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The next war may be due to freshwater, in a new era, the demand for freshwater consecutively increases globally. In earth's atmosphere, the total water supply of about 332.5 million mi3of water whereas 96% of water is saline water and approximately only 3% of water is freshwater. However, of the total freshwater 68% is locked up in glaciers and about 32% of freshwater is in the ground. Drinking water in the earth's atmosphere is 1.2%. Moreover, in the new era, environmental pollution has increased to a noticeable level due to global population growth, rapid industrialization growth increases the demand for products, industrial exploitation of natural resources, and persistent droughts[1-2] and water demand is consecutive increases because the freshwater consumed in many activities of humans i.e. dirking, bathing, fluxing, washing, and cooking, etc. Retaining fresh water in the earth atmosphere is a future challenge globally because of environmental pollution. Moreover, industrial growth and agricultural and municipal activities have contributed directly to the rise in the continuous discharge of dyes, heavy metals, phenolic and organic, and inorganic chemicals into water bodies[3]. Hence, heavy metals can be defined as metallic elements that have a relatively high density compared to water. With the assumption that heaviness and toxicity are interrelated, heavy metals also include metalloids, such as arsenic, that are able to induce toxicity at low levels of exposure[4]. Arsenic is situated in the periodic table in Group 15 and its atomic number is 33. Arsenic is rarely found as a free element in the natural environment, generally, it occurs mainly in four oxidation states (3, 0, +3, +5), while the two oxidation states common in drinking water are oxyanions of trivalent Arsenic (III) and pentavalent Arsenic[5]. Water contamination is one of the most critical issues being faced by society due to industrial and population growth. However, plenty of heavy metals contaminate the groundwater and surface water soil, and atmosphere day by day even in very low concentrations. Heavy metals are a group of trace elements that include metals and metalloids, such as arsenic, cadmium, chromium, cobalt, copper, iron, lead, manganese, mercury, nickel, tin, and zinc[6]. Heavy metals are classified as toxic and carcinogenic; they are capable of accumulating in tissues and causing diseases and disorders. Arsenic contamination in drinking water, as well as groundwater, has become a serious threat to the environment, human health, and animals. The presence of toxic and carcinogenic arsenic in drinking water, as well as groundwater, is increasing continuously due to which a large number of people have potentially threat with various diseases. It is a worldwide problem, some of the most affected regions lie in the flood plains of the great rivers of Bangladesh, Nepal, West Bengal, and India. Arsenic is a poisonous element, odorless and tasteless, which can lead to health effects for humans, animals, and living organisms if present in more than the permissible limit of BIS. Humans and animals consume large quantities of water; therefore, its presence in drinking water is very serious and likely to adversely affect health. Long-term drinking water exposure to arsenic causes skin, lung, bladder, and kidney cancer as well as pigmentation changes, skin thickening, neurological disorders, muscular weakness, loss of appetite, and nausea. Arsenic enters drinking water supplies through natural deposits in the earth, agriculture, and industrial practices. However, contamination of aquatic systems is a serious environmental issue and therefore the development of efficient and suitable technology to remove arsenic from aqueous solutions is necessary. Therefore, it was thought necessary to develop an efficient method to reduce the concentration of arsenic in drinking water to a safe level as prescribed by BIS (10500:1991). A variety of methods has been used in the past to remove arsenic from contaminated water Membrane Filtration, Solvent Extraction, Ion exchange, electrochemical treatment, Reverse Osmosis, and Adsorption, etc. Many of these methods suffer from high capital and operating costs. Adsorption seems to be one of the most suitable methods due to its high efficiency, low cost, and ease of operation. Various adsorbents, such as carbon foam, activated carbon, zeolite, clay minerals, organic polymers, and biochar, even many waste materials, such as fly ash, reused sanding waste, biomass, and water treatment residue (WTR), have been used to removal of heavy metals by adsorption methods. The most effective heavy metal adsorbents, especially for the removal of arsenic, are metal oxides (Fe, Al, Mn oxides), such as WTRs, swamp iron ore, ferrihydrite, goethite, layered double hydroxide, red clay, and fertilizer industry waste.[7-22]. Although several reports have proved that the adsorption technique for the removal of arsenic is one of the best techniques for reducing the arsenic concentration in drinking water as per WHO guidelines or BIS guidelines[23]. The WHO guideline is 10 ppb (0.01 mg/L) for arsenic and as per BIS guidelines, 10 ppb (0.01 mg/L) concentration of arsenic has been prescribed as a permissible limit in the drinking water[24]. For developing countries like India, where people cannot afford costly means, the development of a low-cost and eco-friendly adsorbent is required for the removal of arsenic from water. Hence, in this study, a low-cost adsorbent from fertilizer industry waste has been developed and used for the removal of arsenic from water.

The pollution of water bodies due to the disposal of heavy metal wastes is an increasing worldwide concern for the last few decades. Various methods are currently being investigated with varying degree of success. The most widely applied methods for the metal removal from water/waste waters are chemical and electrochemical precipitation, cationic and anionic ion exchange resins, membrane filtration and sorption. The cost effectiveness of most methods can be questionable. Whereas some methods such as ion-exchange are costly, others such as precipitation techniques have problems for disposal of heavy metal containing sludge. Apart from these, adsorption technique is very popular due to simplicity and low cost. Although activated carbon is frequently used as general adsorbent of inorganic and organic compounds, alternative low cost adsorbents are being developed. Different adsorbents like mesoporous silica have been used for the removal of aqueous heavy metals by adsorption. Some of these include adsorbents like rice husks and rice bran, tartaric acid modified rice husks, chitosan, lignin, modified lignin, cocoa shells, coir fibers, carbon aerogels, coal, fly ash, red mud, clays, zeolites, pottery, bone charcoal, seeds and seed husks etc. that are being actively investigated with varying degree of success [22, 25-27]

All chemicals, used in this study, were of analytical grade obtained either from, Renkem, Merck, Germany or SD Fine Chem. Ltd., India. A stock solution of arsenic was prepared using As2O3 in purified water. Purified analytical grade water was prepared using Millipore Milli-Q water purification systems. As solution of different concentrations were obtained by diluting the stock solution. Standard solution of As (1000 mg/L) for atomic absorption spectrophotometer was obtained from Merck, Germany. Standard acid and base solutions (0.5M H2SO4 and 1M NaOH) were used for pH adjustments. Particle Sizing of Adsorbents: All the powdered adsorbents were passed through different British Standard Sieves (BSS), and fractions corresponding to 100-150, 150-200, and 200-250 mesh were collected. Adsorption on particles of different sizes was studied in order to understand the effect of particle size of adsorbent material. Preparation of Sample: Arsenic trioxide (As2O3) was used as the source of Arsenic. A stock solution (1000 mg/l) was prepared in purified water by dissolving 1.32 g of arsenic trioxide in 30 ml of 3 M sodium hydroxide solution. The resulting solution was neutralized with 3 M sulphuric acid to a phenolphthalein endpoint and diluted to 1L with dilute sulphuric acid. Preparation of Low Cost Adsorbent: Carbonaceous Adsorbent: Carbon slurry waste was collected from the National Fertilizer Limited Plant, Panipat (Haryana). It was found to consist of small, black and greasy granules and treated with hydrogen peroxide to oxidize the adhering organic material and then washed with water and heated to 200ºC till the evolution of black soot stopped. The dried material was then activated at 400ºC temperature in a muffle furnace for 2 h in air atmosphere. After the complete activation the materials was cooled and the ash content was removed by treating the material with 1 M hydrochloric acid and washed with distilled water three to four times and dried. The finished product was called carbonaceous adsorbent.

Batch Studies

Batch sorption experiments were conducted to obtain rate and equilibrium data, using 250-ml Erlenmeyer flasks kept at room temperature; a wrist action shaker was used for shaking the solution. The reaction mixture consisted of a 50 ml arsenic solution of known concentration and the carbonaceous adsorbent which was weighed (50 mg) and added to the solution. After completion of the required reaction time, the flask was removed followed by the solution was filtered using filter paper. All experiments were carried out at room temperature (25^◦C±1^◦C). To study the effect of initial pH (4.0–10.0) on arsenic uptake, experiments were performed with initial arsenic concentrations of 200 μg/l and an adsorbent dose of 1 g/l at a fixed contact time of 6 h. Isotherm studies were conducted with varying initial arsenic concentrations (100–1000 μg/l), fixed adsorbent dose of 1 g/l, and contact time of 6 h at pH 7.0. The effect of contact time was studied with an initial arsenic concentration of 200 μg/l and an adsorbent dose of 1 g/l; pH was kept at 7.0 and contact time was varied from 15 to 720 min. The effect of adsorbent was studied by varying the dose from 1 to 10 g/l at a fixed pH of 7.0 with an initial As the concentration of 200 μg/l and a contact time of 12 h.

Adsorption Isotherms

Adsorption isotherm is a mathematical expression that relates the concentration of the adsorbate in the interface to its equilibrium concentration in the liquid phase. Before calculating the isotherm, equilibrium sorption capacity, qe, was determined by using the following equation:

       ……………………………………………………… (i) 

Where Ci and Ce are the initial and equilibrium concentration of metal element the solution (mg/L), respectively, V is volume of the solution (in Liter) and Ms is the weight of adsorbent (mg). The results from this computation were used in calculating the isotherm. In this research, Langmuir 26 and Freundlich 27 isotherm were studied. 

Langmuir Isotherm:

For Langmuir isotherm the equation (ii) is given below:

      ……………………………………………………… (ii)

Where b is the maximum adsorption capacity corresponding to complete monolayer coverage (mg of solute adsorbed per g of adsorbent) and Keq is the Langmuir constant related to energy of adsorption. The linear form can be used for linearization of experimental data by plotting Ce/qe against Ce. The Langmuir constants b and K can be evaluated from the slope and intercept of the following linear equation.

        …….......................………………………………… (iii)

Freundlich Isotherm

For Freundlish equation as stated in equation (iv),

          ..............……………………………………………… (iv)

where K and n are Freundlish constant. 

A plot of log Ce versus log qe yields a straight line which indicates the conformation of Freundlish isotherm for adsorption. The constant can determined from the slope and intercept.

 ………………………………………………… (v)

Characterization of the Prepared Adsorbents:

The FTIR spectra of carbonaceous adsorbent and adsorbent loaded with arsenic were recorded in dried KBr (0.1g). The FTIR spectra of carbonaceous adsorbent (Fig.1) indicates the presence of prominent bands lying at 1400, 1616, 3128 and 3408 cm^-1. The first strong band at 1400 cm^-1 may be assigned to the presence of methyl and methylene group. The strong band appearing at 1616 cm^-1 might be attributed to C=S, C=C and C=O stretch whereas, the bands appearing  in the region 3128 and 3408 cm^-1 may be assigned to the presence of the alkyl group and OH group (Fig.1). As seen in Fig.1, the intensity of absorbance peaks in the adsorbent loaded with arsenic is slightly lower than that in the native one. The analysis of the FTIR spectra showed the presence of functional groups able to interact with metal, whereas, strong band at 1400 cm^-1 is disappeared after loading arsenic on adsorbent surface.     

The surface area was determined by nitrogen gas adsorption for carbonaceous adsorbent and was found to be 360 m²/g ²⁵. The CHNS analytical data of adsorbent indicate the carbon content as 84.2 % whereas, the carbon content decreased after loading Arsenic on adsorbent surface, where the elemental analysis showed composition of carbon, hydrogen and sulphur as 42.20, 2.52 and 5.01%.

Figure.1. IR spectra of carbonaceous adsorbent before adsorption and after adsorption in KBr.

Batch Studies:

Effect of pH:

The effect of variation of the initial solution pH (4.0–10.0) on the adsorption of arsenic is illustrated in Fig. 2. The efficiency of removal of arsenic increases with an increase in pH from 4.0 to 8.0 and almost constant on further increase in pH up to 10.0. The maximum arsenic removal efficiency at pH 7.0 is 79%. The adsorption capacity increases with an increase in pH, may be due to acidic nature of adsorbent.

Figure.2: Effect of pH on arsenic adsorption by low cost adsorbent.

Effect of Contact Time:

Fig. 3 demonstrates the effect of contact time (15–720 min) on the adsorption of arsenic. It is clear from Fig. 3 that with using developed carbonaceous adsorbent with 200-250 mesh size, adsorption efficiency increases rapidly with increase in contact time up to 180 min; and a maximum removal efficiency of 80% was achieved and remained fairly constant up to the end of the experiment. The results also show that most of the removal (60%) occurs in a contact time of 60 min. The data in Fig. 3 also indicates that the time required for equilibrium adsorption is 6 hr.

Figure.3: Effect of contact time on arsenic adsorption on carbonaceous adsorbent (temperature, 45ºC; particle size 200-250 mesh).

Effect of Initial Arsenic Concentration:

The effect of initial arsenic concentration (100–1000 μg/l) on uptake by carbonaceous adsorbent is illustrated in Fig. 4, which reveals that removal efficiency is higher (74.0%) with a lower initial Arsenic concentration (100 μg/l). A gradual decrease in arsenic uptake by adsorbent was observed at higher initial concentrations of arsenic. The maximum removal efficiencies at an initial arsenic concentration of 1000 μg/L are 16% for carbonaceous adsorbent. The reason for the decrease in arsenic adsorption efficiency at higher initial concentrations may be that the adsorbents porous nature with higher surface area eventually become saturated with adsorbed arsenic and at this point further addition of arsenic to the solution would not be expected to increase the amount adsorbed removed significantly. 

Figure.4: Effect of initial concentration of arsenic adsorption on carbonaceous adsorbent (temperature, 25ºC; particle size 200-250 mesh).

Effect of Temperature:

The adsorptions of arsenic at different temperatures (298, 308 and 318 K) were studied. The adsorption was found to increase with increase in temperature from 298 to 318K (Fig.5). It reveals that the adsorbent system is exothermic in nature for which the evaluations of thermodynamic parameters were carried out. The adsorption data fitted to two adsorption models (Langmuir and Freundlish) to find out the suitable model.

Figure.5: Effect of temperature of arsenic adsorption on carbonaceous adsorbent (particle size 200-250 mesh).

However, from Fig.6, it is observed that the experimental data is fitted in Langmuir model. The maximum adsorption capacity on Langmuir model at temperature 313K was found 166.67 µg/g while, for Freundlich model was 3.78 µg/g . This indicates that the Langmuir model was best fitted than Freundlich model.

Figure.6: Langmuir model fit for adsorption of arsenic at 298K, 303K and 313K (particle size 200-250 mesh).
Effect of Adsorbent Dose:
Adsorption experiments were also performed at different adsorbent dosages from 1 to 10 g/L; a marginal increase is observed on further increase in the adsorbent dose up to 10 g/L. It was observed that upon increases the quantity of adsorbent the efficiency of adsorption increases. The dose of adsorbent increased the adsorption of arsenic also increased up to from 1 to 10 g/L and found at the dose of 10 g/L the remaining arsenic was less than as per BIS (IS: 10500, 1999) limits. This is also noted that as the dose of adsorbent increased the adsorption capacity also increased. The increase in the efficiency of removal may be attributed to the fact that with an increase in the adsorbent dose, more adsorbent surface area is available which turn increases the number for porous and the solute to be adsorbed on the surface.
Effect of Particle Size of Adsorbent:
The adsorption of arsenic at different particle size was studied. The adsorption was found to increase with decrease in the size of particle.  This is mainly true since the surface area and the number of active pores of the adsorbent increase with decrease in particle size. Adsorption onto adsorbent is also dependent on pore size distribution and also depends upon the number of pores in the structure.  The percentage of adsorption was found to be increased for increase the particle size of adsorbent from 100-150 to 200-250 mesh.

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