrates that small changes in reaction parameters have a large effect on the rate of contaminant degradation through changes in nanoparticle aggregation state.