Exchangeable Cation Hydration Properties Strongly Influence Soil Sorption of Nitroaromatic Compounds

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 (fine loamy, mixed, mesic Typic Haplaquoll) soil series A (WA) andB (WB) horizons were determined from batch sorption experiments. Smectite is the most abundant mineral in the horizons of 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 p-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 (1,2,4TCB) 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 generally greater for clays saturated with monovalent cations than clays saturated with 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. NITROAROMATIC COMPOUNDS are commonly used as pesticides, explosives, and intermediates in the synthesis of dyes, ammunition, and solvents. Their uses and manufacture have resulted in contamination of soils, sediments, and aquifers at military training and manufacturing sites. Sorption processes play an important role in determining mobility, (bio)availability and, hence, persistence of NACs in soil. Clay minerals and SOM are generally considered to be the two most active soil components in the sorption of aqueous phase organic contaminants. For many nonpolar organic compounds, sorption is highly correlated with SOM content and is relatively independent of other sorbent properties (Chiou et al., 1979, 1983; Karickhoff et al., 1979). In these instances, sorption has been conceptualized as solute partitioning into SOM (Chiou et al., 1979, 1983; Karickhoff et al., 1979). It has been further suggested that adsorption of such solutes by mineral fractions in soils is suppressed by the preferential adsorption of water (Chiou, 2002). However, many pesticides (e.g., triazines) and organic contaminants (e.g., NACs) containing polar functional groups are substantially adsorbed from bulk water by clays. In fact, comparison on a unit mass basis indicated that adsorption of NACs from water by smectite clays was greater than that by SOM (Sheng et al., 2001). Thus, clay minerals may play a dominant role in sorption of certain polar organic molecules, particularly for geosorbents with low SOM contents (Weissmahr et al., 1999). Several studies have reported the high affinity of NACs for smectites in aqueous solution (Haderlein et al., 1996; Weissmahr et al., 1998; Boyd et al., 2001; Sheng et al., 2002; Johnston et al., 2001, 2002). This has been attributed to the formation of electron donor (clay)–acceptor (EDA) complexes facilitated by the electron withdrawing properties of the nitro (NO2) group (Weissmahr et al., 1997). Alternatively, complexation of the NO2 groups of NACs with exchangeable cations on the clay has been suggested as a primary sorption mechanism (Boyd et al., 2001). Partial solute dehydration associated with sorption in smectite interlayers with optimal distance (|12.3 Å) also provides favorable energy for adsorption (Boyd et al., 2001; Li et al., 2004b). Sorption of NACs by reference smectite clays has been shown to be favored by weakly hydrated exchangeable cations such as K, NH4 , and Cs and disfavored by more strongly hydrated cations such as Na, Ca, Mg, and Al (Haderlein and Schwarzenbach, 1993; Weissmahr et al., 1997; Boyd et al., 2001; Sheng et al., 2002). Cations that possess comparatively smaller hydrated radii manifest larger adsorption domains (i.e., unobscured siloxane surface) between exchange sites on clays. Weakly hydrated cations also have stronger interactions with the partial negative charge of the NO2 groups in NACs (Boyd et al., 2001). Evidence for interactions between weakly hydrated exchangeable cations and NO2 groups of NACs was provided by Johnston et al. (2002), who reported shifts in the NO2 S. Charles, B.J. Teppen, H. Li, and S.A. Boyd, Dep. of Crop and Soil Sciences, and Environmental Science and Policy Program, Michigan State Univ., East Lansing, MI 48824; D.A. Laird, USDA-ARS, National Soil Tilth Lab., Ames, IA 50011. Received 25 July 2005. *Corresponding author ([email protected]). Published in Soil Sci. Soc. Am. J. 70:1470–1479 (2006).