Effects of Soil-to-Solution Ratio on Sorption-Desorption Isotherms of Zinc in a Sandy Soil

Sorption reaction depends on various factors, including number of available sorption sites, total amount of sorptive material in solution and probability of effective contact between sorption sites and sorptive material. The amount of sorption per unit mass of sorbent usually increases with decreasing soil-to-solution ratio. However, there is little information on the effects of soil-to-solution ratio on sorption and particularly desorption isotherms. This research aimed to investigate the effects of different soil-to-solution ratios (1:2.5,1:5,1:10,1:20 and 1:50) on sorption isotherms of zinc at initial concentration range of 10-100 mg Zn/kg soil and 0.01 M Ca(NO3)2 background solution (adjusted to pH 7.3) in a sandy soil. Moreover, the effects of these soil-to-solution ratios on desorption of zinc at three initial concentrations were evaluated. The results indicated large differences in equilibrium concentrations of zinc among the soil-to-solution ratios at low initial concentration of zinc (10 mg/L) and short contact times (< 18 h). The difference decreased with increasing contact time. At high initial concentration of zinc (100 mg/L), the equilibrium concentrations of zinc were almost the same for both short and long contact times. With decreasing the soil-to-solution ratio, the type of sorption isotherms changed from L- to H-type. Fitting the Langmuir equation to the sorption data revealed a direct relation of maximum sorption constant (b) as well as an adverse relation of sorption energy constant (K) to the soil-to-solution ratios. Furthermore, hysteresis values of the sorption-desorption isotherms were increased with decreasing soil-to-solution ratios.