CHAPTER-7. Adsorption characteristics of phosphate-treated Ashok bark (Saraca indica): Removal of Ni(II) from Electroplating wastewater

199 CHAPTER-7 Adsorption characteristics of phosphate-treated Ashok bark (Saraca indica): Removal of Ni(II) from Electroplating wastewater

200 7.1 Introduction Because of heavy metal toxicity and non-biodegradable nature, the introduction of heavy metals in water is becoming a serious environmental and public health concern. Heavy metals in human bodies tend to bioaccumulate, which may result in damaged or reduced mental, central nervous function and blood composition, lungs, kidneys and liver. The heavy metal such as nickel is a naturally occurring element. Small amount of this element is common in our environment and actually necessary for our health. But large amount of it may cause acute or chronic toxicity [1-3].The regulatory level of Ni(II) in drinking water by WHO is 0.07 mg L -1 [4]. A number of technologies have been developed to remove toxic heavy metals from wastewaters. The most important technologies for heavy metals ions removal from wastewaters include precipitation, ion exchange, adsorption, coagulation, evaporation and reverse osmosis. Adsorption on solid matrices has been shown to be an economically feasible alternative method [5-8]. Adsorption play an important role in the elimination of metals ions from aqueous solution in water pollution control [9, 10]. The main advantages of this technique are the low operating cost, improved selectivity for metals of interest, removal of heavy metals from effluent irrespective of toxicity, short operation time and no product of secondary compounds which might be toxic[11]. Recently various biomasses have been used for removal of Ni(II) ions from aqueous solution [12-16]. Ashok tree (Saraca indica) is a plant belongs to Caesalpiniaceae subfamily of Legume family. It is important in the cultural traditions of the Indian subcontinent and adjacent areas. The Ashok tree is prized for its beautiful foliage and fragrant flowers. It is very handsome erect evergreen tree, with deep green leaves growing in dense

201 clusters. Its flowering seasons is around February to April. The Ashok flowers comes in heavy, lush bunches. They are bright yellow in color, turning red before wilting. The bark of the Ashok tree is used to make a drug, which is reported to posses a stimulating effect on the endometrium and ovarian tissue. The present study deals with the adsorption efficiency of phosphate-treated Ashok bark and its possible role in the removal and recovery of Ni(II) from electroplating wastewater.

202 7.2. Experimental procedure 7.2.1. Preparation and activation of adsorbent Bark of Ashok tree (Saraca indica) biomass was collected from A.M.U campus. The biomass was washed several times with double distilled water (DDW) to remove dirt and dust. The washed biomass was dried in an oven at 60 0 C. The dried biomass was then crushed and sieved to 100-300 µm particle size. It was then treated with an aqueous solution of 0.1N Na 3 PO 4.12H 2 O for 24 hrs and then washed several times with double distilled water (DDW) to remove excess phosphate ions. The washed biomass was dried in an oven at 60 0 C and stored in an airtight container in order to avoid moisture and used as such for the adsorption studies. 7.2.2. Preparation of adsorbate solution The stock solution of Ni(II) was prepared (1000 mg L -1 ) by dissolving the desired amount of nickel nitrate (AR grade) salts. Solutions of other salts were prepared by dissolving their nitrates. 7.2.3. Characterization of adsorbent Scanning electron microscopy (SEM) analysis technique was employed to observe the surface morphology of the adsorbent with 3500x magnification. The type of binding groups present on the adsorbent were identified by Fourier transform infrared spectroscopy (FTIR) analysis using Perkin Elmer 1600 infrared spectrometer with pellets of powdered KBr and biomass.

203 7.2.4. Determination of active sites Acidic sites on Ashok bark were determined by acid-base titration method [17]. 0.5g adsorbent was treated separately with 50 ml each 0.1N NaOH, 0.1N Na 2 CO 3 and 0.1N NaHCO 3 in 250 ml conical flasks. The flasks were agitated at constant temperature (20 0 C) and left there for 5 days. Afterwards a sample of 10 ml was titrated with 0.1N HCl solution using ph meter. 7.2.5. Determination of Point of zero charge (ph pzc ) The zero surface charge characteristics of the Ashok bark were determined by using the solid addition method [18] as described in earlier chapters. 7.2.6. Adsorption studies Batch process was employed for adsorption studies. 0.5 g adsorbent was placed in a conical flask having 50 ml Ni(II) solution and the mixture was shaken in a shaker incubator at100 rpm. The mixture was then filtered at predetermined time interval and the final concentration of metal ions was determined in the filtrate by Atomic Absorption Spectrophotometer (GBC 902). Amount of Ni(II) adsorbed was then calculated by subtracting final concentration from initial concentration. Adsorption studies were carried out by varying the adsorbate concentration (10 100 mg L 1 ), the agitation time (1 120 min), adsorbent amount (0.1 1.0 g) and temperature (30, 40 and 50 C). A series of experiments with ph of the initial Ni(II) solution varying between 2 and 9 (by adding 0.1N HCl and 0.1N NaOH solutions) were also carried out using 0.5 g adsorbent at room temperature.

204 7.2.7. Breakthrough studies Breakthrough studies were carried out as follows. 0.5g of adsorbent was taken in glass column (0.6 cm internal diameter) with glass wool support. 1000 ml Ni(II) solution was passed through the column at 1mL min -1 flow rate. The initial Ni(II) ions concentration (C 0 ) was 50 mgl -1. The effluent was collected in 50 ml fractions. The concentration of metal ions (C) in each fraction was determined by AAS. The breakthrough curve was obtained by plotting C/C 0 versus volume of the effluent. 7.2.8. Desorption of Ni(II) by batch process Desorption studies were carried out as follows. 50 mg L -1 Ni(II) solution was treated with 0.5 g adsorbent in a conical flask for 24 hrs. The adsorbent was washed several times with DDW (ph 6.5) in order to remove traces of metal ions remained unadsorbed. The adsorbent was then treated with 50 ml 0.1N HCl for 24 hrs. The desorbed Ni(II) ions were then determined in the solution by AAS. 7.2.9 Analysis of electroplating wastewater Electroplating wastewater was collected from one of the lock factory in Aligarh city. The ph of the waste was measured immediately. It was filtered and stored in a polythene bottle. Analysis of heavy metals ions was carried out by AAS. Total dissolved salts (TDS) were determined by evaporating 100 ml filtered wastewater in a china dish.

205 7.2.10 Treatment of electroplating wastewater by batch process 50 ml wastewater was taken in a conical flask and its ph was adjusted to 5.6 and then 0.5 g adsorbent was added. The mixture was shaken and then kept for 24 hrs. It was filtered and filtrate was analyzed for heavy metals by AAS.

206 7.3. Results and discussion 7.3.1. Characterization of adsorbent Scanning Electron Microscope (SEM) Analysis: A scanning electron microscopy (SEM) was used to examine the surface of the adsorbent. The surface of untreated adsorbent appears to be irregular and porous (Fig. 7.1a).The pores are prominent on the surface of the adsorbent before adsorption. After adsorption of Ni(II) onto untreated bark the pores are filled showing adherence of adsorbate ions on the surface (Fig. 7.1b). Similarly SEM images of phosphate-treated Ashok bark before and after adsorption of Ni(II) are shown in Figs. 7.2a and 7.2b. However the effect of Phosphate treatment is not visible in SEM image. FTIR Analysis: FTIR spectrum of untreated Ashok bark is shown in Fig. 7.3a. The prominent peaks due to the presence of various groups are shown in Table 7.1 [19]. FTIR spectrum of phosphate-treated Ashok bark is shown in Fig. 7.4a. All the dominant peaks remained unaltered except that two new peaks at 1157 and 3786 cm -1 appeared. The appearance of peaks at 1157 may be due to the presence of phosphate [20], while peak at 3786cm-1 may be due to the OH group of PO4-3 [21]. Fig.7.4b represents the FTIR of phosphate treated Ashok bark after Ni(II) adsorption. The peak at 3786 cm -1 disappeared indicating that Ni(II) interacts with OH group of phosphate [21].

207 7.3.2. Effect of phosphate treatment on the adsorption of Ni(II) The % adsorption of Ni(II) increased from 80 to 95% when adsorbent was treated with Na 3 PO 4. Therefore further studies were carried out on phosphate-treated Ashok bark. 7.3.3. Determination of active sites The total numbers of acidic sites matching carboxylic, phenolic, and lactonic sites were neutralized using alkaline solutions (0.1N NaOH, 0.1N NaHCO 3, and 0.1N Na 2 CO 3 ). The carboxylic and lactonic sites were titrated with 0.1N Na 2 CO 3 solution, the carboxylic sites were determined with 0.1N NaHCO 3 solution and the phenolic sites were estimated by the difference [17]. The results shown in Table 7.2 indicated that total numbers of acidic sites are 3.09 meq g -1. These sites suggest the high adsorption capacity of the adsorbent. 7.3.4. Effect of Contact time and initial concentration Adsorption of Ni(II) onto phosphate-treated Ashok bark at various initial concentrations was studied at different time intervals (1 180 min.). The equilibrium uptake capacity (q e ) for Ni(II) was found to be 0.99, 1.97, 4.42, 6.85 and 7.90 mg g -1 at 10, 20, 50, 70 and 100 mg L -1 initial Ni(II) concentrations respectively (Fig. 7.5). When the initial concentration of Ni(II) is increased, the rate of adsorption decreased, but the amount of Ni(II) adsorbed increased. In the first stage, the rate of adsorption is rapid initially and then increases slowly with time until attains an equilibrium. The equilibrium is achieved easily when the initial concentration is low, because at the first stage the ratio of available surface of adsorbent is large for the adsorption of

Ni(II) and as the contact time increases the available sites gradually decreases until it attains equilibrium [22]. 208 7.3.5. Effect of ph and electrolyte concentration The effect of solution ph on the % adsorption of Ni(II) was studied by varying the ph in the range 2-9. It can be observed from Fig. 7.6 that adsorption of Ni(II) was strongly affected by ph. The % adsorption was 75.4 % at ph 2 and then increased by increasing the ph; reached a maximum value (90%) at ph 4. However, beyond ph 4 the adsorption attained the same maximum value (90%). The variation in the adsorption of Ni(II) with change in the ph can be explained by considering the surface charge of the adsorbent and speciation of Ni(II).At ph 2 the H + ions are adsorbed along with Ni(II) ions resulting an increase in final or equilibrium ph (ph f = 6.2).When initial ph (ph i ) is adjusted to 3, the final ph (ph f ) increases rapidly to 7.6 and at same time appreciable amount of Ni(II) is adsorbed (88.4%). However, increasing the initial ph above 3 does not affect final ph value (phf = 7.6) and adsorption of Ni(II) remain maximum (90 %). Fig. 7.7 represents the point of zero charge in presence of varying concentration of electrolyte. The point of zero charge (phpzc) was found to be 8.7. Hence surface of the adsorbent is positive at ph < 8.7; neutral at ph = 8.7 and negative at ph>8.7. Further, Ni(II) is present as Ni 2+ ions at ph 6 and Ni(OH) 2 at ph 7.2 [23]. Therefore adsorption of Ni 2+ in the ph range 2-4 (lower than phpzc) cannot be explained on the basis of electrostatic attraction between adsorbent and Ni 2+ ions. The initial solution ph is not the only factor to explain the adsorption behavior. The increase or decrease of final ph after contact may also affect metal adsorption and may bring about changes in the chemical speciation of the metal [24]. The final ph recorded at which maximum adsorption of

209 Ni(II) occurred was 7.6. Therefore it can be concluded that adsorption of Ni(II) above ph 2 might have occurred in the form of micro precipitation on the surface of the adsorbent. 7.3.6. Adsorption isotherms In order to optimize the design of adsorption system for the removal of Ni(II) from aqueous solution, experimental data were fitted in the Langmuir, Temkin, Freundlich and Dubinin-Radushkeuich models at various temperatures. The linear form of Langmuir isotherm is represented as 1/q e = (1/q m ) (1/b) (1/C e ) + 1/q m ----------------- (1) Where q e is the amount of metal adsorbed per unit weight of adsorbent, q m is the maximum adsorption capacity (mg g -1 ) determined by the number of reactive surface sites in an ideal monolayer system, C e is the concentration of metal ions at equilibrium (mg L -1 ) and b is a constant, related to bonding energy associated with ph dependent equilibrium constant. Plots of 1/q e versus 1/C e at 30 0, 40 0 and 50 0 C gave straight lines (Fig. 7.8) and values of b and q m were calculated from the slope and intercept of the plots (Table 7.3). The essential characteristic of Langmuir isotherm can be expressed in terms of dimensionless constant separation factor or equilibrium parameter R L, given by the following relation R L = 1/(1+ b C 0 ) ------------------- (2) Where b is the Langmuir constant and C 0 is the initial concentration of Ni(II) (mg L - 1 ). R L value predicts the shape of the isotherm. If R L >1 unfavorable; R L = 1 linear; 0< R L <1 favorable and R L = 0 for irreversible adsorption [25]. The R L values at 30 0, 40 0 and 50 0 C are also shown in Table 7.3.The values of R L in the range 0-1 at all these

210 temperatures show favorable adsorption of Ni(II). Table 7.4 presents the comparison of adsorption capacity (q m ) of phosphate-treated Ashok bark with various adsorbents reported earlier [14, 26-32].The Ni(II) adsorption capacity of phosphate-treated Ashok bark is higher than these adsorbents. The Linear form of Freundlich isotherm can be represented as log q e = log K f + (1/n) log C e ---------------- (3) Where K f is Freundlich constant and n is another constant that informs about the heterogeneity degree of the surface sites. Plots of log q e verses log C e gave straight lines at 30 0, 40 0 and 50 0 C (Fig. 7.9) and values of n and K f were calculated from the slope and intercept of these plots (Table 7.3). Temkin isotherm assumes that the decrease in the heat of adsorption is linear rather logarithmic, as implied in the Freundlich isotherm. The linear form of Temkin equation can be represented as [33]. q e = B t ln A t + B t ln C e ----------------- (4) Where B t = (RT/b t ), and R is universal gas constant, T is absolute temperature and b t is another constant. A t (g L -1 ) and B t are Temkin constants related to adsorption potential and heat of adsorption. The values of A t and B t were calculated from the slope and intercept of the plot of q e versus ln C e (Table 7.3). Dubinin-Redushkeuich (D-R) isotherm does not assume a homogenous surface or a constant adsorption potential [34].The linear form of this equation is represented as ln q e = ln q m βε 2 ---------------------- (5)

211 Where ε is the Polyanyi potential, q m is the monolayer capacity (mol g -1 ), C e is the equilibrium concentration (mol L -1 ), and β is a constant related to adsorption energy [mol) 2 (kj) -2 ]. The parameters q m and β can be obtained from the intercept and slope of the plot of ln qe versus ε 2. The Polyanyi potential (ε) and mean free energy of adsorption (E, k J mol -1 ) can be calculated from the equations ε = RT ln (1+1/C e ) ------------ (6) E = 1/ -2 β--------------------- (7) The fitting procedure was performed using R software, version 2.10.1 (2009-12-14). To evaluate the fitness of the data, correlation coefficients (R 2 ), error analysis (residual standard error (RSE), sum of square error (SSE)) and P-values were calculated. The values of constants obtained from different models were fitted and corresponding q e values was calculated (designated as q e(cal) ) from each model. The values of q e found experimentally (designated as q e(exp) ) were compared with q e(cal) using chi-square test (χ 2 ). Chi-square test values were calculated from the following relation χ² = [q e(exp) q e(cal) ] ²/ q e(cal) --------- (8) The SSE values were calculated using the following relation 2 e exp ) / N ----------------(9) SSE= ( q qe cal Where, N is the number of observations. Lower the value of χ 2 and SSE better is the fit. The parameters calculated from different models at 30 0, 40 0 and 50 0 C are reported in Table 7.3. All the models were obeyed by the system at these temperatures as indicated by their regression coefficient values (R 2 vary from 0.98 to 0.99). The p-

212 values in all cases are less than 0.05. However, R 2 is not the only parameter that indicates the fitness of the model because in linear model, experimental values are regressed. The chi-square test (χ 2 ) is more significant because experimental q e values are compared with q e calculated from the model on the same abscissa and ordinate [35]. The Langmuir model for instance shows high correlation coefficient values (R 2 =0.995) at 30 0 C and χ 2 value is least (χ 2 = 0.039) when compared at 40 0 C and 50 0 C therefore it can be concluded that Langmuir model is best obeyed at 30 0 C but Freundlich model shows least χ 2 value (0.003) at 40 0 C. The RSE and SSE are also least (Table 7.3) at this temperature hence Freundlich model is also best fitted at 40 0 C though R 2 at 40 0 C is 0.992. Similarly D-R models are also obeyed at all these temperatures but Temkin model shows higher χ 2 values hence is not a better fit. 7.3.7. Thermodynamic studies The temperature range used in this study was 30-50 0 C.The equilibrium constants (Kc) at 30 0, 40 0 and 50 0 C were calculated from the following relation [36]. Kc = C AC /C e --------------------------------- (10) Where C AC and C e are the equilibrium concentrations (mg L -1 ) of Ni(II) on the adsorbent and in solution, respectively. Free energy change ( G 0 ) can be calculated as G 0 = - RT ln Kc --------------------------------- (11) The value of enthalpy change ( H 0 ) and entropy change ( S 0 ) were calculated from the following relation. ln Kc = ( S 0 /R) ( H 0 /R) (1/T ) -------------------- (12)

213 H 0 and S 0 were calculated from the slope and intercept of linear plot of ln Kc versus 1/T (Fig. 7.10). The values of Kc, H 0, S 0 and G 0 are reported in Table 7.5. The positive value of H 0 indicates endothermic process. The negative values of G 0 indicate that the process is spontaneous and spontaneity increases with increase in temperature. The positive value of S 0 suggests increased randomness at the solidliquid interface during adsorption. The value of mean free energy (energy required to transfer one mole of adsorbate from infinity to the adsorbent surface) gives an idea about the nature of adsorption. If the value of mean energy is greater than 8.0 kj mol - 1, the adsorption is chemical in nature [37], i.e. adsorption occurs due to the chemical bonding between Ni(II) ion and adsorbent. The value of mean free energy was found to be 9.25 k J mol -1 at 30 0 C and increases only slightly with increase in temperature (Table 7.3), indicating that adsorption is chemical in nature. 7.3.8. Adsorption Kinetics In order to analyze the adsorption kinetics, the pseudo-first-order and pseudosecond-order kinetic models were tested using experimental data and rate constants were calculated at different concentrations. Pseudo-first-order kinetics equation as expressed by Lagergren [38] can be written as log (q e q t ) = log q e (K 1 / 2.303) t --------------- (13) Where q e and q t are the amounts of metal adsorbed (mg g -1 ) at equilibrium and at time t respectively and K 1 is the pseudo-first-order adsorption rate constant (min -1 ). A plot of log (q e q t ) versus t gives straight line and rate constant K 1 can be calculated from the slope (Fig. 7.11). Pseudo-second-order kinetics equation may be expressed as [38]

214 t / q t = 1 / (K 2 q e 2 ) + (1/ q e ) t ------------------ (14) Where, K 2 is the pseudo-second-order adsorption rate constant (g mg -1 min -1 ). A plot of t/q t versus t gives straight line (at different concentrations) (Fig. 7.12). The values of K 2 can be calculated from the intercept of the plot. The data show that the correlation coefficient values (R 2 ) for pseudo-first-order are very low in comparison to pseudo-second-order model (Table 7.6). The values of adsorption capacity calculated from the model (q e(cal) ) are very near to experimental values (q e(exp) ) for second-order-kinetics but for first-order-kinetics equation these values are deviated. Higher correlation coefficient (R 2 ) values and similar q e(cal) and q e(exp) values indicate the better applicability of pseudo-second-order kinetics model. For batch process the temporal approach to equilibrium can be illustrated by a plot of the fractional uptake F against time t (figure not shown), where F = q t /q e. The time needed to reach equilibrium increases with increasing the initial Ni(II) concentration. It also shows that the fractional uptake F decreases with increasing the initial Ni(II) concentration, although this tendency is not so obvious within the high concentration range at short adsorption times. Boyd [39] and Webber [40] models are widely used for predicting the nature of adsorption. The detail of the model has been explained in Chapter 2.This model is expressed as. F = 1-6/π²Σ 1/n 2 exp (-n 2 Bt) (15) n = 1 Where, Bt is diffusivity constant. From Eq. (15), it is not possible to calculate the values of Bt for each fraction adsorbed. By applying the Fourier transform and then integration, Reichenberg [41] obtained the following approximation For F > 0.85, Bt = 0.4977 ln (1 F) ------------------ (16)

215 and for F < 0.85, Bt = ( π - (π (π 2 F/3)) 2 --------------- (17) The values of F obtained were F>0.85. This shows that Eq. (16) is applicable in this case. If the plot of Bt versus time is linear and passes through the origin then pore-diffusion controls the rate of mass transfer. If the plot is nonlinear or linear but does not pass through the origin, then it is concluded that film-diffusion or chemical reaction controls the adsorption rate. The plots of Bt versus time for the adsorption of Ni(II) on Phosphate-Ashok bark at different concentrations show that the lines do not pass through the origin. These observations suggest that film diffusion or chemical reaction controls the rate of adsorption during this period. 7.3.9. Intra-particle diffusion The Weber and Morris [40] intra-particle diffusion model can be expressed as q t = Kid t 1/2 + I ----------------------- (18) Where q t (mg g -1 ) is the amount of Ni(II) adsorbed at time t, I (mg g -1 ) is the intercept and Kid (mg g -1 min -1/2 ) is the intra-particle diffusion rate constant. The Kid values obtained from the slope of the curves of different initial Ni(II) concentrations (Fig.7.13) are shown in Table 7.7. The R 2 values (between 0.8502 and 0.9577) suggest that adsorption of Ni(II) follows intra-particle diffusion model to some extent. However, plots did not pass through the origin (the intercept values are between 0.9434 and 5.6894 mg g -1 ) indicating that intra-particle diffusion is not the only rate limiting step. The increase in intercept values with increase in concentration is indicative of increased boundary layer effect [42].

216 7.3.10. Breakthrough studies Fig.7.14 shows breakthrough curve of Ni(II) from the column at an initial concentration of 50 mg L -1.The breakthrough occurred at 50 ml effluent volume. The breakthrough and exhaustive capacities were found to be 5.0 and 65.0 mg g -1, respectively [43]. 7.3.11. Desorption of Ni(II) by batch process In order to make the process more economical, attempt were made to desorb Ni(II). The desorption of Ni(II) by Batch process is shown in Table 7.8. It has been found that 2.22 mg Ni(II) was adsorbed and 2.20 mg was recovered with 0.1N HCl solution. 7.3.12. Removal and recovery of Ni(II) from electroplating wastewater The analysis of electroplating wastewater is reported in Table 7.9. The results after treatment of wastewater by batch processes are shown in Table 7.10. It can be inferred that after treatment the concentration of Ni(II) present in the electroplating wastewater is reduced to much extent. The adsorbed Ni(II) could be recovered to the extent of 69.2 % with 0.1N HCl solution. Further, it is also possible to desorb completely Pb(II), Cu(II) and Zn(II) ions with 0.1N HCl as desorbing agent.

217 7.4. Conclusions Ashok bark showed high affinity towards Ni(II) ions. The adsorption capacity increased remarkably when bark was treated with Na 3 PO 4 solution. Thermodynamic parameters indicated endothermic and spontaneous nature of the adsorbent.the mean free energy value indicated that the adsorption was chemical in nature. Langmuir and Freundlich isotherms were better obeyed at 30 0 C and 40 0 C respectively as compared to Temkin and D-R isotherms as indicated by chi-square test. Adsorption of Ni(II) was appreciable at ph 2 and increased ph favored adsorption due to deprotonation of functional groups. Pseudo-second order kinetics model is better obeyed than pseudo-first order model in all the experimental concentrations. The phosphate-treated Ashok bark could be utilized for the removal and recovery of Ni(II) from electroplating wastewater.

218 Table7.1: FTIR peak values for different functional groups present in native and phosphate- treated Ashok bark before and after adsorption. Functional Ashok bark Phosphate treated Phosphate treated groups Ashok bark Ashok bark Ni(II) loaded 3- -OH( PO 4 ) - 3786.1 - - OH (water) 3427.3 3433.1 3435.7 aliphatic 2927.5 2926.1 2926.9 C H NH 2 2361.9 2361.5 2360.8 C = C 1619.6 1620.8 1621.8 COO- 1450.5 1445.0 (s) 1423.6 phenolic OH 1381.9 1381.0 (s) 1381.0 1320.7 1322.6 1321.6 H 2 PO - 4-1157.3 1157.0 (weak) C-O 1022.6 1019.2 1023.2 Table 7.2: Concentration of active sites on phosphate-treated Ashok bark Actives sites Concentration(meq g -1 ) Carboxylic 0.12 Phenolic 0.57 Carboxylic + Lactonic 2.40 Total sites 3.09

219 Table 7.3: Adsorption isotherm parameters for the adsorption of Ni(II) on phosphate-treated Ashok bark. Isotherms Parameters 30 0 C 40 0 C 50 0 C Langmuir q m (mg g -1 ) 22.900 12.900 11.410 b (L mg -1 ) 0.059 0.186 0.302 R L 0.253 0.097 0.062 R 2 0.995 0.991 0.990 χ 2 0.039 0.082 0.283 RSE 0.008 0.009 0.014 SSE 0.121 0.204 0.342 P-value <0.05 <0.05 <0.05 Freundlich K 1.742 2.330 2.680 n 1.520 1.730 1.810 R 2 0.980 0.992 0.994 χ 2 0.016 0.003 0.052 RSE 0.038 0.006 0.223 SSE 0.077 0.005 0.018 P-value <0.05 <0.05 <0.05 Temkin A t 0.698 1.386 2.280 B t 4.440 3.310 2.930 R 2 0.994 0.984 0.989 χ 2 0.114 0.112 0.261 RSE 0.321 0.421 0.741 SSE 4.08 10-6 4.47 10-7 5.96 10-6 P-value <0.05 <0.05 <0.05 D-R q m (mol g -1 ) 0.002 0.001 0.001 β ( mol 2 (kj) -2 ) 5.83 10-9 4.28 10-9 3.88 10-9 E (k J mol -1 ) 9.250 10.80 11.350 χ 2 2.40 10-6 4.86 10-8 1.21 10-7 RSE 0.068 0.042 0.056 SSE 1.05 10-6 1.77 10-8 1.213 10-6 R 2 0.998 0.996 0.993 P-value <0.05 <0.05 <0.05

220 Table 7.4: Adsorption capacities of various adsorbents reported earlier. Adsorbent Adsorption capacity Reference (mg g -1 ) Peat 8.52 [14] Modified pine bark 9.50 [23] Wheat Straw 7.90 [24] Bagasse 0.001 [25] Fly ash 0.03 [25] Bituminous coal 6.47 [26] Coir pith 9.50 [27] Sheep manure wastes 7.20 [28] Nanoiron 11.53 [29] Phosphate-treated Ashok bark 22.90 Present study Table 7.5: Thermodynamics parameters at different temperatures for the adsorption of Ni(II) on phosphate-treated Ashok bark. Temperature Kc G H S R² ( C) (kj molˉ¹) (kj molˉ¹) (kj molˉ¹ Kˉ¹) 30 19.0-7.42 40 30.3-8.87 55.59 12.476 0.9733 50 40.7-9.95 Table 7.6: Pseudo-first-order and pseudo-second-order rate parameters for the adsorption of Ni(II) at different concentrations on phosphate-treated Ashok bark. Concentration Pseudo-first-order Pseudo-second-order (Co) (mglˉ¹) q e(exp) q e(cal) K 1 R² q e(exp) K 2 h R² (mg gˉ¹) (mg gˉ¹) 10 0.99 0.057 0.746 0.8687 0.99 28.360 27.8 1.0000 40 1.97 0.140 0.108 0.9555 1.98 10.200 40.0 0.9997 50 4.42 2.144 0.052 0.9089 4.43 0.606 11.83 0.9925 80 6.85 1.056 0.036 0.9058 6.54 0.3290 15.46 0.9999 100 7.90 1.960 0.103 0.9573 8.00 0.1470 9.20 0.9995

221 Table 7.7: Intra-particle diffusion parameters for the adsorption of Ni(II) on phosphate- treated Ashok bark. Concentration Kid I R² (mg L -1) (mg g -1 min -1/2 ) (mg g -1 ) 10 0.0230 0.9434 0.9542 40 0.0395 1.8007 0.9577 50 0.2019 3.6334 0.9246 80 0.1829 5.5448 0.8904 100 0.4274 5.6894 0.8502 Table 7.8: Desorption of Ni(II) from aqueous solution by batch process (eluent = 0.1N HCl) on phosphate- treated Ashok bark. Amount % Adsorption Amount Amount %Recovery loaded adsorbed desorbed (mg) (mg) (mg) 2.5 88.8 2.22 2.20 99.1 Table 7.9: Analysis of electroplating wastewater. Parameters Concentration (mgl -1 ) ph 5.60 TDS 710.00 Na + 165.00 K + 3.10 Ca 2+ 30.00 Cu(II) 1.60 Zn(II) 0.42 Ni(II) 1.70 Cr(VI) 27.00

222 Table 7.10: Removal and recovery of Ni(II) and some other heavy metals from electroplating wastewater by batch process. Heavy metals Amount %Adsorption Amount Amount %Recovery loaded adsorbed desorbed (mg) (mg) (mg) Pb(II) 0.035 100.0 0.035 0.035 100.0 Cu(II) 0.080 81.3 0.065 0.065 100.0 Ni(II) 0.085 76.5 0.065 0.045 69.2 Cr(VI) 1.000 15.0 0.150 0.070 46.7 Zn(II) 0.021 85.7 0.018 0.018 100.0

223 Fig.7.1a SEM image of Ashok bark (native). Fig.7.1b SEM image of Ashok bark after Ni(II) adsorption.

224 Fig.7.2a SEM image of Phosphate-treated Ashok bark before Ni(II) adsorption. Fig.7.2b SEM image of Phosphate-treated Ashok bark after Ni(II) adsorption.

225 %T 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 3427.3 2927.5 2361.9 1513.4 1381.9 1450.5 1320.7 1619.6 781.9 668.5 1022.6 515.0 4000 3500 3000 2500 2000 1500 Wavenumbers (cm -1 ) 1000 500 Fig.7.3a FTIR spectrum of native Ashok bark.

226 61 60 59 58 57 56 %T 3786.1 55 54 2926.1 53 52 51 50 3433.1 49 4000 3500 3000 778.7 408.4 1157.3 649 1019.2 1322.6 2361.5 1620.8 2500 2000 1500 1000 500 Wavenumbers (cm -1 ) Fig.7.4a FTIR spectrum of phosphate-treated Ashok bark before Ni(II) Adsorption. 35 34 33 32 31 30 29 %T 28 27 26 25 24 23 22 21 20 19 4000 3500 3435.7 3000 2926.9 2360.8 1423.6 1321.6 1621.8 2500 2000 1500 Wavenumbers (cm -1 ) 433.9 779.6 659.5 1023.2 1000 500 Fig.7.4b FTIR spectrum of Phosphate-treated Ashok bark after Ni(II) Adsorption.

227 8 7 6 5 4 3 qt(mg/g) 2 100 80 conc(mg/l) 60 40 20 0 5 10 15 20 1 25 30 0 Time(min) Fig.7.5 Effect of conc and contact time

228 100 10 90 80 8 70 % Adsorption 60 50 40 6 4 phf 30 20 10 DDW 0.1N KNO3 ph change 2 0 2 3 4 6 8 9 0 phi Fig.7.6 Effect of ph and electrolyte on the adsorption of Ni(II) 2 1 phi-phf 0-1 -2-3 -4-5 -6 0 2 4 6 8 10 0.1N KNO3 0.01N KNO3 DDW phi Fig.7.7 Point of zero charge

229 1 0.9 0.8 0.7 30C 40C 50C 0.6 1/qe 0.5 0.4 0.3 0.2 0.1 0 0 0.2 0.4 0.6 0.8 1 1.2 1/Ce Fig. 7.8 Langmuir isotherms at different temperatures 1.4 1.2 1 30C 40C 50C log qe 0.8 0.6 0.4 0.2 0 0 0.5 1 1.5 log Ce Fig. 7.9 Freundlich isotherms at different temperatures

230 4 3.5 3 2.5 1/T 2 1.5 1 0.5 0 0.003 0.0031 0.0032 0.0033 0.0034 ln Kc Fig. 7.10 Van't Hoff Plot 2 0 0 10 20 30 40-2 log (qe-qt) -4-6 -8-10 10mg/L 20mg/L 30mg/L 80mg/L 100mg/L -12 time(min) Fig. 7.11 Pseudo first order kinetics for the adsorption of Ni(II) at different concentrations

231 t/qt 9 8 7 6 5 4 3 10mg/L 20mg/L 30mg/L 80mg/L 100mg/L 2 1 0 0 5 10 15 20 25 30 35 time(min) Fig. 7.12 pseudo-second order kinetics for the adsorption of Ni(II) at different concentrations 10 9 8 7 6 10mg/L 20mg/L 30mg/L 80mg/L 100mg/L qt 5 4 3 2 1 0 0 2 4 6 8 10 t 1/2 Fig. 7.13 Intraparticle diffusion

232 1.2 1 0.8 C/Co 0.6 0.4 0.2 0 0 200 400 600 800 1000 volume (ml) Fig. 7.14 Breakthrough capacity curve for the adsorption of Ni(II) on phosphste-treated Ahsok bark