Abstract
Batch adsorption-desorption equilibrium techniques were used to investigate the adsorption capacity and influence of salinity on partitioning of the insecticide chlorpyrifos between water and soil or water and sediments from the Rufiji Delta. The data were fitted to different adsorption-desorption models and the hysteresis index was calculated using the ratio between the Freundlich exponents for desorption and adsorption, and secondly, the difference in area under the normalized adsorption and desorption isotherms using the maximum adsorbed and solution concentrations. The data showed non-linear adsorption and that chlorpyrifos was strongly adsorbed to soil and sediments from the Rufiji Delta. The linearized adsorption coefficient (KD) and Freundlich adsorption coefficient (Kf) correlated significantly with organic carbon content. Chlorpyrifos adsorption as well as hysteresis calculated by both methods decreased with salinity (i.e. the sediment adsorbs increasing amounts of chlorpyrifos with decreasing salinity). This indicates that settling of freshwater sediments is among the major removal pathways of the chemical from the water column, but increased turbulence during high tides may resuspend settled sediment simultaneously increasing salinity and re-dissolve chlorpyrifos. However, discharge of fresh water, particularly during heavy rains, increases the trapping efficiency of the sediments. The theoretical approach developed showed that the Langmuir model describes the desorption data better than the Freundlich model, and that a better index of hysteresis is one that considers areas under the adsorption and desorption isotherms, provided the desorption isotherm is described by the normalized Langmuir isotherm and the adsorption isotherm by the normalized Freundlich isotherm.
Highlights
The highest percentages of clay and organic carbon contents were measured in Ruaruke soil (RSO) followed by Matosa soil (MSO)
Nonlinear isotherms were obtained for all adsorbents indicating that chlorpyrifos has a preferential adsorption to soils and sediment initially, and adsorption decreases as more pesticide is adsorbed (Fig. 2)
N values which indicate the dependence of adsorption on concentration were 0.78, 0.88 and 0.70 for Ruaruke Soil (RSO), Matosa Soil (MSO) and Ruaruke Sediments (RSE), respectively
Summary
Linearized sorption coefficients (KD1, KD2 and KD3) from the three approaches and KD from the linear isotherm (Eq 1) were normalized to the organic carbon conata analysis andDinattearparnetaaltyisonis and interpretation ll adsorption dataAwllearedsfiottrepdtitoonthdealtianweaerrme foidtteeld(Etoq.th1e):linear model (Eq 1): ata analysis and interpretation tent of the corresponding sorbents to give KDOC, KD1OC, KD2OC and KD3OC that were used to compare between low and high saline soil and sediment samples. The OC rich soil and sediment from the Rufiji Delta stand out to some extent in that the clay content contributes to the adsorption of chlorpyrifos.
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