Low-Molecular-Weight Organic Acids Influence the Sorption of Phenanthrene by Different Soil Particle Size Fractions

The impact of low-molecular-weight organic acids (LMWOAs) on the sorption of phenanthrene (a representative polycyclic aromatic hydrocarbon) by different particle size fractions of a soil was investigated using a batch technique. Citric and malic acids were used in experiments. Four soil fractions were fractionated: fine sand, silt, coarse clay, and fine clay. Laser granulometry confirmed the suitability of the fractionation method used for the particle size distributions in this investigation. The sorption of phenanthrene by the different soil fractions was described well using a linear distribution-type model, and the distribution coefficients (K-d) followed a descending order of fine sand > fine clay > coarse clay > silt, irrespective of the addition of organic acids. This order was significantly positively correlated with the organic carbon content of test soil solids. The K-d values for phenanthrene sorption by soil fractions initially increased but then decreased as the concentrations of citric and malic acids increased (0-1000 mmol L-1). The presence of citric and malic acid at lower concentrations (< 100 mmol L-1) generally promoted the sorption of phenanthrene, while higher concentrations (>100 mmol L-1) inhibited sorption irrespective of the soil fraction. The mechanism of the LMWOA-influenced sorption of phenanthrene by test solids is discussed based on the observed sorption of organic acid, the dissolution of metal cations and minerals in soil, and the competition from dissolved organic matter in solution that were released from soil solids.