Flow and reactivity effects on dissolved organic matter transport in soil columns

Dissolved organic matter (DOM) plays a prominent role in the transport of contaminants in porous media. As DOM has to be considered as a reactive component, now regime and sorbent reactivity should affect overall DOM transport in an important way. We focused on DOM transport in unsaturated column experiments using quartz sand (QS) and goethite-coated quartz sand (GS). Rate constrictions to DOM sorption were investigated by varying the volumetric now rate, while extent and reversibility of sorption were studied in consecutive adsorption and desorption steps. In the QS, DOM retention was low and unaffected by changes in now rate. Desorption-step breakthrough curves (BTCs) and mass balances show full reversibility of the sorption process. However, DOM retention in GS was significant and sensitive to now variation, indicative of nonequilibrium sorption. At lower now rates, DOM breakthrough exhibited a change in curvature (shoulder) due to the superimposition of two BTCs representing reactive and nonreactive DOM fractions. Transport was successfully modeled assuming these two fractions governed overall DOM mobility. At higher flow rates, the BTC shoulder vanished due to reduced contact time between the DOM and the solid phase (rate-limited sorption). Sorption of DOM on GS is accompanied by a marked rise in effluent pH, indicative of a ligand-exchange mechanism. Recovery of DOM during desorption was incomplete due to either partially irreversible sorption or strongly rate-limited desorption. Increased DOM mobility in the consecutive adsorption step resulted from partial blocking of sorption sites by the initial pulse of DOM.