Document Type : Research Article
Authors
1 Faculty of Natural Resources and Environment, Science and Research Branch, Islamic Azad University, Tehran, Iran
2 Graduate Faculty of Environment, College of Engineering, University of Tehran, Tehran, Iran
Abstract
Introduction: Increasing freshwater consumption caused to reduce renewable freshwater resources in recent years, and one of the basic strategies would be use of non-conventional water resources. Arsenic is one of the natural elements widely distributed in the Earth’s crust. It is commonly found in compounds with oxygen, chlorine, or sulfur, which generally contain inorganic arsenic compounds. Arsenic organic compounds also contain hydrogen and arsenic carbon. There are several methods that can limit the amount of arsenic in water and wastewater; one of these methods is surface adsorption. In this process, any solid that tends to absorb the fluid environment on its surface is considered as an adsorbent. Absorption capacity, selectivity, reproducibility, kinetics, compatibility, and cost are the most essential characteristics of the adsorbent.
Materials and Methods: In this study, activated carbon derived from agricultural waste was used as tertiary treatment. The heated coal powder used in this study was obtained from the almond and walnut peel waste (from Tuyserkan city of Hamedan province). Activated carbon powder was used in laboratory-scale experiments and was performed for arsenic removal from synthetics samples. Physically activated carbon was obtained and then chemically activated by acidification. Characterization tests (i.e., XRD, FT-IR, BETand SEM tests) were carried out on both types of the adsorbent. Arsenic removal was carried out in batch experiments. The effect of laboratory parameters (i.e., contact time, pH, adsorbent dose, and initial concentration) on the removal process was studied. Experiments are carried out step by step, and after optimizing each parameter and keeping the other parameters constant, all the parameters are optimized accordingly.
Results and Discussion: The contact time for the adsorption process was considerably decreased in comparison with previous studies. Kinetic and equilibrium studies showed that the adsorption process followed by Langmuir isotherm and second-degree kinetic models. Chemical activation, improved performance, and characteristics of the adsorbent. Acidified charcoal and raw charcoal were compared, and it was found that acidic charcoal had the finest cavities and had a uniform distribution. Although the volume of the cavities has not changed significantly, the structure of the cavities has changed substantially, with the most enormous volume of cavities (0.5 cm3 / g) being less than 5 nm in diameter and the average diameter of the cavities Decreased by 2 nm. According to the results of the Coal Structure Morphological Survey (SEM), in crude coal, the cavities have large openings. Their number is small, but in acidified coal, the number of cavities is increased, and the surface area of the coal is high. The high internal surface area and the presence of microstructural cavities lead to high adsorption of arsenic at the acid-adsorbed sites. With increasing contact time from 0 to 3 minutes, the removal rate of arsenic increases, and after 3 minutes to 10 minutes, it grows with a low gradient and then the removal percentage slightly. In other words, after 10 minutes, there is a balance between the solid and liquid phases. The arsenic removal rate reaches 90% at the third minute and 100% in approximately 13 minutes. As the retention time increases, the contact time of the arsenic with the adsorbent increases, and the adsorption rate increases as the opportunity for contact with the adsorption sites increases. Due to the high specific surface area of the adsorbent and its morphological characteristics, the removal rate reaches 100% with time.
Conclusion: The XRD experiment shows that improved coal is closer to the stable structure than the raw coal. According to the FT-IR experiment, the acidified charcoal decreased the oxygen and aliphatic functional groups and increased the hydrophobicity of the charcoal. The BET experiment revealed that the cavity surface size increased, and the cavity diameter decreased. The cavity distribution was such that the largest volume of cavities was in the range of nanomolecular size. The SEM image also shows an increase in the fine cavities. As a result, the adsorbent has a good morphology and reduces the adsorption time. Also, its special surface is high and has uniform cavities distribution, which can be one of the main causes of high removal percentage. The results showed that at concentrations of less than 120 µg / L and 10 min retention time, the removal rate was 100%. At higher concentrations, drinking water standards can be reached by increasing the retention time or adsorbent dose. The rate of uptake depends on both the concentration of arsenic and the amount of the adsorbent
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