REMOVAL OF ARSENIC FROM WATER BY SURFACE MODIFIED ADSORBENTS AND IMMOBILIZED WHOLE CELLS

Abstract

Various types of arsenic related human cancers have been reported in recent years around the world due to the continuous consumption of arsenic contaminated groundwater for drinking purpose specially in Bangladesh and India. Thus, supply of arsenic free drinking water at an affordable cost to common mass is of paramount important in these countries. Arsenic enters naturally into groundwater by geothermal, geohydrological and biogeochemical processes. Whereas, surface water, which is also a source of drinking water, is contaminated by arsenic through industrial effluents as well as acid mine drainage necessitating the treatment of these streams. Under the above backdrop, the present study has been undertaken to develop a suitable method for removal of arsenic from groundwater as well as acid mine drainage. For this purpose, simulated groundwater containing As(III), As(V), Fe and Mn in the concentrations and pH normally encountered in arsenic contaminated groundwater of West Bengal and Jharkhand, India, has been considered with total As0 and pH as 188 ug/1 and 7.1 ±0.1 respectively. Whereas, the simulated acid mine drainage containing As(III), As(V), Fe, Mn, Cu and Zn has been considered based on available literature data with a total As0 concentration of 25 mg/1 and pH6.5. A review on arsenic removal technologies reveals that amongst various treatment options, like adsorption, ion-exchange, coagulation-precipitation, membrane filtration, biological treatments etc., adsorptive removal process as well as biological treatment with immobilized whole bacterial cells have attracted considerable attention in recent years. However, the efficiency of these techniques largely depends on relative amount of As (III) and As(V) present in the solution. Therefore, quantitative detection of various arsenic species in water in addition to the total arsenic is a prerequisite for prescribing a technology. To address the above requirement, in the present work, an arsenic speciation technique has been developed in which As(T) of the solution is first measured by ICP-MS and then the same solution is treated in a strong base anion resin (AG1 X8) in column to separate As(III) and As(V) fractions. The eluent from the column which primarily contains As(III) is then analyzed by ICP-MS again. Infact, Fe and Mn ions, which are normally available in groundwater, influence the separation of As(III) and As(V) in resin column. To compensate these interfering effects, models have been developed which can predict As(III) in eluent with around 97 % accuracy. Though, granular activated carbon (GAC) and activated alumina have widely been used for the treatment of water and wastewater, GAC has been less exploited due to its poor performance. This is primarily due to the fact that at pH value of7the GAC surface and arsenic both exist as predominantly negatively charged and thus do not attract each other. Thus, GAC shows very poor removal efficiency (45-65 %) at this pH range. However, the predominantly negatively charged surface ofGAC, which is the prime cause for its poor performance, can be converted to predominantly positively charged surface if metal ions having more than one positive charge are impregnated on the surface ofGAC. Infact, this metal ion gets chemisorbed on the surface of GAC and during this process attaches itself to GAC surface by neutralizing one negative charge and subsequently generates a positive charge in this location and thus helps in increasing the positive charge density of GAC. In the present investigation the surface of GAC has been modified by impregnating Fe3+, Mn +/Mn +, Cu2+ and Ca2+ ions one at atime on its surface to create different surface modified adsorbents which have positive charge density on its surface at neutral pH. These surface modified GACs as well as untreated GAC have been used for the removal of arsenic from simulated groundwater in batch reactors. Effects of process parameters like adsorbent concentration, agitation time, pH and temperature on the removal of arsenic species from simulated groundwater have been investigated using the above adsorbents to determine the optimum process conditions for maximum removal of arsenic. Effects of initial arsenic concentration as well as the effect of the presence of Fe and Mn on the removal of arsenic species have also been investigated. Optimum adsorbent concentrations for GAC, GAC-Fe, GAC-Mn, GAC-Cu and GAC-Ca for the removal of arsenic species at agitation period of 24 h are found to be 16 g/1, 5g/1, 5g/1, 6g/1, and 8g/1 respectively. All the above stated surface modified GACs have been able to reduce the arsenic concentration in treated water below 10 u.g/1, which is the MCL value for arsenic in drinking water as per WHO standard. However, as per Indian standard this value is 50 u.g/1. It has also been observed that GAC can reduce the As(T) concentration in treated water to 84 fj.g/1, which is above the MCL value for arsenic in drinking water and thus can not be used as it is without any surface modification. Further, these adsorbents including untreated GAC are capable of removing Fe completely from the solution. However, this is not so for Mn. Amongst the above adsorbents GAC-Fe and GAC-Mn have been found to be most efficient in terms of its capacity to remove arsenic (~ 95 %). It has been observed that presence of Fe and Mn ions in water oxidizes As(III) to As(V) to some extent and thus influences the adsorption process of arsenic species. Therefore, interaction effects of Fe & Mn ions and As0 on the % removal of arsenic species have also been studied to compute the % removal of arsenic species at various As0, Fe and Mn concentrations using GAC-Fe as adsorbent. To facilitate mathematical modeling adsorption kinetics models as well as adsorption isotherms have been developed. Adsorption of both As(III) and As(V) on GAC and surface modified GACs has been found to follow pseudo 2nd order kinetic model. Amongst the conventional isotherms such as Langmuir, Freundlich and Temkin isotherms, the Freundlich isotherm gives better prediction of specific uptakes for As(III) and As(V) adsorption at equilibrium for all of the above stated adsorbents. Though, empirical polynomial isotherms have less acceptance amongst investigations due to the fact that these do not explain the physics of adsorption process, these provide more accurate prediction of specific uptakes for As(III) and As(V) adsorption at equilibrium for all of the adsorbents investigated. Thus, polynomial isotherms have also been developed in the present study. To investigate the re-use capability of adsorbents, spent adsorbents have been regenerated using various regenerating liquids like 5N H2S04, IN NaOH, 30 % H202 in 0.5 N HN03 & H20 and have been put to reuse for arsenic removal from simulated groundwater. Amongst these regenerating liquids, 5N H2S04 has been found regenerate adsorbents more efficiently than other regenerating liquids. in Biological treatments with bacterial whole cells is another important route for arsenic removal from water in which whole cells are used either in bulk phase or are immobilized on a solid support (adsorbent). Immobilized whole cells produce bio-layer on adsorbent surface and when treated with solution having arsenic, the bacterial cells accumulate it within the cell mass as well as it gets adsorbed on cell surface and adsorbent surface. This process is termed as simultaneous adsorption bioaccumulation (SABA) process. In the present work whole bacterial cells ofRalstonia eutropha MTCC 2487 has been screened as most efficient arsenic resistant bacteria amongst Ralstonia eutropha MTCC 2487, Bacillus indicus MTCC 4374, and Pseudomonas putida MTCC 1194 on the basis of their arsenic removal capacity in bulk phase and has been used for the removal of arsenic from simulated acid mine drainage. Effects of process parameters like agitation period and pH on the removal of arsenic species using R. eutropha and GAC have also been investigated in batch reactor to determine the optimum process conditions. Effects of initial arsenic concentration as well as the types of adsorbents on arsenic removal have also been investigated. Optimum agitation period and pH for the bio-removal ofarsenic from simulated acid mine drainage are found to be 100 hand 6-7 respectively. Initially, %removal ofarsenic increases with the increase in As0 value from 0.1 mg/1 to around 15 mg/1 and thereafter it decreases gradually. Use of GAC-Fe in place of GAC along with R. eutropha increases the %removal ofAs(T) from 86 %to 92 %at As0 value of15 mg/1. Contribution of different arsenic removal mechanisms such as physical adsorption, chemisorption, bio-adsorption on bacterial cell walls and bioaccumulation in cells on SABA process has been determined. Further, the efficiency of SABA process has also been compared with adsorption of arsenic on GAC surface. The SABA process is found to be superior to adsorption process when As0 value is less than 50 mg/1. Simulated acid mine drainage has also been treated in bio-column reactor using immobilized R. eutropha on GAC bed. Effect of empty bed contact time (EBCT), bed height and regeneration (back washing) of bio-column reactor bed on the removal of arsenic species has been IV investigated. It has been found that an EBCT of six hours is sufficient to reduce the concentration of As(T), Fe, Mn, Cu and Zn from 25 mg/1, 10 mg/1, 2 mg/1, 5 mg/1 and 10 mg/1 respectively to 0.15 mg/1, 1.7 mg/1, 0.09 mg/1, 0.6 mg/1 and 0.4 mg/1 respectively. These concentrations are less than the permissible limits of As(T) (0.2 mg/1), Fe (3 mg/1), Mn (0.2 mg/1), Cu (3 mg/1) and Zn (5 mg/1) in wastewater as per WHO as well as Indian standard.