Abstract
Dissolved silicate ions in wet and dry soils can determine the fate of organic contaminants via competitive binding. While fundamental surface science studies have advanced knowledge of binding in competitive systems, little is still known about the ranges of solution conditions, the time dependence, and the molecular processes controlling competitive silicate–organic binding on minerals. Here we address these issues by describing the competitive adsorption of dissolved silicate and of phthalic acid (PA), a model carboxylate-bearing organic contaminant, onto goethite, a representative natural iron oxyhydroxide nanomineral. Using surface complexation thermodynamic modeling of batch adsorption data and chemometric analyses of vibrational spectra, we find that silicate concentrations representative of natural waters (50–1000 μM) can displace PA bound at goethite surfaces. Below pH ∼8, where PA binds, every bound Si atom removes ∼0.3 PA molecule by competing with reactive singly coordinated hydroxo groups (−OH) on goethite. Long-term (30 days) reaction time and a high silicate concentration (1000 μM) favored silicate polymer formation, and increased silicate while decreasing PA loadings. The multisite complexation model predicted PA and silicate binding in terms of the competition for −OH groups without involving PA/silicate interactions, and in terms of a lowering of outer-Helmholtz potentials of the goethite surface by these anions. The model predicted that silicate binding lowered loadings of PA species, and whose two carboxylate groups are hydrogen- (HB) and metal-bonded (MB) with goethite. Vibrational spectra of dried samples revealed that the loss of water favored greater proportions of MB over HB species, and these coexisted with predominantly monomeric silicate species. These findings underscored the need to develop models for a wider range of organic contaminants in soils exposed to silicate species and undergoing wet–dry cycles.
Highlights
Silicate is one of the most widely distributed major oxyanion in soils and natural waters.[1]
Our finding that dry goethite predominantly exposed coexisting MB phthalic acid (PA) species with monomeric silicate species even aligns with our SCM model of wet goethite pastes from which these products were made. We find these results encouraging for pursuing future studies dedicated to bridging the speciation of wet and dry interfacial systems, which are becoming crucial for understanding how wet/dry cycling impacts competitive binding at mineral surfaces
Our surface complexation model can adequately predict competitive binding for reactive −OH functional groups of the goethite surface only using formation constants of HB/MB PA species and monomeric/polymeric silicate species obtained from subsystems and whose presence was assessed by Attenuated Total Reflectance (ATR)-Fourier transform infrared (FTIR) spectroscopy
Summary
Silicate is one of the most widely distributed major oxyanion in soils and natural waters.[1]. As such, understanding how silicates alter mineral surface site reactivity,[3,8] including associated mineralogical transformations,[9−11] is key to improving mass transport predictions in terrestrial and aquatic environments. Such predictions deserve a special focus on nanosized iron (oxyhydr)oxides given the reactivity and widespread occurrence of these particles in nature. Less is, known about (i) how silicate species impact the fate of other compounds especially organic contaminants competing for the same mineral surface sites, (ii) whether polymeric silicate species[5,13,20] appearing over the course of a long reaction time (days to weeks) affect this competition, and (iii) how the loss
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