Cosorption of atrazine and a lauryl polyoxyethylene oxide nonionic surfactant on smectite.

Commercial atrazine formulations commonly contain nonionic surfactants that serve as solubilizing and wetting agents for enhancing the stability and efficacy of the herbicide. The fate of atrazine in soils has been extensively investigated; yet, few studies have considered the effects of formulation components on the fate of atrazine in soils. In this study, we investigated the influence of the nonionic surfactant, Brij 35 (Brij), on the sorption of atrazine on Ca- and K-saturated samples of a reference smectite, Panther Creek (PC). In general, Brij concentrations of 50 and 200 mg L(-1) had little effect on atrazine sorption, but sorption was substantially inhibited by Brij concentrations of 2100 mg L(-1). For Brij concentrations of 6300 mg L(-1), atrazine sorption was intermediate between that observed for the 200 and 2100 mg L(-1) Brij systems. Brij molecules themselves were very strongly sorbed by PC, with sorption maxima exceeding 200 g kg(-1). X-ray diffraction analysis of Brij-treated PC indicated that the sorbed Brij was intercalated into interlayers of the smectite. At Brij concentrations of 2100 mg L(-1), Brij competed with atrazine for interlayer sorption sites. In contrast, at the initial Brij concentration of 6300 mg L(-1), the clay interlayers were largely filled with Brij, and excess Brij probably accumulated on external surfaces of the clay as surface micelles. We hypothesize that atrazine partitioning into surfactant micelles on external surfaces of the clay led to enhanced retention by the solid phase.

Potentiometric-spectroscopic evaluation of metal-ion complexes by humic fractions extracted from corn tissue.

Humic fraction (HF) functional group-type and content are expected to depend on molecular size, which in turn, is expected to influence formation of heavy-metal complexes. In this study, corn (Zea mays L.) stalks and leaves were decomposed for an 8-month period to produce water-soluble humic substances. These substances were separated into three water-soluble fractions, HF1, HF2 and HF3, from highest to lowest relative molecular size. Functional group determination showed that carboxylic, and phenolic OH acidity increased as relative molecular size of humic fractions decreased. We also observed decreasing C/O ratios from larger to smaller corn tissue-derived humic fractions, whereas N/C and H/C ratios remained relatively unaffected. Furthermore, using potentiometric titration and FTIR spectroscopy we studied formation of Ca2+-, Cd2+-, and Cu2+-humic fraction complexes and how they were affected by pH and molecular size. We determined that metal-humic complexes exhibited at least two types of functional group-sites with respect to Ca2+, Cd2+, and Cu2+ complexation. Strength of metal-ion humic complexes followed the order Cu2+ > Cd2+ > Ca2+ and was affected by pH, especially for low affinity sites. Carboxylic groups were most likely the dominant group-sites involved in complex formation. Magnitude of the metal-humic formation constants in the logarithmic form at the lowest equilibrium metal-ion concentration, under the various pH values tested, varied from 5.39 to 5.90 for Ca2+, 5.36 to 6.01 for Cd2+, and 6.93 to 7.71 for Cu2+. Furthermore, the formation constants appeared to be positively influenced by decreasing molecular size of water-soluble humic fraction, and increasing pH. However, our molecular spectra showed that the pKa of corn humic fractions increased with decreasing relative molecular size and that Cu2+ was more covalently bonded by humic fractions than were Ca2+ and Cd2+, and the nature of the covalent bond character was independent of pH.

Stability of Ca2+-, Cd2+-, and Cu2+-illite complexes.

In this study, using ion selective electrode techniques, we investigated the influence of pH on metal-ion, Ca2+, Cd2+, and Cu2+, adsorption by Fithian illite. The results showed that Fithian illite exhibited at least two types of metal-ion adsorption sites, high and low strength with the strength of metal-ion-illite surface complexes following the order of Cu2+ > Cd2+ > Ca2+ (strongest to weakest acids) at any of the pH values tested. The ability of metal-ions to form complexes with illite surfaces was affected by type of metal-ions and pH, especially for low metal-ion affinity sites. These sites formed stronger metal-ion complexes at high pH than at low pH, which implicated clay edge sites and indicated that H+ competed with metal-ions for available complexation sites. The data also showed that illite functional groups forming the strongest metal-ion complex did not appear to be pH-sensitive, which implicated wedge siloxane cavities or extremely low pKa clay-edge OH functional groups, but the total number of such sites were very small. The magnitude of the metal-ion-illite stability constants, as metal-ion solution concentration approached zero, on a log-scale, varied from 3.52 to 4.21 for Ca2+, 4.38 to 5.18 for Cd2+, and from 5.23 to 5.83 for Cu2+. These constants were approximately an order of magnitude smaller than those representing illite with sorbed humic fractions. The above results along with the results from our previous studies imply that metal-ion mobility and bioavailability would be affected by soil mineral surface properties, which would be significantly influenced by sorption of humic substances.