Soil-water equilibria for nonionic organic compounds

Determination of influencing factors on historical concentration variations of PAHs in West Taihu Lake, China

The adsorption of polycyclic aromatic hydrocarbons (PAHs) by components such as elemental carbon (EC), total organic carbon (TOC), and particles is different, and EC and PAHs are good materials for reconstructing historical human activity patterns and pollution conditions. In this study, the effects of EC (soot and char), TOC and particles of different grain size on PAHs in surface sediments were quantitatively analysed, and their historical concentrations in a sediment core from western Taihu Lake were reconstructed. The contents of soot, TOC, clay, EC and char explained 57.2%, 27.6%, 26.0%, 24.0% and 16.4%, respectively, of the PAH concentrations in surface sediments. The correlation between the soot and PAH levels was significantly higher than that between the char, TOC, and clay contents and PAH levels, and PAHs were mainly affected by the local economic development and human activity, as indicated by metrics of population, highway mileage, coal burning, and industrial output. With the development of the economy of the Taihu Lake Basin, the composition of PAHs in the sediments has changed: the proportion of low-molecular-weight PAHs decreased from 42.4% to 17.5%, and that of high-molecular-weight PAHs increased from 58.7% to 82.5%. The concentration of PAHs in pore water from Taihu Lake over the past 100 years was reconstructed and ranged from 43.1 to 961.2 μg L^-1, with an average of 180.7 μg L^-1. After China's reform and opening up, the concentrations of various PAHs in Taihu Lake changed from safe to chronic pollution levels. The ratios of lead (Pb) isotopes and the diagnostic ratios of PAHs showed that the main sources of PAHs in western Taihu Lake sediments were human activities such as coal and petroleum combustion.

Improved prediction of the bioconcentration factors of organic contaminants from soils into plant/crop roots by related physicochemical parameters

There has been an on-going pursuit for relations between the levels of chemicals in plants/crops and the source levels in soil or water in order to address impacts of toxic substances on human health and ecological quality. In this research, we applied the quasi-equilibrium partition model to analyze the relations for nonionic organic contaminants between plant/crop roots and external soil/water media. The model relates the in-situ root concentration factors of chemicals from external water into plant/crop roots (RCF(water)) with the system physicochemical parameters and the chemical quasi-equilibrium states with plant/crop roots (αpt, ≤1). With known RCF(water) values, root lipid contents (flip), and octanol-water Kow's, the chemical-plant αpt values and their ranges of variation at given flipKow could be calculated. Because of the inherent relation between αpt and flipKow, a highly distinct correlation emerges between log RCF(water) and log flipKow (R2 = 0.825; n = 368), with the supporting data drawn from 19 disparate soil-plant studies covering some 6 orders of magnitude in flipKow and 4 orders of magnitude in RCF(water). This correlation performs far better than any relationship previously developed for predicting the contamination levels of pesticides and toxic organic chemicals in plant/crop roots for assessing risks on food safety.

Resolution of Adsorption and Partition Components of Organic Compounds on Black Carbons

Black carbons (BCs) may sequester non-ionic organic compounds by adsorption and/or partition to varying extents. Up to now, no experimental method has been developed to accurately resolve the combined adsorption and partition capacity of a compound on a BC. In this study, a unique "adsorptive displacement method" is introduced to reliably resolve the adsorption and partition components for a solute-BC system. It estimates the solute adsorption on a BC by the use of an adsorptive displacer to displace the adsorbed target solute into the solution phase. The method is validated by tests with uses of activated carbon as the model carbonaceous adsorbent, soil organic matter as the model carbonaceous partition phase, o-xylene and 1,2,3-trichlorobenzene as the reference solutes, and p-nitrophenol as the adsorptive displacer. Thereafter, the adsorption-partition resolution was completed for the two solutes on selected model BCs: four biochars and two National Institute of Standards and Technology (NIST) standard soots (SRM-2975 and SRM-1650b). The adsorption and partition components resolved for selected solutes with given BCs and their dependences upon solute properties enable one to cross-check the sorption data of other solutes on the same BCs. The resolved components also provide a theoretical basis for exploring the potential modes and extents of different solute uptakes by given BCs in natural systems.

The ratio of clay content to total organic carbon content is a useful parameter to predict adsorption of the herbicide butachlor in soils

Thirteen soils collected from 11 provinces in eastern China were used to investigate the butachlor adsorption. The results indicated that the total organic carbon (TOC) content, clay content, amorphous Fe2O3 content, silt content, CEC, and pH had a combined effect on the butachlor sorption on soil. Combination of the data obtained from the 13 soils in the present study with other 23 soil samples reported by other researchers in the literature showed that Koc would be a poor predictive parameter for butachlor adsorption on soils with TOC content higher than 4.0% and lower than 0.2%. The soils with the ratio of clay content to TOC content (RCO) values less than 60 adsorbed butachlor mainly by the partition into soil organic matter matrix. The soils with RCO values higher than 60 apparently adsorbed butachlor by the combination of the partition into soil organic matter matrix and adsorption on clay surface.