Quantification and characterization of mobile colloids: their potential role in carbon cycling under varying redox conditions
University of Delaware
Mobile colloids, 1-1000 nm particles, have attracted much research attention because they have small size and large specific surface area therefore the potential to facilitate the transport of contaminants in the subsurface environment. Despite colloids’ potential importance, the role of mobile colloids in carbon cycling under varying redox conditions is largely unknown. This dissertation, combining laboratory investigations and field measurements, focused on (1) identification and understanding of the key processes and pathways leading to colloid and colloidal organic carbon (COC) release under dynamic redox conditions and (2) quantification of the actual colloidal load and assessment of colloids’ role in organic carbon retention and mobilization in representative environmental systems. ☐ In Chapter 2, I investigated the complex interplay of soil colloid release, organic matter (OM) content, and redox conditions via batch experiments. I found that reducing conditions promoted colloid release, but colloid release largely depended on the dynamic interactions between OM and colloids. Under aerobic conditions, the addition of dissolved OM (DOM) increased colloid release, while under anaerobic conditions, the release of indigenous DOM (DOM in) by iron reduction inhibited colloid release. In Chapter 3, I developed a simple and efficient methodology to quantify mobile colloids in < 0.1, 0.1-0.45 and 0.45-1.0 µm fractions using size-dependent correlations between nephelometric turbidity and colloid mass concentration. I found that colloid size strongly affected concentration-turbidity correlations, while colloid composition played a less important role in shifting the correlations. The relatively insignificant particle composition effect indicates the practically “universal” applicability of the reported correlations. The method developed in Chapter 3 was applied in Chapter 4 to quantitatively examine the actual colloidal load and COC pool in agricultural, forestry, wetland and estuarine systems. A special focus was on colloids in < 0.45 µm fraction, which has been operationally defined as part of the "dissolved" phase in most previous studies. I found that the amounts of colloids and COC in 0.1-0.45 µm fraction were substantial and COC accounted for 8-19% of the operationally defined dissolved organic carbon in < 0.45 µm fraction. Additionally, wetland was found to be a hotspot of both colloid and COC release and mobilization, compared to other investigated sites.