ARS | Publication request: Exchangeable Cation Hydration Properties Strongly Influence Soil Sorption of Nitroaromatic Compounds

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 7, 2006
Publication Date: September 18, 2006
Citation: Charles, S., Teppen, B.J., Hui, L., Laird, D.A., Boyd, S.A. 2006. Exchangeable Cation Hydration Properties Strongly Influence Soil Sorption of Nitroaromatic Compounds. Soil Science Society of America Journal. 70(5):1470-1479.

Interpretive Summary: Nitroaromatic compounds (NACs) are used in the plastics industry as components of military ordinance and as pesticides in agriculture. When these compounds are present as contaminants in soils they may have a negative effect on both human and ecosystem health. NACs are strongly adsorbed by soils. However, there has been a debate in the scientific literature as to whether NACs, and various other organic contaminants, are more strongly adsorbed by organic matter or by clay minerals in soils. We discovered that the type of cation saturating a soil has a big influence on the soil's ability to adsorb NACs but has little effect on the soil's ability to adsorb trichlorobenzene (TCB). We interpret this as evidence that polar compounds, such as NACs, are primarily adsorbed by soil clays while nonpolar compounds like TCB are primarily adsorbed by soil organic matter. This information will be used by scientists to more accurately predict whether an organic contaminant will be adsorbed by soils or whether it is likely to be leached into ground water.

Technical Abstract: Nitroaromatic compounds (NACs) are commonly found as soil contaminants in military training sites and manufacturing facilities, and may adversely affect human and ecosystem health. Exchangeable cation effects on p-nitrocyanobenzene (p-NCB) and 1,4-dinitrobenzene (1,4-DNB) sorption by the Webster soil A(WA) and B (WB) horizons were determined from batch sorption experiments. Smectite is the most abundant clay mineral in this soil. Soil organic matter (SOM) removal increased p-NCB sorption to the WA and WB horizons by 1.5 times, and increased 1,4-DNB sorption to the A and B horizons by 1.2 to 2.2 times, respectively. Greater sorption of NACs by whole soils after SOM removal suggests that SOM suppresses -NCB and 1,4-DNB sorption by soil mineral components. Clay (<2-mm fraction) removal decreased p-NCB sorption to the WA and WB horizons by 8 and 11 times, respectively; clay removal decreased 1,4-DNB sorption to the WA and WB horizons by 4.8 and 6.7 times, respectively. Nitroaromatic compound sorption to different soil fractions was measured to identify the independent effects of soil components and exchangeable cations on sorption. For this purpose, 1,2,4-trichlorobenzene (1,2,4-TCB) and 1,4-DNB sorption to two organic soils and a soil devoid of smectites, and p-NCB sorption to whole soil and the soil clay-sized fraction were determined. Exchangeable cation type was found to minimally affect sorption of 1,2,4-TCB, p-NCB, and 1,4-DNB by SOM. Sorption of p-NCB to homoionic soil clay-sized fractions was greater for clays saturated with monovalent cations than divalent cations. Greater p-NCB sorption followed decreases in cation hydration energy in the order Na< Li << K < Cs and Ca < Ba < Mg. Similar trends were observed for whole-soil samples exchanged with these cations. This indicates that differences in nitroaromatic sorption are due to exchangeable cation effects on the clay mineral fraction.