Partitioning Behavior of Insecticides in Soil-Water Systems: I. Adsorbent Concentration Effects. 1985

Adsorption studies were conducted to determine whether adsorbent concentration (soil-solution ratio) affected insecticide adsorption by soils and clays. Adsorption isotherms were obtained for several insecticides in Bondhead sandy loam and Ca-saturated illite suspensions using adsorbent concentrations that were varied over several orders of magnitude. There was no evidence that altering adsorbent concentration exerted any influence on partitioning of insecticides between the adsorbent and solution phases. The centrifugation process, which in effect greatly increased adsorbent concentration at the bottom of the centrifugation tube, did not change the partitioning of the insecticides between the two phases. Dieldrin (1,2,3,4,10,10-hexachloro-exo-6,7-epoxyl, 4,4a,5,6,7,8,8a-octahydro-1,4-endo, exo-5,8 -dimethanophthalene), unlike the other insecticides, exhibited strong concentration-dependent adsorption by glass surfaces, which invalidated the use of the normal "blank" (no adsorbent) sample for determining the initial solute concentration. Since glass adsorption appeared quite irreversible in aqueous solution, a sequential blank/sample adsorption technique was adopted whereby the stock solution was initially equilibrated in the glass bottle, then a small aliquot was removed for analysis before adding the adsorbent for its equilibration period. This technique produced consistent adsorption data that showed no evidence of being influenced by adsorbent concentration. Additional Index Words: soil-solution ratio, isotherm, adsorption, parathion, dieldrin, chlorpyrifos, fensulfothion sulfone. Bowman, B. T., and W. W. Sans. 1985. Partitioning behavior of insecticides in soil-water systems: I. Adsorbent concentration effects. J. Environ. Qual. 14:265-269. 1 Contribution from the Research Centre, Canada Agriculture, University Sub Post Office, London, Ontario, N6A 5B7. Received 27 Jan. 1984. 2 Research scientist and research assistant, respectively. 3 See the Appendix for a listing of the chemical names of pesticides used in this article. It is only recently that adsorption studies involving water-sediment (soil or clay) systems have examined the effect of changing the adsorbent concentration on partitioning of neutral organic compounds between the two phases. Adsorbent concentrations used in adsorption studies (expressed as milligrams adsorbent per liter solution) reported in the literature have ranged from approximately 10 (11) to 1 x 10 (9). Factors involved in selecting experimental adsorption concentrations include (i) relative partitioning of compound between adsorbent and water phases, (ii) detection limits and working range of analytical methods, and (iii) concentrations at which the compound and adsorbent occur in natural aqueous environments. Lotse et al. (10) reported that the sediment concentration (2 x 10 to 5 x 10 mg L) as well as the lindane/sediment ratio affected lindane adsorption by several lake sediments. They showed that relative adsorption increased with an increasing lindane/sediment ratio. Karickhoff et al. (8) reported that their isotherms for pyrene and methoxychlor on several sediments were independent of sediment concentration (10 to 4 x 10 mg L) in dilute suspensions. Koskinen and Cheng (9) found that decreasing the adsorbent concentration from 10 mg L(10 g/10 mL) to 3 x 10 mg L(10 g/33 mL) resulted in a significant increase in 2,4,5-T adsorption, but that a further decrease in adsorbent concentration to approximately 3 x10 mg L (1g/33 mL) resulted in no further change in adsorption. Farmer and Aochi (6) found that decreasing the adsorbent concentration from 5 x 10 to 2 x 10 mg L resulted in decreased picloram adsorption. These references, dealing primarily with adsorbent concentrations exceeding 10 mg soil L solution, have produced no consistent conclusions on a possible relationship between adsorbent concentration and partitioning of neutral organic compounds between sediment or soils and water. Recently O'Connor and Connolly (11), DiToro et al. (5), and Horzempa and DiToro (7) described experiments in which, at low adsorbent concentrations (10-1000 mg L), the relative adsorption of organochlorine insecticides and polychlorinated biphenyls apparently increased by orders of magnitude as a result of comparable decreases in the sediment (or clay) concentration. DiToro and Horzempa (4) have also reported that when a portion of the supernatant solution of hexachlorobiphenyl (HCBP) in equilibrium with lake sediment, was removed and the remaining solution used to resuspend the sediment (with no new solution addition), there appeared to be a significant increase in HCBP solution concentration. They attributed this increase to HCBP desorption in response to the increased sediment concentration following the resuspension step. The authors concluded that these results were important to understanding the adsorption/desorption status of these compounds in river or lake systems. They hypothesized that as dilute sediment suspensions containing adsorbed pesticide settle to the river bottom, the "sediment concentration" would increase, thereby lessening its ability to retain the pesticide, resulting in a partial desorption. Conversely, if the pesticide-loaded sediments were resuspended by stream currents, the "sediment concentrationwould decrease, thereby increasing their ability to readsorb more pesticide. They suggested that this process could significantly influence the manner in which pollutants are flushed from river systems. The implications of these proposals, if proven correct, could be far-reaching. For example, adsorption isotherms could not be directly compared unless they had similar adsorbent concentrations, thereby making comparisons among many published isotherms difficult, if not impossible. Traditionally, the centrifuge has been used to separate the aqueous and adsorbent phases in adsorption experiments. This research casts doubts on the validity of this technique, since, according to their hypothesis, the sediment concentration would increase tremendously as it was thrown to the bottom of the centrifuge tube where a partial desorption would supposedly occur, destroying the equilibrium attained before centrifugation.