Materials and Methods
4.2 METHODS
4.2.1 Batch Experiments with Mono-metal Ion Systems
The feasibility of PWC and PS for metal removal from mono-metal ion systems was evaluated through a series of batch experiments as a function of different parameters such as pH, dissolved oxygen (DO), adsorbent dose, contact time and shaking speed. The studies were carried out in order to obtain the optimum values of the adsorbent dose, shaking speed and equilibrium time which were then used to
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investigate the mass transfer properties of the adsorbent(s) through kinetic experiments and for the development of adsorption isotherms using equilibrium adsorption studies.
4.2.1.1 Solubility of Metal Ion with pH from Mono-metal Ion Systems
Experiments were designed to assess solubility of metal ions from mono-metal ion systems [i.e. Fe(II) or F− or As(III)] in pH range of 2 to 12. An adequate volume of mono-metal ion [either Fe(II) or F− or As(III) as the case may be stock solution was taken in 100 mL specimen tube, added with appropriate amount of acid (0.1N HNO3) or lime solution (Rao and Rekha, 2004) for adjusting the desired pH with the help of a digital pH meter (Model: µpH system 361, M/S Systronics India Ltd., India). The final volume was made to 100 mL with de-ionized water to give a desired initial concentration of the mono-metal ion in solution [i.e. initial concentrations of Fe(II), F− and As(III) as 5, 2 and 0.5 mg/L respectively]. This mixture was mounted on a shaker having reciprocating horizontal type motion (Model: ICT, M/S International Commercial Trading, Kolkata, India; Horizontal shaking length = 4 cm) and thoroughly mixed by shaking at a speed of 120 horizontal shaking per minute (hspm) for approximately 5-8 min. and then the mixture was allowed to stand for 20 min. at room temperature (22 ± 1 ºC). Thereafter, samples were drawn from the supernatant for analysis of mono-metal ions remaining in the solution of respective mono-metal ion systems. All experiments were carried out in duplicate.
4.2.1.2 Solubility of Metal Ion with Dissolved Oxygen from Mono-metal Ion Systems
Experiments were planned to assess solubility of mono-metal ions present in the mono-metal ion systems [i.e. Fe(II) or F− or As(III)] in the presence of dissolved oxygen (DO). 10–12 L of de-ionized water was taken in a plastic bucket and pH was initially adjusted near to 6 with the help of 0.2N HNO3 acid. Initial DO concentration was estimated and then it was brought down to zero by adding appropriate amount (slightly more than the stoichiometric requirement) of sodium sulfite (Na2SO3). The required amount of mono-metal ion stock solution was added and the final liquid volume was made to 15 L using de-ionized water to achieve a desired initial
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concentration of the mono-metal ion in solution [i.e. initial concentrations of Fe(II), F− and As(III) as 5, 2 and 0.5 mg/L respectively]. Then aeration was carried out with the help of an air pump (Model: APM 414, M/S Vyubodhan Upkaran Pvt Ltd, New Delhi, India). Samples were drawn at regular intervals from the bucket using a tap provided at the bottom. The samples for DO concentration estimation was directly collected in a 300 mL BOD bottle from the bucket and appropriate chemical reagent (such as alkali- iodide-azide reagent and manganous sulfate solution) were added immediately after collection to fix the DO and thereafter DO concentrations were estimated according to Standard Methods (APHA, 1998). The samples for mono-metal ions estimation were withdrawn into a 100 mL specimen tube and analyzed for respective mono-metal ion concentrations immediately. During experimentation, variation in pH was controlled near to 6 with the help of buffer prepared using acetic acid and sodium acetate (1 mL of 0.2M acetic acid and 9 mL of 0.2M sodium acetate mixed to prepare 10 mL of buffer solution) (Jeffery et al., 1996). All experiments were conducted in duplicate.
4.2.1.3 Effect of Variation in Contact Time on Mono-metal Uptake from Mono-metal Ion Systems
In order to investigate the effect of variation in contact time on mono-metal uptake by PWC and PS, a fixed dose of adsorbent (0.3 g of PWC or 3 g of PS) added in 10 glass conical flasks of 150 mL capacity having 50 mL of mono-metal ion solution of the desired initial concentrations [i.e. the initial Fe(II) concentrations of 2.5, 5 and 10 mg/L; initial F− concentrations of 2, 3, 4 and 5 mg/L; and initial As(III) concentrations of 0.25 and 0.5 mg/L] and covered to block the entrance of air from outside. The conical flasks containing the solution and adsorbent were mounted in a shaker having reciprocating horizontal type motion (Model: ICT, M/S International Commercial Trading, Kolkata, India; Horizontal shaking length = 4 cm) and mixed thoroughly at a shaking speed of 120 and 140 horizontal shaking per minute (hspm) for PWC and PS respectively for 3 h at room temperature (22 ± 1 ºC). Conical flasks were taken out one by one at pre-determined time intervals and the two phases were separated immediately using a 0.40 µm Whatman filter paper. The residual mono-metal ion concentrations in the liquid portion were estimated thereafter. All the experiments were conducted in duplicate.
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4.2.1.4 Effect of Variation in Adsorbent Dose on Mono-metal Uptake from Mono-metal Ion Systems
In order to select the dose of adsorbent (PWC or PS) for obtaining residual mono-metal ion concentration of less than or equal to guideline values for drinking water [i.e. guideline values of Fe(II), F− and As(III) less than or equal to 0.3, 1 and 0.01 mg/L respectively as per IS 10500 (1991)] for subsequent experiments; desired doses of PWC [0.1, 0.2, 0.3 and 0.4 g for mono-metal ion Fe(II) and As(III) but 0.2, 0.3 and 0.4 g for mono-metal F−] and PS [2, 3, 4 and 5 g for mono-metal ion Fe(II) and As(III) but 2, 3 and 4 g for mono-metal F−] were added in different sets of glass conical flasks [i.e.
four flasks for mono-metal ion systems of Fe(II) and As(III) but three flasks for mono- metal ion system of F−] having 150 mL capacity with 50 mL of mono-metal ion solutions having desired initial concentrations [i.e. initial Fe(II), F− and As(III) concentrations of 5, 2 and 0.5 mg/L respectively]. The conical flasks were covered and mounted in the horizontal shaker and the contents were mixed thoroughly at a shaking speed of 120 and 140 hspm for PWC and PS respectively, continuously for 3 h in the case of mono-metal ion systems of Fe(II) and As(IIII) but for 4 h at room temperature (21 ± 1 oC) in the case of mono-metal ion system of F−. After shaking, the two phases were separated using 0.40 µm filter paper and the respective residual mono-metal ion concentrations in the liquid portions were estimated. The amount of PWC and PS yielding residual mono-metal ion concentration less than or equal to guideline value was selected as the dose for subsequent studies. All the experiments were conducted in duplicate.
4.2.1.5 Effect of Variation in Shaking Speed on Mono-metal Uptake from Mono-metal Ion Systems
One of important parameters affecting the adsorption process is intensity of mixing. In the present investigation, the intensity of mixing has been varied by varying the horizontal shaking speed (i.e. number of horizontal shakings per minute) using a horizontal shaker. A fixed dose of adsorbent (0.3 g of PWC or 3 g of PS) added in different sets of 4 glass conical flasks of 150 mL capacity having 50 mL of desired initial concentration of mono-metal ion [i.e. initial Fe(II), F− and As(III) concentrations of 5, 2 and 0.5 mg/L respectively] and covered to block the entrance of air from
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outside. The conical flask were then mounted in the horizontal shaker and the contents were mixed thoroughly at different horizontal shaking speeds [i.e. 80, 100, 120 and 140 hspm for mono-metal ion system of Fe(II), and 100, 120, 140 and 160 hspm for mono- metal ion systems of F− and As(III)]. After shaking, the two phases were separated using 0.40 µm filter paper and respective residual mono-metal ion concentration in the liquid portions were estimated. All the experiments were conducted in duplicate.
4.2.1.6 Adsorption Kinetic Studies of Mono-metal Uptake from Mono-metal Ion Systems
The data obtained while investigating the effect of contact time on mono-metal uptake from mono-metal ion systems (as described in section 4.2.1.3) were used to study the adsorption kinetics of the respective mono-metal ion systems.
4.2.1.7 Adsorption Equilibrium Studies of Mono-metal Uptake from Mono-metal Ion Systems
The adsorption equilibrium studies were carried out for PWC and PS by varying the dose of adsorbent in a fixed solution volume of 50 mL. Required amount of adsorbent dose [i.e. 0.2 to 0.4 g of PWC for mono-metal ion systems of Fe(II), F− and As(III), but 3 to 8 g of PS for mono-metal ion system of Fe(II) and F−, and 2 to 6 g of PS for mono-metal ion system of As(III)] were added in different sets of glass conical flasks of 150 mL capacity with 50 mL of desired initial mono-metal ion concentrations [i.e. the initial Fe(II) concentrations of 5 and 10 mg/L; initial F− concentrations of 2 and 3 mg/L; and initial As(III) concentrations of 0.25 and 0.5 mg/L]. The conical flasks were covered and mounted in the horizontal shaker and the contents were mixed thoroughly at a shaking speed of 120 and 140 hspm for PWC and PS respectively, continuously for 3 h at room temperature (22 ± 1 ºC). After shaking, the two phases were separated using 0.40 µm filter paper and respective residual mono-metal ion concentration in the liquid portions were estimated. All the experiments were conducted in duplicate.
The detailed experimental conditions used for the mono-metal ion systems of Fe(II), F− and As(III) are presented in Tables 4.2 to 4.4 respectively.
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Table 4.2 Experimental conditions used for batch studies for removal of Fe(II) from mono-metal ion system comprising of Fe(II) with processed wooden charcoal (PWC) and processed sand (PS).
Adsorbent Experiments Initial Fe(II)
conc. (mg/L) Solution volume
(mL) Adsorbent
Dose (g) Shaking speed
(hspm) Temp
(oC) Initial pH
Processed wooden charcoal (PWC)
Variation in contact time 2.5, 5, 10 50 0.3 120 22 ± 1 5.5
Variation in adsorbent dose 5 50 0.1, 0.2, 0.3, 0.4 120 22 ± 1 5.5
Variation in shaking speed 5 50 0.3 80, 100, 120, 140 22 ± 1 5.5
Equilibrium studies 5, 10 50 0.2–0.4 120 22 ± 1 5.5
Processed sand (PS)
Variation in contact time 2.5, 5, 10 50 3 140 22 ± 1 5.5
Variation in adsorbent dose 5 50 2, 3, 4, 5 140 22 ± 1 5.5
Variation in shaking speed 5 50 3 80, 100, 120, 140 22 ± 1 5.5
Equilibrium studies 5, 10 50 3–8 140 22 ± 1 5.5
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Table 4.3 Experimental conditions used for batch studies for removal of F− from mono-metal ion system comprising of F− with processed wooden charcoal (PWC) and processed sand (PS).
Adsorbent Experiments Solution
volume (mL) Initial F−
conc. (mg/L) Adsorbent
dose(g) Shaking speed
(hspm) Temp
(oC) Initial pH Processed
wooden charcoal (PWC)
Variation in contact time 50 2, 3, 4, 5 0.3 120 22 ± 1 6.0
Variation in adsorbent dose 50 2 0.2, 0.3, 0.4 120 22 ± 1 6.0
Variation in shaking speed 50 2 0.3 100, 120, 140, 160 22 ± 1 6.0
Processed sand (PS)
Variation in contact time 50 2, 3, 4, 5 3 140 22 ± 1 6.0
Variation in adsorbent dose 50 2 2, 3, 4 140 22 ± 1 6.0
Variation in shaking speed 50 2 3 100, 120, 140, 160 22 ± 1 6.0
Equilibrium studies 50 2, 3 2– 4 140 22 ± 1 6.0
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Table 4.4 Experimental conditions used for batch studies for removal of As(III) from mono-metal ion system comprising of As(III) with processed wooden charcoal (PWC) and processed sand (PS).
. Adsorbent Experiments Solution volume
(mL) Initial As(III)
conc. (mg/L) Adsorbent
Dose(g) Shaking speed
(hspm) Temp
(oC) Initial pH
Processed wooden charcoal (PWC)
Variation in contact time 50 0.25, 0.5 0.3 120 22 ± 1 6.0
Variation in adsorbent dose 50 0.5 0.1, 0.2, 0.3, 0.4 120 22 ± 1 6.0
Variation in saking speed 50 0.5 0.3 100, 120, 140, 160 22 ± 1 6.0
Equilibrium studies 50 0.25, 0.5 0.2–0.4 120 22 ± 1 6.0
Processed sand (PS)
Variation in contact time 50 0.25, 0.5 3 140 22 ± 1 6.0
Variation in adsorbent dose 50 0.5 2, 3, 4, 5 140 22 ± 1 6.0
Variation in shaking speed 50 2 3 100, 120, 140, 160 22 ± 1 6.0
Equilibrium studies 50 0.25, 0.5 2–6 140 22 ± 1 6.0
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