Experimental Study and evaluation of kinetic models of cadmium desorption from bed sediment load in a circular Flume

Document Type : Research Paper

Authors

• Mohsen Nasrabadi ¹
• Mohammad Hosein Omid ²

¹ Department of Water Science and Engineering, Faculty of Agriculture and Environment,, Arak University, Arak, Iran.

² Department of Irrigation and Reclamation Engineering,, Faculties of Agricultural and Natural Resources, University of Tehran, Karaj, Iran.

Abstract

Keywords

• Desorption rate
• cadmium
• river sediments
• desorption kinetic models

Main Subjects

• ..

Introduction

The importance of the adsorption and desorption of chemicals between sediment particles and dissolved materials can be examined from different points of view. First, a significant number of pollutants, especially heavy metals, are adsorbed by suspended sediments or bed sediments after entering the waterway. Second, resuspension of sediments due to dredging, tides, floods, or washing of sedimentation basins may cause pollutants to re-enter the waterway. Also, contaminated sediment particles may release heavy metals under some environmental conditions and become a source of secondary pollution (Huang, 2003a, b). Therefore, it is necessary to understand the mechanism of adsorption/desorption of heavy metals by sediments and the parameters affecting it. However, in general, the results have shown that the same factors affect adsorption and desorption, and the desorption capacity of cadmium is controlled by the cadmium adsorption capacity (Loganathan et al., 2012).

Although many studies have been conducted on the adsorption/desorption of heavy metals, there are still gaps in the literature to be filled. For example, it is still unclear how the flow velocity of a stream can control the desorption rates of cadmium at different concentrations. Therefore, compared to the numerous studies on the desorption of heavy metals from sediments, relatively few studies have investigated the cadmium desorption kinetics from river sediments (especially the Karaj River as a case study) and even fewer have determined the effect of flow turbulence, a common phenomenon in natural rivers, on the desorption rate. Also, almost all studies so far have investigated the adsorption and desorption phenomena under reactor conditions, and no study has been conducted in an open channel, where the flow conditions are similar to those in a river. In addition, in this study, the desorption kinetics were evaluated using eight models developed in the literature. The findings of this study may be useful for predicting the efficiency of heavy metal pollution cleanup methods in rivers. Therefore, the overall objectives of the present study are: 1. To investigate the factors affecting cadmium desorption (pollution concentration and flow velocity) in an open channel and 2. To evaluate different kinetic models in describing cadmium desorption.

Methodology

In this study, a circular flume with an average diameter of 1.6 m, a width of 0.2 m, and a depth of 0.15 m was used (Figure 1). This flume was placed on a fixed platform with dimensions of 2 × 2 m². Sediments were artificially contaminated with cadmium in the circular flume. For this purpose, cadmium solutions with concentrations of 0.15, 0.46, 0.77, and 1 ppm were added to the flow with a sediment concentration of 20 g/L. Circular flume flow was maintained at two constant speeds of 6 and 10 rpm. After the sediments were exposed to different concentrations of cadmium for 12 hours to determine the amount of cadmium absorbed, a sample of the supernatant was taken and the remaining solution was separated from the sediments. After adjusting the temperature, pH and EC, the experiment began by taking samples at specific time intervals from a specific point in the center of the flume.

The flume was then completely filled with distilled water to a depth of 0.13 m. Before the desorption experiments began, a 0.01 M solution of calcium chloride CaCl₂ was used to adjust the electrical conductivity and a dilute solution of HNO₃ was used to adjust the pH. After adjusting the electrical conductivity and pH, to create a concentration of 20 g/L, 2600 g of cadmium-contaminated sediment samples were poured into a circular flume with a water volume of 130 L. The flow in the circular flume was maintained for 12 h until equilibrium was reached. The cadmium ion concentration was measured by taking 50 mL samples at specific time intervals (0, 5, 15, 30, 60, 120, 180, 240, 480, and 720 min) from a specific point in the center of the flume. These samples were immediately transported to the laboratory for analysis by ICP-OES.

Results and Discussion

The results if adsorption experiments showed that as the cadmium concentration in the solution increases, the adsorption rate per unit mass of sediment increases. Meanwhile, the percentage of cadmium removed from the solution also increased with increasing concentration. It can be said that this time is the time to reach equilibrium, which is independent of the cadmium concentration and the sediment concentration. The time for the cadmium desorption process to reach equilibrium is about 2 hours, and from then the desorption rate reaches a constant value. The negligible amount of cadmium desorption indicates a strong and strong bond between cadmium ions and sediment particles. The predictable result in studies where sediments are artificially contaminated is that cadmium adsorption and desorption are reversible processes (physical bonding). However, the results of the experiments in this study showed that most sediment particles and cadmium have established a strong chemical bond. The results of the experiments show that the adsorption and desorption processes of heavy metals by Karaj River sediments are irreversible.

With increasing flow velocity, the amount of cadmium desorption also increased slightly. Also, at low concentrations, the difference in cadmium desorption at both flow velocities is very small and negligible. One of the main differences in sediment movement at 0.18 and 0.35 m/s was that at 0.18 m/s, the sediments had not reached the threshold of movement and were almost stationary at the bottom of the flume. This is while at 0.35 m/s, the sediments were completely moving in the bed and had a lot of rolling and sliding. As a result, increasing the flow velocity and consequently increasing the turbulence causes an increase in the collision of sediment particles with each other, and as a result, a greater amount of cadmium will be released from the sediments. These results show that in seasonal rivers that always have flood currents flowing in them, the turbulence caused by floods can cause the release of heavy metals from the sediments into the flow. This phenomenon is also significant for contaminated sediments accumulated upstream of dam reservoirs.

Conclusions

The results of experiments showed that the initial desorption of cadmium from sediments is a rapid process. Most of the desorbable cadmium is removed from the sediments in about 0.5 hours, although some of the desorption process is still in progress by the end of the experiment.

The results of the experiments in this study showed that most of the sediment and cadmium particles have established a strong chemical bond. The results of the experiments also show that the processes of adsorption and desorption of heavy metals by the sediments of the Karaj River are irreversible.

With increasing cadmium concentration, the rate of its desorption from the sediments has also increased. So that the percentage of cadmium desorption (R) for low concentrations is up to about 10.7 percent, and with increasing cadmium contamination, the percentage of cadmium release reaches 30 percent.

With increasing average flow velocity, the rate of cadmium desorption has also increased. Although the increase in the rate of increase in turbulence has been very small. Also, at high concentrations, the difference in cadmium desorption at both flow rates is very small and about 5%.

The results of the desorption kinetics models showed that the double parabolic diffusion models (with R² = 0.896 and SSE = 1.26), the two-constant rate (with R² = 0.737 and SSE = 1.72) and the parabolic diffusion (with R² = 0.99 and SSE = 5.76) are suitable models for estimating the desorption rate, but after calculating the mean relative error (MAE), it was observed that the double parabolic diffusion model with an average error of 7.02 percent is better than the two constant rate models (with an average error of 10 percent) and the parabolic diffusion (with an average error of 57 percent).

Author Contributions

“Conceptualization, M.N.and M.H.O.; methodology and validation, M.N. and M.H.O.; formal analysis, M.N.; investigation, M.N.; resources, M.H.O.; data curation, M.N.; writing—original draft preparation, M.N.; writing—review and editing, M.H.O.; visualization, M.N.; supervision, M.H.O.;

All authors have read and agreed to the published version of the manuscript.”

Data Availability Statement

Not applicable

Acknowledgements

The authors would like to thank all participants of the present study.

Ethical considerations

The study was approved by the Ethics Committee of the Arak University. The authors avoided data fabrication, falsification, plagiarism, and misconduct.

Conflict of interest

The author declares no conflict of interest.