Adsorption Of Toxic Metal Pollutants From Aquatic Environment

Abstract

Rapid industrialization and urbanization in Chhattisgarh as well as in others states of India are responsible for accumulation of toxic metals in our daily life through waste water. Consumption of polluted drinking water over a long period of time causes many serious health issues in human beings. Adsorption is an environment friendly and economic tool for removal of toxic metals from aquatic environment. This paper will focus on importance of Tinospora Cordifolia as adsorbent for removal of toxic metals such as nickel, manganese, cadmium and lead.

Introduction

Water is essential for existence of life on earth. Availability of pure drinking water for use has become a big challenge today. Discharge of industrial waste water, agriculture run off, Sewage leakages, High population density, mining, house hold chemicals in water body has made the situation worse1. These activities introduce many toxic metals in water bodies and make them polluted2. Consumption of such polluted water is responsible for severe diseases in human beings. Cholera, hepatitis, diarrhea and typhoid are common water borne diseases3, but higher contamination can cause cancers, birth defects and diseases related to skin, lungs, brain, kidneys and liver4. Therefore removal of toxic pollutants from water is essential for providing safe drinking water to society.

Several commercial and non commercial methods are available for removal of metal pollutants from water. The traditional methods include use of activated carbon, oxidation, nano filtration, reverse osmosis5, but choice of method depends on availability of material involved in method in endemic area, disposal of waste material and cost of the method used. Adsorption is a very good option for removal of metal pollutants from aquatic environment as it is cost effective and causes no harm to environment6. Adsorbents like activated carbon7, fly ash8, cow bone charcoal9, synthesized chitosan10,11, Moringa olifera12, Calotropis procera13, Tectona grandis14, Scolymus hispanicus15 and dried water Hyacinth16. A particular adsorbent is recognized to remove a particular pollutant. In this study bio adsorbent Tinospora Cordifolia was considered as it has very good adsorptive properties of removal of more than one metal pollutant from water.

Water pollutants are mainly classified as inorganic and organic pollutants. This paper will focus on inorganic water pollutants nickel, manganese, cadmium and lead and their removal by a bioadsorbent Tinospora Cordifolia.

Methods for removal of toxic pollutants from water

Wide range of methods is available for removal of toxic pollutants from water. These methods are based on physical, chemical and biological process. Every method has its own advantages and disadvantages, some methods are efficient but costly, some are economic but causes harm to environment. Therefore there is a need of a method with good efficiency, cost management and positive environment perspectives.

Bioadsorption is a better solution

Bioadsorption is a process of removal of toxic pollutants from water by using biological material. Plant materials are very good option for adsorption of pollutants. The use of these materials are beneficial because of their low cost, easy availability, high efficiency, regeneration property, no production of huge sludge, removal capacity over a wide range of pH and temperature and easy disposal of used adsorbent18,19.

Toxic pollutants and Health risk

Toxic pollutants in aquatic environment generally arises from effluents discharged from industries such as discharge from fertilizer industry, dyes and pigment industries, film and photography, galvanometry, metal cleaning, electroplating, herbicides, pesticides, leather and mining industries20,21. Accumulations of these toxic elementss discharged in water bodies cannot be eliminate and thus persist there and consumption of such water by human beings creates lots of health issues. This paper considers the toxicity of nickel, manganese, cadmium and lead in water.

At low concentration some of the metals are required for some biological process, but as their concentration increases they become toxic. In humans these metals accumulate in living tissues and thus enhance the threat to good health.

Manganese is a naturally-occurring element, which is an essential nutrient for humans and animals22,23 as it is required by many cellular enzymes (e.g., manganese superoxide dismutase, pyruvate carboxylase) and can serve to activate many others e.g., kinases, decarboxylases, transferases, hydrolases, etc.24-26. Although manganese is an essential nutrient at low doses, chronic exposure to high doses may be harmful and is responsible for neurological disorders.

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Tinospora Cordifolia

Tinospora Cordifolia is a member of Manispermeace family28. It is a large, deciduous extensively spreading climbing shrub with several elongated twining branches. It is a fairly common plant of deciduous and dry forests, growing over hedges and small trees. Tterpenoids, alkaloids, lignans and steroids are the major chemical constituents of the plant. Stem, root and leaves all parts of the plant are important from phramacognostical point of view. It is an essential component of many medicines and used as anti cancer, anti diabetics, anti oxidants, anti-inflammatory, anti ulser and anti stress medicine in pharma industry. The plant is widely distributed throughout the tropical region of India. It is easily found plant of Kumaon, Assam, West Bengal, Bihar, Deccan, Konkan, Karnataka and Kerala29. It is well spread in various parts of Chhattisgarh also28,30. This study focused on adsorption of metals by stem part of the plant.

Factors affecting adsorption process

Effect of pH

pH plays an important role in adsorption process of metals on bioadsorbent. pH of the solution reflect the competition of hydrogen ions with metal ions to occupy the active sites of adsorbent. Nickel30, manganese28 and lead31 exhibit maximum adsorption on Tinospora Cordifolia at pH 4 whereas cadmium showed maximum removal at pH 221 for various concentration of metal ions. As the pH of solution increases, removal efficiency of bioadsorbent decreases. This may be due to the fact that in alkaline media metal ions get precipitated in the form of respective metal hydroxides. Therefore acidic environment is preferential for adsorption of these metals on Tinospora Cordifolia.

Effect of adsorbent dose

Adsorbent dose is related with the available surface area of adsorbent for adsorption. In case of nickel adsorption on Tinospora Cordifolia adsorption dose was varied from 0.5 g to 20 g per litre and maximum removal is obtained at 5g/L for metal ion concentration of 50 mg/L30. For manganese dose was varied from 0.5 g to 7 g per 50 ml and maximum removal is obtained for 1g of biomass28. For cadmium dose was varied from 1 to 8 g for metal ion concentration of 100 mg/L and maximum removal is obtained for 5g of biomass21. For all these metal ions removal capacity of biomass initially increases with increase in adsorbent dose which in turn provides more active sites for adsorption of metal ions but after optimum dose of adsorbent further increase do not show any marked effect on removal efficiency.

Effect of initial concentration

When initial concentration of metal ion solution increases, more and more metal ions are readily available to capture the active sites of bioadsorbent. As a result removal efficiency increases with increase in concentration of metal ion solution. But increase in metal ion concentration after optimum value will reduce the ratio of sorptive surface and ion concentration which results in decrease of removal efficiency of biomass30. For Ni (II) ion, percentage adsorption decreases from 92.3% to 72.2% with increase in metal ion concentration from 50 to 500 mg/L30 For Mn (II) ion, metal ion concentration was varied from 50 to 500 mg/L. Removal efficiency increases up to 200 mg/L concentration and after that further increase in metal ion concentration percentage removal of metal decreases28. For Cd (II) ion, metal ion concentration was varied from 100 to 1000 mg/L and percentage removal efficiency varied from 90% to 70%21. Although the biomass is efficient to remove 70% of 1000 mg/L cadmium ion solution, but maximum removal efficiency (90%) was obtained at 100 mg/L. Similarly the biomass showed 92% removal effiency for 450 mg/L Pb (II) ion31. This indicates that the biomass Tinospora Cordifolia has very good adsorptive property to adsorb these metal ions from their higher concentrations. Therefore it can be used as effective adsorbent for removal of these metals for industrial effluents and waste water.

Effect of contact time

Contact time in adsorption process is the time for which adsorbate is kept in contact with the adsorbent. Removal of pollutant occurs efficiently when it get sufficient contact time with the adsorbent. For Ni(II), Mn(II) and Cd(II) 30 min time was sufficient for maximum removal of metal ions by Tinospora Cordifolia 21,28,30, on the other hand optimum contact time for removal of Pb(II) was obtained at 60 min31. After this optimum value of contact time further increase in time has no marked effect on removal of these ions by biomass which may be due to the saturation of active sites of adsorbent material.

Isothermal and kinetic study of metal adsorption on biomass

Isothermal study of adsorption gives relation between amounts of adsorbate adsorbed on the surface of adsorbent. Distribution of metal ions in liquid phase and solid phase was described by Langmuir and Freundlich, where qe is the metal amount adsorbed per unit mass of adsorbent (mg/g), Ce is the equilibrium concentration of metal in the solution (mg/L), Q0 is the maximum adsorption capacity (mg/g), KL is the constant related to the free energy of adsorption, KF is adsorption capacity of the adsorbent and n is a characteristic constant for the adsorption system. Langmuir model describes monolayer adsorption whereas Freundlich model describes multilayer adsorption phenomenon. Higher the value of correlation coefficient R2 for a model, more will be the suitability of data with the model. Adsorption of Ni(II), Cd(II) and Pb(II) ions were follow Freundlich multilayer adsorption21,30.31, but for Mn(II) ion data was best fitted with Langmuir model indicating towards monolayer adsorption.

Kinetics study of adsorption gives explanation of the rate of solute uptake by biomass. It helps in determinig the efficiency of adsorption. pseudo-first order and pseudo-second order kinetic models were taken into account for metal adsorption on biomass. Description of these models are the amount of metal ion adsorbed in in mg/g at time t (min) and at equilibrium respectively and k1 and k2 are the pseudo-first order and pseudo-second order rate constants. Adsorption of Ni(II) and Pd(II) ions on the biomass obeyed pseudo-second order kinetics (Sao et al., 2014; Sao et al., 2017), but such kinetic studies were not made in case of manganese and cadmium ions28,21 .

Column adsorption of metal ions by Tinospora Cordifolia

Column adsorption study has wider applicability in adsorption process. In batch study only a small volume of effluent is kept in contact with biomass, but in column study a continuous flow of effluent is maintained in biomass packed column with the help of peristaltic pump. Therefore adsorbate gets sufficient time to get adsorbed on the surface of adsorbent. This enhances the removal efficiency of adsorbent. This study found that in case of column adsorption of Cd(II) and Pb(II) ions through Tinospora Cordifolia packed column, adsorption capacity was 43.06 and 63.77 mg/g respectively which was much higher than their adsorption capacity in batch made that is 38.91 and 20.83 mg/g at a constant effluent flow rate of 13ml/min . But in case of Ni(II) ion column adsorption capacity 2.49 mg/g is lower than that obtained in batch mode i.e. 4.48 mg/g at a constant effluent flow rate of 13ml/min. This may be due to the fact that the chosen flow rate or bed depth might not be suitable for column adsorption of the ion.

Mechanism of adsorption on Tinospora Cordifolia

FTIR analysis of native and loaded biomass confirmed the presence of hydroxyl, amine, carbonyl, nitrile, nitro groups along with carbon – carbon double bond and triple bond in the biomass. Shifting in the position of peaks of such groups indicates that presence of hydroxyl, aliphatic -C-H and carbonyl groups in biomass is responsible for its metal binding property28,30,31.

Conclusion

Present study conclude that Tinospara cardifolia an indigenous plant has good removal efficiency for inorganic pollutants nickel, manganese, cadmium and lead from aquatic environment. Therefore it can be used for removal of these ions from industrial effluents. The plant has many medicinal properties, so treated water can be used for drinking purpose without causing any harm to human health and the biomass can be easily disposed off without causing any damage to environment. Presence of hydroxyl, aliphatic -C-H and carbonyl groups in biomass is responsible for its metal binding property. Biomass is easily available and has multi ion adsorption capasity; hence its use is cost effective.

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