Liquid chromatographic approach to investigation of dissolved organic matter adsorptive fractionation and competitive displacement on minerals

Abstract

Although physicochemical processes at liquid/solid boundaries are critical in several increasingly important areas of environmental science and engineering, research methods used to contact solutes and adsorbents have changed little in recent decades. We describe an emerging liquid chromatographic (LC) experimental approach which can be used to investigate various phenomena at liquid/solid boundaries, and we report its application in studies of Suwannee River Humic Acid (SRHA) adsorptive fractionation and phosphate competitive displacement on aluminum oxide coated quartz (AlQ). This LC-based approach is similar to continuous-flow column (i.e., breakthrough) methods. However, using the LC auto-injection system, `pulsed' (i.e., non-continuous) additions of adsorbate (or displacement agent) are made onto adsorbent-packed columns, and non-adsorbed (or displaced) ligands are monitored in multiple optical chromatographic detection channels. We applied the LC-based method in this area of research because (i) this method promises to be more efficient (in materials and analytical/experimental time) than conventional (batch and continuous-flow column) methods, and (ii) it generates unique data that reveal changes in of solution-phase adsorbate mixture compositions due to preferential adsorption or displacement effects. In an SRHA-AlQ adsorption study, detector signal ratios suggested preferential adsorption of high molecular weight (MW) SRHA constituents. When phosphate ions reacted with SRHA-saturated AlQ surfaces, these ratios indicated preferential displacement of low MW SRHA constituents. Adsorption and displacement results agreed, indicating relatively high MW SRHA constituents bound with greatest affinity. Results agree with previous studies of similar adsorbate–adsorbent systems conducted using conventional methods, which validates this method. Each of these experiments, which provided data on both adsorptive fractionation and competitive displacement, was completed in the timeframe of a typical chromatographic experiment and consumed <1mg adsorbate and < 2g adsorbent. Based on its temporal and materials efficiencies and its ability to yield unique forms of data pertaining to the interactions of complex organic ligand mixtures with adsorbent surfaces, this LC-based method will be an important alternative and/or complementary technique that will provide new insights into the interplay of processes that ultimately govern humic substances adsorption.