Removal of heavy metals from aqueous medium and their impact on biological system

Abstract

The presence of arsenic in groundwater has been recognized as a major problem for Bangladesh. Due to its high toxicity, even in low concentrations, it is a threat to human health. There are many techniques, which can be effectively applied to remove arsenic from water streams. Many technologies, including co precipitation with iron or alum, adsorption onto coagulated floc, ion-exchange resin, reverse osmosis, and membrane techniques, have been used to remove arsenic from aqueous solution. Moreover, these existing techniques are costly and they are not economically applicable in small community systems. Therefore, there is a need for developing cheap efficient methods for the removal of arsenic from drinking water. Adsorption techniques are simple and convenient and have the potential for regeneration and sludge free operation. The present work was performed to evaluate the use of duckweed (Lemna minor) and neembark (Azadirachta indica) without any pretreatment as alternate adsorbents for removing arsenic (III) from aqueous mediums. Moreover, the duckweed and neem bark were applied to the removal of arsenic from Bangladeshi As-contaminated drinking water samples in a single-step column operation. For the removal of arsenic (III) using duckweed, various parameters were investigated that affect arsenic adsorption/desorption. Optimum arsenic (III) removal was obtained under the following conditions: initial As (III) concentration, 100 μg/L; duckweed amount, 3 g; average particle size, 0.595 mm; treatment flow rate, 1.67 mL/min; and pH, 5.5,respectively. The desorption efficiencies with 1M H2SO4 was observed 97.67%. The adsorption characteristics of arsenic on neem bark, a low-cost natural adsorbent, were studied in the laboratory scale using real-life sample. Neem bark has tremendous potential as a remediation material for the removal of arsenic from groundwater. The present work investigates the possibility of the use of neem bark adsorption technology without any pretreatment in the removal of arsenic from aqueous media. Various conditions that affect the adsorption/desorption of arsenic are investigated. Adsorption column methods show the optimum removal of As (III) under the following conditions: initial arsenic concentration, 100 μg/L; neembark amount, 5 g; average particle size, 0.595 mm; treatment flow rate, 1.67 mL/min; and pH, 6.9; respectively. The desorption efficiencies with 1M of HCl after the treatment of groundwater were in the range of 79%. The present study might provide new avenues to achieve the arsenic concentration for drinking water recommended by Bangladesh and the World Health Organization (WHO). Tanning industries are one of the main economic activities in Bangladesh. It has been well documented that waste water discharged from tanneries without appropriate treatment that results in detrimental effects on the ecosystem. No eco-toxicity evaluation of any aquatic environment in Bangladesh has been conducted so far. There are various methods to remove Cr (III) and Cr (VI) including chemical precipitation, membrane process, ion exchange, liquid extraction and electrodialysis. These methods are non economical and have many disadvantages such as incomplete metal removal, high reagent and energy requirements, generation of toxic sludge or other waste products that require disposal or treatment. In contrast, the adsorption technique is one of the preferred methods for removal of chromium because of its efficiency and low cost. In this study, groundnut shell (Arachis hypogaea) and flax (Linum usitatissimum) have been used for Cr (III) and Cr (VI) removal from aqueous solution. The present study is intended to use of these locally available as conventional cheap materials as chromium adsorbents. Adsorption and de-sorption of chromium (III) ions on groundnut shell from aqueous solutions have been studied using batch adsorption techniques with respect to the influence of contact time, pH, adsorbent dose, initial chromium concentration and particle size. Appropriate adsorption isotherm and kinetic parameters of chromium (III) adsorption on groundnut shell have also been determined. The results of this study showed that adsorption of chromium (III) by groundnut shell reached to equilibrium after 360 minutes of the experiment and after that a little change of chromium removal efficiency was observed. Maximum chromium removal (87.5 %) was obtained at pH 7.0. The adsorption of chromium by groundnut shell was found to decrease with the higher chromium concentrations in aqueous solutions, lower adsorbent doses and higher particle sizes. The desorption efficiencies with 0.5M KOH was observed 78%. It is observed that the adsorption of chromium (III) by groundnut shell follows Langmuir and Freundlich isotherm equation. The kinetic of the adsorption process follows the first order kinetics with a rate constant of 0.01min-1. The results indicate that groundnut shell can be employed as a low cost alternative to commercial adsorbent in the removal of chromium (III) from water and wastewater. The removal of hexavalent chromium from aqueous solution using nutshell has been best suited for the treatment of real tannery effluents since Cr salt is used for tanning. Batch adsorption studies were carried out under varying experimental conditions of contact time, pH, adsorbent dose, initial chromium concentration and particle size. Appropriate adsorption isotherm and kinetics parameters of chromium (VI) adsorption on nutshell have also been determined. The adsorption followed first order rate expression. The equilibrium data fit well into Langmuir and Freundlich isotherms. Adsorption was maximum at an initial pH of 2.0. The desorption efficiencies with 1 M KOH was observed 87%. It is expected that the developed rate equation could likely be used for design and performance predictions of adsorption processes for treating chromate wastewaters. Furthermore, adsorption of chromium (III) on flax from aqueous solutions has been studied using batch adsorption techniques. Batch adsorption studies were carried out under varying experimental conditions of contact time, pH, adsorbent dose, initial chromium concentration and particle size. Appropriate adsorption isotherm and kinetics parameters of chromium (III) adsorption on flax have also been determined. The results of this study showed that adsorption of chromium (III) by flax reached to equilibrium after 420 minutes. Maximum chromium removal (70 %) was obtained at pH 2.0. The adsorption of chromium by flax was found to decrease with the higher chromium concentrations in aqueous solutions, lower adsorbent doses and higher particle sizes. The desorption efficiencies with 0.5M KOH was observed 90%. It is observed that the adsorption of chromium (III) by flax fitted Langmuir and Freundlich isotherm equation. The kinetic of the adsorption process follows the first order kinetics with a rate constant of 0.01min-1. The results indicate that flax can be employed as a low cost adsorbent for the removal of chromium (III) from water and waste water. A 60-day experiment was conducted to compare the accumulation and toxicological effects of arsenic in muscle, intestine and liver of shingi fish, H. fossilis (Bloch) after exposure to two concentrations (7.0 and 20.0 ppm) of arsenic trioxide. The highest/maximum level of accumulation of arsenic was observed in the liver whereas the lowest level of arsenic was found to accumulate in the muscle tissues at the end of exposure period. It is apparent from the study that the damage of the liver of test fish due to 15 days exposed period was less compared to the damage caused by 60 days exposure periods. The intensity of histological alterations was observed to increase gradually with the arsenic concentration and the exposure time.