Low-Molecular-Weight Organic Acids Influence The Sorption Of Pahs By Different Particle Size Fractions Of Soils

Extensive industrial manufacturing, agricultural production, and military practice operations have discharged enormous quantities of toxic compounds into the soil environment. Among these compounds, polycyclic aromatic hydrocarbons (PAHs) are regarded as one such class of persistent organic contaminants in soils that are of major concerns due to their recalcitrance to degradation and strong carcinogenic and/or mutagenic properties. Various studies have found that PAHs at high concentrations occur in soils all around the world. PAHs are readily fixed in soils; for this reason, insight into how to activate PAHs, enhance their availability, and consequently increase the efficiency of soil remediation has become an important area of research.Sorption, as well as other chemical and biological processes involving organic pollutants, depends on the soil particle size. Unfortunately, little published information is available on the sorption of PAHs and other organic pollutants by different particle size fractions of soil in the presence of root exudates as well as organic acids. To this end, the aim of this work was to fractionate the different particle size fractions of soil and examine the impacts of low-molecular-weight organic acids on the sorption of phenanthrene, as a representative PAH, by these soil fractions. The main novel findings are as follows:(1) Utilizing sieving, sedimentation, and centrifugation as well as cross-flow filtration (CFF), the particle size in yellow-brown soil was sequentially fractionated into five scales, i.e., fine sand (50-250μm), silt (5～50μm), coarse clay (1～5μm), fine clay (0.1～1μm), and nanocolloids(<0.1μm). Laser granulometry, transmission electron microscopy (TEM) and x-ray diffraction (XRD) were used to characterize these particles with different size. Results showed that the determined size of each particle was consistent with the theoretical fractionation value, indicating the availability and reasonableness of the fractionation method. TEM test displayed that nanocolloids were uniformly spherical with size range of20-40nm and well dispersion and distribution, but presented a few large granular aggregates. As the particle size became smaller, the contents of quartz and other primary minerals in the former four fractions decreased, but clay minerals increased. The main mineral in fine clay fraction was illite. One notes that, nanocolloids had low peak intensity of XRD, bad dispersion particles and poor crystal, and halloysite and kaolinite were the main minerals in nanocolloids.(2) The effect of LMWOAs with three typic acid (citric, malic and oxalic acid) on the PAHs sorption was also investigated in soil and its fractions using a batch approach. The sorption of phenanthrene could be well described by a liner isotherm irrespective of the presence of LMWOAs. Results shown the addition of LMWOAs with the concentration of0-100mmol·L-1, promoted the sorption of phenanthrene by soil fraction; The promotion increased at first and reduced, and the maximum sorption by yellow-brown soil, fine sand and slit appear when the concentration of LMWOAs is8mmol·L-1; while coarse clay and fine clay is10mmol·L-1. The addition of LMWOAs with the concentration of·L-1, inhibited the sorption. Citric, malic and oxalic acid had different effect on the sorption, and malic acid resulted in more change on the Kd value. Take citric acid for example, the Kd value for fine sand make little change, while slit change more, indicating the higher content of organic matter in soil samples, the smaller effect on sorption of phenanthrene. Furthermore, the addition of LMWOAs could result in the increment of DOM concentration from soil examples in solution at first and decrement soon after, which behave a significant negative correlation with sorption of phenanthrene.(3) Results for sorption of phenanthrene by soil nanoparticle with concentration of3550mg-L"1shown the sorption balance time was two hours, and acidic and alkaline could improve their sorption amounts, which may benefit from its more dispersion for nanoparticle. Sorption of phenanthrene by soil nanoparticles was fitting F-type equation, and the Kd value was1634.2L·kg-1.