MATERIALS AND METHODS
4.2 Methods
Stock solutions of known concentrations of Cu(II), Pb(II) and Cr(III) mono-metal ions were prepared by dissolving required amounts of analytical grade copper sulfate [Cu(SO4).5H2O)], lead nitrate [Pb(NO3)2] and chromium nitrate [Cr(NO3)3.9H2O]
respectively in de-mineralized water (DMW) as indicated in Table 4.3. The copper sulfate, lead nitrate and chromium nitrate salts were dried overnight in a hot air drying oven (Model:
PSI, M/S Mahindra Scientific Instrument Co., India) at 50 oC, 70 oC and 38 oC respectively and cooled in a dessicator before weighing out the required quantity using an analytical balance (Model: AB304S, M/S Mettler Instrument AG, Switzerland). The drying temperatures for the metal ions were adopted based on the thermal decomposition analysis of the metal ions reported in published literatures (Galwey & Brown, 1999; Melnikov et al., 2013; Vratny & Gugliotta, 1963). The DMW was prepared through RO process (Model:
Milli-Q Water, M/S Millipore S.A.S., Molsheim, France). The characteristics of de- mineralized water are presented in Table 4.4.
Table 4.3: Stock solutions of selected metal ions.
Metal ion
Cu(II) Pb(II) Cr(III) Dissolved 3.9292 g of
Cu(SO4).5H2O in de- mineralized water and made final volume to 1000 mL.
1.00 mL of stock solution contained 0.0315 meq (1.00 mg) of Cu(II).
Dissolved 1.5985 g of Pb(NO3)2 in de- mineralized water and made final volume to 1000 mL.
1.00 mL of stock solution contained 0.0097 meq (1.00 mg) of Pb(II).
Dissolved 7.6958 g of Cr(NO3)3.9H2O in de- mineralized water and made final volume to 1000 mL.
1.00 mL of stock solution contained 0.0577 meq (1.00 mg) of Cr(III).
Table 4.4: Characteristics of de-mineralized water (DMW) used.
Parameters Value Electrical conductivity (mmho/cm) 0.002±0.001
Total alkalinity (mg/L as CaCO3) 9.00±0.10 Total hardness (mg/L as CaCO3) 2.00±0.20
Temperature (°C) 23.0±1.0
pH (pH unit) 6.60±0.30
Sulphate (mg/L) 0.58±0.10
Chloride (mg/L) 1.00±0.10
Calcium (mg/L) 1.29±0.03
Sodium (mg/L) 0.11±0.05
Potassium (mg/L) 0.07± 0.04
Cu(II) (mg/L) Not detected
Pb(II) (mg/L) Not detected
Cr(III) (mg/L) Not detected
Data presented in xx±yy format where xx is the average value and yy is the standard deviation estimated using a minimum of three independent data points.
4.2.1 Combinations of mono-, binary- and ternary-metal ion systems
The studies for metal removal and metal uptake from mono-, binary- and ternary- metal ion systems were carried out by selecting different combinations of metal ion systems with a fixed desired total initial metal ion concentration in the solution. The selection of combinations was primarily based on metal ion system (either mono or binary or ternary), metal ion(s) present in the system and desired initial concentration of metal ions in the system (expressed in meq/L). In the case of mono-metal ion systems comprising of Cu(II), Pb(II) and Cr(III); all the systems were having a fixed desired initial metal ion concentration of 0.60 meq/L in the solution [which on mass basis equaled to 19.06 mg/L for Cu(II), 62.16 mg/L for Pb(II) and 10.40 mg/L for Cr(III)]. The combination of mono-metal ion systems were designated as M[Cu(0.60)], M[Pb(0.60)] and M[Cr(0.60)] respectively for Cu(II), Pb(II) and Cr(III) metal ion systems where M represented the mono-metal ion system while square brackets included the metal ion along with its desired initial concentration expressed in meq/L. The selected metal ions for the study [viz. Cu(II), Pb(II) and Cr(III)] yielded three different binary-metal ion systems of Cu(II)+Pb(II), Pb(II)+Cr(III) and Cr(III)+Cu(II). Each of the binary-metal ion systems have been further grouped into four different combinations by selecting different desired initial concentration of each of the metal ions present in the combination. The four different combinations of binary-metal ion system of Cu(II)+Pb(II) on the basis of different desired initial concentration of metal ions were (a) 0.45 meq/L of Cu(II)
Cu(II) + 0.60 meq/L of Pb(II) – designated as B[Cu(0.60)+Pb(0.60)] where B represented the binary-metal ion system while square brackets included the metal ions along with its desired initial concentration expressed in meq/L. In the first three combinations, the desired total initial metal ion concentration were fixed at 0.60 meq/L – similar to the metal ion concentrations maintained in mono-metal ion systems while in the fourth combination, the desired total initial metal ion concentration was fixed at 1.20 meq/L – double of the concentration maintained in mono-metal ion systems. The selection of initial concentration of a mono-metal ion as 0.60 meq/L was helpful in making the first three combinations of binary- metal ion system by maintaining a definite proportion of each of the metal ions in the combination. Similarly, four different combinations for each of binary-metal ion systems of Pb(II)+Cr(III) and Cr(III)+Cu(II) were also grouped and designated. Further, the selected metal ions yielded two different combinations of ternary-metal ion systems of Cu(II)+Pb(II)+Cr(III) on the basis of different desired initial concentration of metal ions as (a) 0.20 meq/L of Cu(II) + 0.20 meq/L of Pb(II) + 0.20 meq/L of Cr(III) which was designated as T[Cu(0.20)+Pb(0.20)+Cr(0.20)] and (b) 0.60 meq/L of Cu(II) + 0.60 meq/L of Pb(II) + 0.60 meq/L of Cr(III) – designated as T[Cu(0.60)+Pb(0.60)+Cr(0.60)] where T represented the ternary-metal ion system while square brackets included the metal ions along with its desired initial concentration expressed in meq/L. In the first combination, the desired total initial metal ion concentration was fixed at 0.60 meq/L – similar to the metal ion concentrations maintained in mono-metal ion systems while in the second combination, the desired total initial metal ion concentration was fixed at 1.80 meq/L – triple of the concentration maintained in mono-metal ion systems. The details of selected combinations of mono-, binary- and ternary-metal ion systems with unique designations are presented in Table 4.5.
4.2.2 Solution pH conditions in combinations of mono-, binary- and ternary-metal ion systems
The studies of metal removal from aqueous phase and metal uptake by adsorbent from mono-, binary- and ternary-metal ion systems have been carried out under two different conditions of solution pH. In the first condition, the initial pH of the solution was adjusted to
51
Table 4.5: Details of combinations of mono-, binary- and ternary-metal ion systems selected for investigation in the present study.
System Metal ion Combination and desired initial metal ion concentration Designation assigned Mono
(M)
Cu(II) 0.60 meq/L of Cu(II) M[Cu(0.60)]
Pb(II) 0.60 meq/L of Pb(II) M[Pb(0.60)]
Cr(III) 0.60 meq/L of Cr(III) M[Cr(0.60)]
Binary (B)
Cu(II)+Pb(II)
0.45 meq/L of Cu(II) + 0.15 meq/L of Pb(II) B[Cu(0.45)+Pb(0.15)]
0.30 meq/L of Cu(II) + 0.30 meq/L of Pb(II) B[Cu(0.30)+Pb(0.30)]
0.15 meq/L of Cu(II) + 0.45 meq/L of Pb(II) B[Cu(0.15)+Pb(0.45)]
0.60 meq/L of Cu(II) + 0.60 meq/L of Pb(II) B[Cu(0.60)+Pb(0.60)]
Cr(III)+Pb(II)
0.45 meq/L of Cr(III) + 0.15 meq/L of Pb(II) B[Cr(0.45)+Pb(0.15)]
0.30 meq/L of Cr(III) + 0.30 meq/L of Pb(II) B[Cr(0.30)+Pb(0.30)]
0.15 meq/L of Cr(III) + 0.45 meq/L of Pb(II) B[Cr(0.15)+Pb(0.45)]
0.60 meq/L of Cr(III) + 0.60 meq/L of Pb(II) B[Cr(0.60)+Pb(0.60)]
Cu(II)+Cr(III)
0.45 meq/L of Cu(II) + 0.15 meq/L of Cr(III) B[Cu(0.45)+Cr(0.15)]
0.30 meq/L of Cu(II) + 0.30 meq/L of Cr(III) B[Cu(0.30)+Cr(0.30)]
0.15 meq/L of Cu(II) + 0.45 meq/L of Cr(III) B[Cu(0.15)+Cr(0.45)]
0.60 meq/L of Cu(II) + 0.60 meq/L of Cr(III) B[Cu(0.60)+Cr(0.60)]
Ternary
(T) Cu(II)+Pb(II)+Cr(III) 0.20 meq/L of Cu(II) + 0.20 meq/L of Pb(II) + 0.20 meq/L of Cr(III) T[Cu(0.20)+Pb(0.20)+Cr(0.20)]
0.60 meq/L of Cu(II) + 0.60 meq/L of Pb(II) + 0.60 meq/L of Cr(III) T[Cu(0.60)+Pb(0.60)+Cr(0.60)]
M, B and T represented mono-, binary- and ternary-system respectively while square brackets included the metal ion along with its desired initial concentration expressed in meq/L.
maintained at the same level/value during the course of experiment, hence the experiments were termed as carried out under uncontrolled pH conditions in this study. In the second condition, the initial pH of the solution was buffered to a pre-decided level/value of 5.00 in the beginning of the experiment by adding 8 mL of acetate buffer {prepared by mixing 680 mL of 0.5M sodium acetate tri-hydrate with 320 mL of 0.5M acetic acid (Asnin et al., 2010)}
thereby variations in the solution pH level/value were checked or maintained at the same level/value during the course of experiment. Hence, the experiments were termed as carried out under controlled pH conditions in this study.
4.3 Metal ions availability in aqueous phase with variation in solution pH from