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Temperature and pH Effects on Adsorption Materials used for Arsenic Removal from Drinking Water.

Byline: Saeeda Yousaf, Shahida Zakir and Sardar Khan

Summary: This study focused on removal of arsenic from drinking water using different media such as bauxite, lime and plastic clay. Media were treated with different temperature and pH to check adsorption capacity changes. According to the results obtained, temperature (30 degC and 40 degC), pH 9 and 1h retention time has resulted maximum removal of arsenic on the media.

The adsorption data was analyzed and it was found to obey Langmuir model with a good value (0.99) of correlation coefficient.

Keywords: Adsorption, bauxite, lime, plastic clay, temperature.

Introduction

In developing countries, most of the diseases are related with the consumption of contaminated water [1, 2]. Contaminated water is a serious environmental issue that has further aggravated with the recent discovery of arsenic in drinking water. The groundwater contamination with arsenic has occurred in a number of Asian countries (China, India, Pakistan, Bangladesh etc) that has led to a major environmental concern. Conventional water treatment processes remove toxic metals through various mechanisms such as sorption and particle removal. Oxidation/reduction reactions reduces (add electrons to) or oxidizes (remove electron from) chemicals, altering their chemical form [3].

Sedimentation is the gravity separation of solids from liquid by settling process. It is generally used in conjunction with precipitation or coagulation. Conventional filtration is the separation of solid particulates from a liquid by passing the mixture through a medium, like sand, anthracite coal, activated carbon, cloth, paper, which retains the solid on its surface and allows the liquid to pass through them. Common particulates remove infiltration include silt, clay, colloidal and precipitated natural organic matter, naturally- occurring iron and manganese precipitates, metal salt precipitates or polymer coagulation, and microorganisms. Bacteria are playing an important role in catalyzing many of the above mentioned processes.

The process of evaporation and recondensation separates all chemicals, including arsenic, from the water. Membrane separation uses semi-permeable membranes that are selectively permeable to water and certain solutes to separate impurities from water. Like evaporation and recondensation processes, membranes also remove many different kinds of dissolved solids, including arsenic, from water. However, they are usually expensive and therefore, are typically considered in applications such as desalination, brackish water conversion and for removal of specific ions, such as arsenic, that are difficult to remove by other technologies [3].

Coagulation/Filtration is an effective treatment process for removal of As (V). Alum performance is slightly lower than ferric sulfate. Ferric chloride gave the most favorable results ((greater than)90%) for removal of arsenic [4].

Arsenic removal in conventional treatment plants (i.e., coagulation plants with alum or ferric chloride) is suitable for large- scale arsenic treatment. It was found that one of the mechanisms for arsenate removal during coagulation was arsenate adsorption onto iron or aluminium hydroxide flocs [5]; while in another experiment it was found that arsenic removal during coagulation is also the result of precipitation of arsenic with iron or aluminum hydroxides [6]. Overall, ferric coagulants seem to be more effective; one of the problems with using alum as coagulants is that there is incomplete precipitation of soluble aluminium hydroxide [5-7].

The adsorption of the oxyanions of As+3 and As+5 on goethite has been studied in presence of various inorganic macro-elements (Mg, Ca, PO4, CO3). The presence of Ca and Mg has no significant effect on As+3 oxyanion (arsenite) adsorption in the pH range relevant for natural groundwater (pH 5-9).

In contrast, both Ca and Mg promote the adsorption of PO4-3. A similar (electrostatic) effect is expected for the Ca and Mg interaction with As+5 oxyanions (arsenate). Phosphate is a major competitor for arsenate as well as arsenite [8].

Bauxite is a rock composed mainly of aluminum oxide and aluminum hydroxide minerals. Alum (potassium aluminum sulfate) is a white mineral.

A particularly stable oxide of aluminum, aluminum oxide (Al2O3) can be obtained from alum. Activated alumina is a media produced by the controlled calcinations of alumina trihydrate. Plastic clay is an extremely rare minerals found in very few places around the world. It is also sometimes referred to as ball clay. Plastic clays are sedimentary in origin. Plastic clay usually contain three dominant minerals; kaolinite, mica, and quartz.

Plaster of paris, CaSO4.1/2 H2O or (CaSO4)2.H2O It occurs in nature as gypsum and the anhydrous salt as anhydride. Limestone is defined as a rock of sedimentary origin composed principally of calcium carbonate or the double carbonate of calcium and magnesium, or a combination of these two minerals. Calcined bauxite was suggested a solution to the real life problem of arsenic poisoning of potable ground- water. Calcined bauxite, a locally available adsorbent in a modified form, acting as scavenger for As+5.

The modified calcined bauxite, (MCB) exhibited excellent As+5 removal (99-100%) over a wide range of pH from 2- 8 in batch studies. The column study results have shown that at a sorbent bed depth of 10 cm and feed flow rate of 8 ml min[?]1, the MCB media was capable of treating 8.16 l (260 bed volumes) of As+5 spiked water (C0 = 2 mg l[?]1) before breakthrough. The values of adsorption rate coefficient (K) and adsorption capacity coefficient (N) are calculated by Logit method of column design and were found to be 0.2467 l mgL-1 h[?]1 and 865.85 mgL-1, respectively [9].

The pH of the water also has a strong effect on adsorption efficiency. As pH changes, the charge associated with the arsenic anion also changes. The H2AsO4- anion carries a single negative charge at or below pH 7, but it loses a proton at higher pH, resulting in a doubly charged anion, HAsO4 .

The singly charged anion is adsorbed more effectively from solution than the doubly charged species. In a similar manner, the charge state on the surface of the adsorbent varies with pH also. As temperature increased, arsenic adsorbent performance significantly improved (P _ 0.001, (Alpha) = 0.001, n = 6 for each adsorbent) for Fe modified activated carbon, chabazite, and clinoptilolite. The arsenic removal efficiency improvements with increasing temperature suggest the Fe modified activated carbon and chabazite could operate successfully as arsenic adsorbents from 23* to 35*C. Decreased maximum loading capacity with increasing temperature was reported for As (V) adsorption on goethite [10].

Temperature and pH has great impacts on removal efficiency of arsenic; therefore, the present study is designed to find out the effect of temperature and pH on selected media such as bauxite, plastic clay and lime on adsorption of arsenic from drinking water.

Results and Discussion

Adsorption Experiment

Solution with standard addition of arsenic was taken in column and its pH was adjusted by 1N HCl. Known weight of media was added to arsenic solution of known concentration and it was shaked with the help of motor for 1 h. After 1 h, an aliquote was taken from column, filtered through Whatman filter paper No 42 and the filtrate was analyzed for arsenic.

Kinetics of Adsorption

The rate of the adsorption can be described as the amount of adsorbate that is adsorbed in a unit time. The adsorption kinetics was studied by choosing 25 mgL-1 arsenic standards -solutions. To a 450 g of media 600 ml of arsenic solution was added and kept for various time intervals. The pH was maintained at 7 by 1N HCl. The batch experiment was conducted at room temperature 18 degC. After an adsorption time of 1, 2, 3, 4, 5, 24, 48, 119 and 143 h, samples were taken, filtered and analyzed for arsenic (Fig. 1).

The surface area analysis, explained in detail in experimental section, indicated that specific surface area of the media after adsorption (25 mgL-1at pH 7) decreased from 36.05 m2 to 13.77m2. But there was an increase of adsorption energy from 2.16 KJ/m to 2.55 KJ/m and beside decrease in the micro pore volume from 0.01 cc/g to 0.00 cc/g. According to literature pore volume and size are considered to be more important factor rather than surface area for effective adsorption processes.

Effect of Temperature

Media supplemented with different additives (constituents) were subjected to different temperatures for the removal of arsenic (Fig. 2). Known concentration of arsenic was added to different media at different temperatures.

It was observed that the temperature has a significant effect on the adsorption of arsenic. Keeping temperature at 30degC, the maximum removal of arsenic was 93% whereas at 40degC the removal was 94% obtained. On increasing the temperature up to 50degC, the removal decreased, therefore temperature of 30degC and 40degC was the ideal temperature for maximum removal of arsenic (Table-1).

Table-1: Adsorption of arsenic by media at different temperature.

###C0 (mgL-1)###Ce (mgL-1)###Temperature (C0)###Removal (%)

###5.30###2.23###30###42

###9.31###8.69###30###93

###13.87###11.52###30###83

###21.02###15.57###30###74

###26.63###22.15###30###83

###5.30###4.97###40###93.7

###9.31###8.63###40###92.6

###13.87###11.4###40###82

###21.02###14.96###40###71

###26.63###19.72###40###74

###5.30###3.98###50###75

###9.31###8.55###50###91.8

###13.87###11.1###50###80

###21.02###12.28###50###58

###26.63###17.01###50###63.8

Adsorption Isotherm

The adsorption data was analyzed in terms of Langmuir model (Fig. 3) and found to obey the same with a correlation coefficient of 0.99. Maximum adsorption Qm and binding energy K calculated according to the well-known Langmuir's linear form 1 equation Ce/qe= 1/KaQm + 1/Qm Ce where Ce is the equilibrium concentration of adsorbate, K the adsorption coefficient, qe and Qm the amount of adsorbate (mg gm[?]1) at equilibrium and saturation, respectively. From the Langmuir adsorption isotherm using the ratio of slope to intercept of the linear plot, the value of adsorption coefficient was calculated.

The results are expressed in terms of Langmuir adsorption model. For each adsorbent dose, a solid phase concentration X (mg/g) was calculated from initial (C0) and equilibrium (Ceq) concentration (mg/g) of arsenic resulting from the given adsorbent dose (C0-Ce) V/m. The data for each experiment at 30, 40, and 50were plotted in two form of a classic isotherm (Fig. 4).

The ln b versus 1/T was plotted as shown in Fig. 5. From the slope, DS, DH and DG were calculated. It was concluded that the first phase was by diffusion but changed to chemical later.

DS= R intercept

DS=8.314 31.475= 261.68 JK-1mole-1

Slope = -DH/R DH= slope R

DH = - (-9.4923) 8.314

DH = 78.91 JK-1mole-1

DG = DH-TDS

DG = 78.91 - (303 261.68)

30 degC DG=-79.210 KJ

40 degC DG= -81.826 KJ

50 degC DG=-84.443 KJ

Effect of pH on Arsenic Adsorption

The effect of pH on the adsorption of arsenic at the surface of the media (bauxite+ lime+ plastic clay) was studied with an initial concentration of 25 mgL-1 arsenic solutions (Fig. 6). The desired pH values of 1, 3, 5, 7, 9, 11, 12, and 13 were established with buffer. 30 g of media was added to each of test solution and equilibrated for 1 hour.

The volume of solution and weight of media were kept constant. pH was measured before and after passing the solution through media. It was noted that pH decreased after passing the solution through media. After equilibration, un-reacted arsenic and the pH of the solution were measured and results are shown in Table-2.

From these results it is apparent that maximum adsorption of arsenic on the media was observed around pH 9. It was also observed that pH of the standard -solution was shifted slightly towards higher pH i.e. towards basic.

This slight shift in pH may be attributed to the lime as the lime containing carbonates have alkaline nature and as such the pH is shifted to the basic range.

Contact Time Effects

Effect of contact time of the arsenic solution with a constant weight of the media was studied. Volume of standard solution 240 ml, weight of the media 147g and recirculation time was kept constant whereas the contact time was varied. pH of the standard solution and elute were also noted. The initial and equilibrated concentration of arsenic was determined. It was observed that keeping the contact time at 1 hour, arsenic removal was observed to be varied from 17.3-91.9%. Increasing the retention time to 2 hours, arsenic removal was observed to be varied from 67-100% (Table-2). However, 1 h retention time has resulted maximum removal of arsenic (92-100%) in 04 media 1, 2, 6, 8, 9.

Zeta Potential Measurement

The pH ZPC was determined by measuring zeta potential (ZP) as a function of pH. A zeta meter was used to measure ZP as a constant ionic strength (established by KNO3); interpolation of the ZP versus pH provided an estimate of pH ZPC; corresponding to the pH where the ZP is zero. PZC values were found 2 (Fig. 7). At pH 2, PZC of aluminum was observed and 2nd PZC at 11 was associated with lime.

Table-2: Removal of arsenic at different contact time using different media.

###Media###Contact time (hour) pH of Standard Arsenic Sol. pH of Elute Solution C0 (mgL-1) Ce (mgL-1) As+5 Removal (%)

Bauxite, Lime and PC###1###2.1###12.15###8.188###0.661###91.9

Bauxite, Lime and PC###1###2.3###11.12###11.809###3.224###72.6

Bauxite, Lime and PC###1###2.05###11.19###18.093###7.161###60.4

Bauxite, Lime and PC###1###2.08###12.05###25.606###11.966###53.2

Bauxite, Lime and PC###1###3.5###12.06###32.700###27.012###17.3

Bauxite, Lime and PC###2###2.1###12.05###8.188###0###100

Bauxite, Lime and PC###2###2.2###12.01###11.809###2.742###76.7

Bauxite, Lime and PC###2###3.02###11.93###18.093###1.167###93.5

Bauxite, Lime and PC###2###3.05###12.03###25.606###.04###99.8

Bauxite, Lime and PC###2###2.85###12.01###32.700###10.774###67

Experimental

Materials

A stock solution of 500, 5, 10, 15, 20 and 25 mgL-1 arsenic were prepared from a standard solution (1000 mgL-1) of arsenic. Bauxite Al2O3 (with less Si, Fe, Ti), plastic clay Kaoline: Al2O3.2SiO2.2H2O Mica: XY2-3 Z4O10 (OH, F) 2: X= K, Na, Ba, Ca, CS (H3O), NH4 Y= Al, Mg, Fe+2, Li, Cr, Mn, V, Zn Z= Si, Al, Fe+3, Be,Ti Quartz: SiO2 PLASTER OF PARIS (Gypsum cement): CaSO4. 1/2 H2O limestone Ca (Mg) CO3 Alum Al2 (SO4)3.12(H2O) Activated Alumina Al2O3 Commercial alumina Al2O3 were used for different media preparation for arsenic adsorption.

Media Preparation

The above stated materials were ground and mixed thoroughly with the help of motor and sufficient distill water was added to the mixture to form thick slurry. Lumps of irregular shapes were prepared from the mixed materials, dried in oven at temperature 600degC, ignited at 900degC in a muffle furnace for 2 h and cooled. The lumps were then filled in a glass column of length of 28 inch and diameter is 5.5mm fitted with a stopper at the lower end.

The standard solution of a known concentration of arsenic was poured in the column filled with a media at a flow rate of 16.4 L/h. The solution was retained for different interval of times and then collected in a beaker through the stopper at the end of the column.

Atomic absorption spectrometer (hydride generation mode) was used for the analysis of arsenic in water samples. All samples were analyzed on HFS-3 (Hydride Formation System) Hitachi model 8000.

Surface Characteristics Measurements

For the better understanding of the adsorption process, the surface area and pore size of the media were analyzed on Analyzer NOVA 2200e.

The representative images are given in Fig. 8.

Surface Area Measurement

Samples were degasified under vacuum at a higher temperature 400 degC. The surface area was measured by nitrogen adsorption.

Pore Size, and Volume Measurements

Micropores (i.e. those of molecular size) were analyzed by gas adsorption at very low pressures on state-of-the-art high vacuum systems using surface area and pore size analyzer.

Resulting data were appropriately processed by the largest commercially available range of advanced computational methods. Large macropores (i.e. over 0.5 microns, and upto -900 (Mu)m diameter) were analyzed by the adsorption of nitrogen gas on the surface of adsorbent. Pores of intermediate size (mesopores) were characterized by gas sorption.

Micropores require gas sorption technology. Before performing gas sorption experiments, solid surfaces were freed from contaminants such as water and oils. Surface cleaning (degassing) was carried out by placing a sample in a glass cell and heating it under vacuum. Once clean, the sample was brought to a constant temperature by means of an external bath i.e., Liquid Nitrogen (b.p.-196 degC). Then, small amounts of a gas usually Nitrogen N2 the adsorbate, were admitted in steps into the evacuated sample cell. Gas molecules (adsorbate) that adsorb on the surface of the solid sample, the adsorbent is supposed to be adsorbed and tend to form a thin layer that covers the entire adsorbent surface.

Based on the well-known Brunauer, Emmett and Teller (BET) theory, number of molecules required to cover the adsorbent surface was estimated with a mono-layer of adsorbed molecules (Nm). By multiplying Nm by the cross- sectional area of an adsorbate molecule yielded the sample's surface area. Continued addition of gas molecules beyond monolayer formation led to the gradual stacking of multiple layers (or multilayers). The formation occurred in parallel to capillary condensation.

The latter process was adequately described by the Kelvin equation, which quantifies the proportionality between residual (or equilibrium) gas pressure and the size of capillaries capable of condensing gas within them. Methods such as the one by Barrett, Joyner and Halenda (BJH) allow the computation of pore sizes from equilibrium gas pressure.

Experimental curves (or isotherms) was generated linking adsorbed gas volumes with relative saturation pressure at equilibrium, and convert them to cumulative or differential pore size distributions. As at the equilibrium adsorbate pressures approached saturation, the pores become completely filled with adsorbate molecules. Knowing the density of the adsorbate, the volume it occupies was calculated.

Zeta Potential Measurement

Volumetric flasks (07) were taken and in each flask 30 ml of 0.1M KNO3 was added. Then 0.2g of media was also added to the flask and its pH was adjusted to 1.18, 3, 5, 7, 9, 11, and 12 with 0.1M HNO3, 1M HNO3, 0.1M KOH, and 1M KOH, respectively and pH were noted after one h.

Conclusion

The materials applied for the preparation of different media are inexpensive and abundantly available locally. The method is simple and economically viable as no complicated operational units are involved in the process. The method could be applied for the removal of arsenic from potable and waste water.

Temperature (from 30degC - 40 degC ) is ideal for removal of arsenic from drinking water. It was observed that maximum adsorption of arsenic on the media was around pH 9.

Acknowledgements

This study was financially supported by (HEC) Higher Education Commission Pakistan.

References

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Department of Environmental Sciences, University of Peshawar, 25120 Peshawar, Pakistan. saeedayousaf@hotmail.com
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Publication:Journal of the Chemical Society of Pakistan
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Date:Dec 31, 2013
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