- Thesis objective
- Thesis organization
- presents a brief review on the geochemical properties and mobility of arsenic in the natural groundwater system. A brief discussion is made in this chapter to
- deals with the groundwater quality parameter estimation methods which are generally collected from study area (Nalbari district, Assam) at different time intervals
- aims at delineation of the unsafe aquifer of the study area. Seasonal variation of arsenic, spatial distribution of arsenic and correlation of arsenic with other water
- focuses on the characterization and optimization of the selected adsorbent
- discusses the finding from the column experimental results of the selected materials at different loading rate, bed depth and initial arsenic concentration. In this
- concludes the entire thesis work with findings in brief. It produces views on possible studies involving the developed arsenic removal filter for future modification or
The preparation, development and testing of the arsenic removal filter is described in detail at the end of this chapter. In this chapter a cost analysis of the newly developed arsenic removal filter is performed.
Arsenic in natural environment
- Geochemistry of arsenic
- Sources of arsenic
- Speciation of arsenic
- Cause of arsenic contamination in groundwater
- Factors controlling arsenic concentration and transport
- Mobility of arsenic
In the atmosphere, the availability of arsenic is mainly due to volcanic eruptions or human activities. In addition to the diverse toxicity of arsenic, the chemical forms of arsenic determine its mobility in the environment.
Arsenic distribution in groundwater
- Arsenic around the World
- Arsenic in India
Concentrations of arsenic in groundwater exceeded 50 μg L-1 in some parts of Assam (18 of 24 districts), Tripura (3 of 4 districts), Arunachal Pradesh (6 of 13 districts), Nagaland (2 of 8 districts) and Manipur (1 of 9 districts). In a recent study, Mahanta et al. 2009) expressed that of the presence of arsenic in the upper Brahmaputra floodplains, a relatively recent discovery, but extensive study, drinking water quality has been made covering all the districts of Assam.
Health impact of arsenic
Arsenicosis is a general term used to identify various symptoms resulting from toxic effects in the human body (Signes-Pastor et al., 2008). Depigmentation: Colorless spots the size of a grain of millet, to densely aggregated raindrop-sized spots, mostly on the stem (Supapong et al., 2004).
Arsenic treatment technologies
- Coagulation/filtration/Precipitation process
- Membrane filtration process
- Electro-coagulation process
- Ion exchange process
- Adsorption process
- Indigenous filters
- Role of iron compounds in arsenic adsorption
- Role of sand in arsenic adsorption
- Role of laterite soil in arsenic adsorption
- Basic criteria for development of household level arsenic removal filter
Sometimes a combination of the media mentioned above is used together to maximize the adsorption of arsenic compounds. In arsenic removal techniques involved, precipitation of iron and immobilization of arsenic by sorption on the Fe(III) precipitates have received much attention.
Preparing the filter The filter will be simple; which can be done by the villagers in the country. Filter life The filter should serve arsenic-free water for a fairly long period of time. Adaptability If the filter will be similar to the existing iron removal filter then it will be easily adaptable.
Laterite soil shows good potential to be used as adsorbent for arsenic removal from aqueous media. This chapter pointed out the need to develop a new arsenic removal technology that is affordable, sustainable, low cost and can be made by local people using locally available materials.
MATERIALS AND METHODS
Delineation of unsafe aquifer in the study area
- Study area
- Field parameters estimation and water sampling procedure
- Seasonal variation of arsenic concentration
- Groundwater quality parameters estimation methods
Collection station for water samples and soil samples in study area (Madhupur block of Nalbari district of Assam, India). Water samples were collected from the tube wells installed by the PHED Department of Assam. Samples were collected after minutes of pumping before collecting water samples until electrical conductivity stabilized.
Alkalinity titration method Standard method (APHA, 1998) Magnesium AAS in flame mode EPA 7000B; Standard Method Calcium AAS in flame mode EPA 7000B; Standard method Arsenic AAS in VGA mode Hung et al., 2004; Standard Method Sodium AAS in flame mode EPA 7000B; Standard Method Potassium AAS in flame mode EPA 7000B; Standard Method D.O. Electrodemeter Standard Method (APHA, 1998) Hardness Titration Method Standard Method (APHA, 1998) Nitrate UV Visible Spectrophotometer Standard Method (APHA, 1998) Ammonia Colorimetric Method Standard Method (APHA, 1998) Phosphate Spectrophotometer Standard Method (APHA, 1998) Sulphate Turbidimetric Method Standard Method (APHA, 1998) Turbidity Turbidimeter Standard Method (APHA, 1998) Chloride Argentometric Method Standard Method (APHA, 1998) Iron AAS in flame mode EPA 7000B; Standard Method Lead AAS in Flame Mode EPA 7000B; Standard Method Fluoride UV Visible Spectrophotometer Standard Method (APHA, 1998) Chromium AAS in flame mode EPA 7000B; Standard Method Cadmium AAS in flame mode EPA 7000B; Standard method Mercury AAS in flame mode EPA 7000B; Standard Method Other Anoins Standard Method Standard Method, APHA, 1998 Other Cations AAS in flame mode EPA 7000B; Standard method.
Comparative evaluation of arsenic adsorption potential of the locally available materials
- Adsorbents materials
- Characterization of the adsorbents
- Reagents and analytical methods
The sand particles were then washed with distilled water to ensure that no impurities remained on the outer surfaces of the sand particles. From the seeding test, the sand particles were divided into three groups to determine the relationship of the sorption mechanism with the size of the sand particles. Energy dispersive X-ray (EDX) analyzes of the sand and red earth were performed simultaneously with a scanning electron microscope (SEM) to determine the chemical and mineralogical composition of the prepared adsorbent.
The major oxide composition of the sand, red soil and NOIS was determined by X-ray fluorescence (XRF) spectrometry (Model: AXIOS, PANalytical, Brand: Philips) analysis. Distilled water as well as tap water from the Environmental Engineering Laboratory (IIT Guwahati) are used for the preparation of arsenic point water for other studies.
- Determination of pH zpc (Point of zero charge)
- Batch experimental procedures
- Continuous mode laboratory scale column experimental procedures
- Co-precipitation test methodology
- Desorption and regeneration studies
Before starting the experiments, pH of the arsenic-spiked solutions [both As(III) and As(V)]. Data obtained from experiments plotted in Excel spreadsheet to analyze the effect of temperature on arsenic sorption. Arsenic concentrations of all the generated samples were analyzed in AAS after experiments were completed.
The mass of the added adsorbent media was estimated from the bulk density and the volume of adsorbent applied to the column. The volume of the distilled water present in the pipe and the column was subtracted from the estimated effluent quantity.
Development of porous media
- Preparation methodology of circular porous disk (CPD)
- Batch experiments procedures of CPD
- Column experiments procedures of CPD
- Porous pot preparation procedures
- Porous filter preparation procedure
- Arsenic removal filter (ARF) test procedure
The development mechanism of both the porous disc and porous media is discussed in this chapter. For example, the upper part of the upper chamber was made non-porous using white clay to increase its water retention capacity. The efficiency of the removal of arsenic from the adsorbent (NOIS) together with the filtration media (sand) was investigated.
The influent flow rate was kept slightly lower than the filtration flow rate of the filter. The adsorbent (NOIS) and filtration media (sand) were placed in the chamber as discussed in Chapter 7, prior to testing the arsenic removal capacity of the filter.
Before adsorbents were placed in the upper chamber, both chambers were properly washed with distilled water. At constant loading rate, arsenic challenged water (200 μg L-1) was allowed to pass through the adsorbents and the filter. The effluent was drained from the lower chamber to prevent the overflow as shown in Fig.
Delineation of Unsafe Aquifers in a Major Arsenic Contaminated Area of Madhupur Block in Nalbari District,
- Spatial distribution of arsenic
- Field parameters variations
- Geochemistry and distribution of arsenic in groundwater
- Groundwater geochemistry
- Correlation of arsenic with co-existing ions
- Arsenic versus calcium and sodium
- Arsenic versus manganese and iron
- Arsenic verses pH
- Arsenic versus phosphate and bicarbonate
- Groundwater characteristics over the different seasons
- Seasonal variation of arsenic
The geo-chemical properties of the groundwater in the study area were estimated and presented in Table A.2. An attempt was made to estimate the correlations of arsenic with other water quality parameters in order to understand the arsenic contamination in the groundwater of the study area. Box and Whisker plots showing variations of major ion concentrations in the shallow groundwater of the study area.
While during the winter season, the arsenic concentration is quite high and the maximum concentration is reached in the month of March. During the winter season, the arsenic concentration was quite high and the maximum concentration was reached in the month of March.
Comparative Evaluation of Arsenic Adsorption Potential of the Locally Available Materials
Characterization of adsorbent
- Point of zero charge (pH ZPC )
- SEM and XRF analysis of adsorbents
- XRD analysis of adsorbents
- FTIR analysis of adsorbents
To analyze the surface morphology and structural features of the sand and red soil particles prior to adsorption, scanning electron microscope (SEM) images were examined (Fig. 5.2-5.3). Highly amorphous nature of the particles may be an increased arsenic adsorption capacity of NOIS. The composition of the main oxides of the sand, red earth and NOIS was determined by X-ray fluorescence (XRF) spectrometry.
The IR bands at 1635 and 1161 cm-1 are assigned to the hydroxyl bending mode of water and the bending vibration of the surface hydroxyl group, respectively (Mohapatra et al, 2011; Ruan et al, 2002). The presence of the minerals and their corresponding bonds in the samples can be identified from the peaks of the IR spectra.
Theory of batch study
- Adsorption capacity
- Kinetic modeling
- Statistical measure
- Thermodynamic study
- Equilibrium study
The plot of versus t will give a straight line and value can be obtained from the slope of the graph. If the plot of versus t follows a straight line, can be obtained from the slope of the graph. The value of kfAs, which is known as global external transport coefficient (m3s-1), can be calculated from the slope of the linear plot of.
The values of ¨Hº and ¨Sº can be evaluated from the slope and intercept of the plot of vs. The maximum adsorption capacity ( ) of the sorbent can be evaluated from Halsey equation using constant initial concentration C.
Batch experimental results and discussions
- Effect of agitation speed on arsenic sorption
- Effect of pH on arsenic sorption
- Effect of contact time
- Kinetic modeling
- Effect of temperature
- Thermodynamics evaluation of arsenic sorption
- Effect of adsorbent dose
- Equilibrium study
- Effect of initial arsenic concentration
- Effect of coexisting ions on arsenic removal
Second-order kinetic plot of arsenic sorption [As(III) and As(V)] adsorption on different adsorbents: (a) NOIS, (b) sand and (c) murum. The initial part of the plots was found to be liner due to rapid sorption of arsenic. Low values of SE confirm that the kinetics of the arsenic sorption of the selected materials follow a pseudo second-order reaction model.
Intraparticle diffusion rate constant parameters for arsenic sorption on red soil, sand and NOIS. Parameters of the intraparticle diffusion rate constant for arsenic sorption on different sizes of specific adsorbents. The correlations of determination values (R2) of the arsenic sorption by different adsorbents reveal that the Temkin model can represent the equilibrium data satisfactorily.
According to the adsorption capacity (Tables and Table B.1), the materials can be arranged in the following order: NOIS> red soil > murum> sand.
Maximum adsorption capacity was found for NOIS and the adsorbents can be ordered according to adsorption capacity as: NOIS>red soil>murum>sand. The initial strength of the arsenic in the aqueous medium affects the sorption capacity of the adsorbents. Arsenic removal efficiency of the NOIS was not much affected by the initial arsenic concentration due to higher adsorption capacity.
At higher concentration, Ca2+ and Mg2+ ions slightly increase As(III) and As(V) adsorption capacity on the adsorbents. Both As(III) and As(V) adsorption on the adsorbents are adversely affected by the presence of PO43-. anions but no significant variation on arsenic sorption was observed for SO42- ion.
Comparative Evaluation of Arsenic Adsorption Potential of the Locally Available Materials Locally Available Materials. Column experiments, co-precipitation, desorption/regeneration).
Continuous mode laboratory scale column studies
- Theory of column performance
- Breakthrough curve modeling
- Results and discussions of column studies
The performance of the column is usually determined by the concept of the breakthrough curve, which is obtained by plotting the normalized arsenic concentration (Ct/C0) against the throughput volume (Vt) at time t for a given bed depth (h) and flow rate (Q ). Where Q is the volume flow rate (ml min-1) of the column and t is the running time (min) of the column, respectively. Thomas (1944) developed one of the most general and widely used models in column performance theory (Baral et al., 2009).
6.1, it is clear that the volume of water treated increased with increasing bed height of the column. This is because the columns with smaller bed depth get exhausted quickly compared to the others.