Implications of Free and Occluded Fine Colloids for Organic Matter Preservation in Arable Soils

Influence of organic matters on the adsorption-desorption of 1,2-dichloroethane on soil in water and model saturated aquifer

1,2-Dichloroethane (1,2-DCA) is a typical organic chlorinated compound largely utilized in chemical manufacturing and industrial production and also a common pollutant in organically contaminated sites. The adsorption of 1,2-DCA on soil grains significantly influences its environmental fate and removal process. This study investigated the influence of fulvic acid (FA) and humic acid (HA) on the adsorption-desorption of 1,2-DCA in solid-liquid interfaces in water or constructed porous media. Experimental findings demonstrated the influence of organic matter on the adsorption of 1,2-DCA at the solid-water interface. 1,2-DCA adsorption increased in the FA or HA-treated soils when organic matter was present on the solid surfaces. The 1,2-DCA adsorption in the mixture of FA and HA was slightly lower than that in single organic acids, depending on the binding of FA and HA to the soil grains/colloids. Basic conditions reduced the adsorption of 1,2-DCA on soils, whereas acidic conditions enhanced adsorption due to the increased interactions via adsorption sites and hydrogen bonds. Conversely, the presence of organic matter in solutions (liquid phase in constructed porous media) will reduce the adsorption of 1,2-DCA on solid surfaces and increase the transport in the model aquifer. The combination of FA, HA, and rhamnolipids is helpful for the removal of 1,2-DCA from solid surfaces. Additionally, because of the enhanced desorption, the risk of 1,2-DCA contamination in groundwater can be increased when the organic matter or surfactant is present in the liquid phase if the eluent is not collected. This study helps to better understand the cooperative interaction of soil organic matter and chlorinated hydrocarbons at solid-water interfaces and the environmental fate and potential removal strategies of chlorinated hydrocarbons in contaminated sites.

Redox oscillations destabilize and mobilize colloidal soil organic carbon

The onset of oxic-anoxic events in soil induces biogeochemical processes that may have profound influences on the fate and transport of soil organic carbon (OC). An 18-wk long laboratory soil column study was conducted to investigate the influence of static oxic (SO), static anoxic (SA), or redox oscillation (RO) events on the release of OC. Column leachate samples were collected after 9-wk and 18-wk of incubations and separated into dissolved (<2.3 nm), natural nanoparticle (2.3-100 nm), fine colloid (100-450 nm), and particulate (> 450 nm) fractions that were characterized through spectrofluorometric analyses. The concentration of released OC from different treatment columns followed the order of SA > RO > SO. After 9-wk, total OC concentrations in the leachate samples were increased by 4-fold and 54-fold in the SO and SA columns compared to the time-zero control columns, respectively. However, after 18-wk, the released amount of OC doubled in SO but decreased by 50 % in SA columns compared to the 9-wk incubation samples. The RO columns had intermediate OC concentrations between the SO and SA treatments. The RO events further led to widely varied dynamics in the release and molecular composition of the size-fractionated OC compared to static conditions, indicating the effects of redox oscillation on the organo-mineral association in soil. The wide variations in the aromaticity of OC released after the 1^st and 2^nd RO events further support the notion that alternating redox processes regulate OC cycling differently than the SO or SA condition. The observed increase in fine colloid and particulate OC fractions (i.e., >100 nm) from 7 % to 40 % between 1^st and 2^nd RO suggests the clustering of nanoaggregates and/or formation of colloidal size aggregates. The composition of the released OC as influenced by redox fluctuations provides a baseline for the size continuum of soil OC and its potential ecological and environmental roles.