Study of Cadmium Desorption Kinetics from Riverine Sediments

In natural streams, the majority of heavy metal ions are generally associated with sediment particles. Under 21 some environmental conditions, these metal ions may release from the sediment particles. In such conditions, the 22 desorption rate of heavy metals is very important for decision-makers of water quality assessment. In this study, 23 the effect of cadmium desorption from the river bed sediments has been experimentally investigated. Artificially 24 contaminated sediments were used for performing batch desorption experiments. The experiments were conducted 25 by adding 1 gr of contaminated sediment (D 50 = 0.53 mm) with a known concentration and shaking until observing 26 a roughly constant cadmium concentration in the solution. It was concluded that the cadmium ions were strongly 27 bond to the river bed sediment; meanwhile, at the equilibrium time, up to about 7 to 29 percent of cadmium ions 28 were released from the artificially contaminated sediments. The experiments were followed by two agitation rates 29 of 100 and 200 rpm. It was revealed that by increasing the flow turbulence, the amount of desorbed cadmium is 30 slightly increased. Besides, the desorption kinetics was evaluated using eight models of Zero-, first-, second-, 31 third-order, parabolic diffusion, double parabolic diffusion, two constant rate, and simple Elovich. The results of 32 the evaluation showed that simple Elovich (with R 2 = 0.991), double parabolic diffusion (with R 2 = 0.9882), two 33 constant rate (with R 2 = 0.983) and parabolic diffusion models (with R 2 = 0.846) have respectively the best 34 performance in calculation of Cd desorption rate from the sediments. metals (Cd, Cr and Cu) in a water-level fluctuation zone of TGR. They also simulated the adsorption and desorption behavior of heavy metals on soils. Their results showed that adsorption of Cd(II) was a chemical 80 process and dissolved organic matters (DOM) in soils strengthened the combination of Cd(II) to soil surface which 81 inhibited the desorption process. Cr(VI) was physically adsorbed and readily to be desorbed and DOM enabled 82 deposition of Cr(VI) in soils. They found that the cation exchange was dominate mechanism in Cu(II) adsorption 83 process, whereas DOM presented positive effects on desorption of Cu (II).


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The contamination of aquatic systems by heavy metal ions, especially in natural rivers, has become one 39 of the most important environmental problems. Therefore, the existence of sediment particles in the water column  nickel, and zinc) from artificially contaminated kaolinite soils with different organic matter content. The data obtained from desorption kinetics experiments were fitted with four kinetics models: pseudo-second-order second-order equation was higher than that of other model.

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The main purpose of this study is to provide quantitative information about the behavior of desorption 113 cadmium in river bed sediments. The artificially contaminated samples were subsequently subjected to a wide 114 range of conditions that may occur in natural systems to study their effects on heavy metal desorption. In addition,  The sediment samples were collected from shallow waters nearer to the bank of the Karaj River and were 122 taken from upper 0-15 cm layer of the deposits at the places with low flow rates as sedimentation was assumed to 123 occur (Jain and Ram 1997b). The samples were washed several times with distilled water to remove physical earthen impurities. Then, prior to experiments, they were dried in a hot air oven at 110°C for 24 hrs. A sediment size of 0.53 mm was selected by using standard sieving apparatus (particles remained between sieves No. 30 and 126 40). The cation exchange capacity (CEC) of the riverine sediments was measured using Bower's method (Bower 127 et al. 1952) equal to 13.873 meq/100gr. In addition, the physicochemical composition and particle size distribution where, Co is the initial cadmium concentration (mg L -1 ), Ce is the cadmium concentration after adsorption 150 (mg L −1 ), V is the volume of the aqueous phase (50 mL), and W is the amount of the sediment (gr). Cadmium

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In the next stage, new solutions with the same Cd concentration were again added to the sediment 153 samples and the mixtures were shake for another 5 hrs. This process was repeated for five stages and the amount 154 of Cd adsorbed at each stage were determined (Eq. 1). After contamination of the sediments for five stages, they 155 were removed from the flasks. Then, they were allowed to air-dry for one week at ambient temperature of the 156 laboratory .The results of these measurements are given in Table 3.

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Cd, was shake with 50 mL of distilled water for different times (0, 5, 15, 30, 60, 120, 300, and 720 min) to reach 166 an equilibrium desorption rate. The amount of desorbed Cd from unit weight of the sediments (Ds, in mg g -1 ) and desorption percent (R, %) were determined using the following equations: where, V is the volume of the solution (lit), C is the Cd concentration at equilibrium condition (mg/lit) In this study, to investigate the effect of turbulence on cadmium desorption the mixtures were shake with

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As mentioned before, in this study, eight desorption kinetic models including Zero-order, First-order, 8 Elovich equations were evaluated using the experimental data Table 4  ko, zero-order rate constant αs, initial Cd desorption rate (mgCdkg -1 S -1 ) and βs, Cd desorption constant In order to analyze the accuracy of the best fitted model, the coefficient of determination (r 2 ) and the standard error of estimate (SE) were calculated as follows: After the sediments were artificially contaminated by different concentrations of cadmium ions,

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desorption experiments were conducted to evaluate the effect of Cd concentrations and flow turbulence on the 195 desorption rate of cadmium from the river bed sediments. After that, the analysis for the desorption kinetics models 196 are represented in order to select the best fitted model for cadmium release from the sediments.

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The results of cadmium desorption from the artificially contaminated sediments were given in Fig. 1

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On the other hand, simple Elovich, double parabolic diffusion, two constant rate, and parabolic diffusion models

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The authors declare that they have no competing interests.

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Some or all data, models, or code that support the findings of this study are available from the 344 corresponding author upon reasonable request.