Investigation of activated carbon denaturation in order to create material for treatment of several hazard ions in water environment

orption and ion exchange 1.3.1. The over view of adsorption - Classification of adsorption isotherm curves - The adsorption technics 1.3.2. The over view of ion exchange - The basic theory of ion exchange process - The basic principal of ion exchange reaction Chapter 2. THE RESEARCH METHODS 2.1. The research objectives - Activated carbon: Activated carbon used for studies of the thesis is activated carbon made from coconut shell of Tra Bac Company Ltd. Some physico-chemical characters of Tra Bac activated carbon are: Density at 25oC and 1 atm: 0.45 g/cm3, Humidity: 3.3%, Ash: 2.5%, Grain size: 0.5 – 1.0 mm. - Water samples were artificial samples containing designed concentration of ammonia, calcium, Cr(III), Cr(VI), As(III), As(V). - Underground water samples containing ammonia ware collected from Hoang Liet, Hoang Mai, Hanoi. 2.2. Chemicals, devices and equipments for experiments All chemicals used for research have purity for analysis. The devices and equipments have precision and accuracy satisfying requirement for experiments of a doctoral thesis. 2.3. Experimental methods 2.3.1. Methods for activated carbon oxidation 2.3.1.1.Oxidation of activated carbon by nitric acid The activated carbon modification process was carried out as follow: Step 1. Sieve raw activated carbon in order to take the portion with the size of 0.5 – 1.0 mm. Step 2. Wash out recovered activated carbon sample by distilled water, then dry for 24 h at 105oC (the sample is signed as AC). Step 3. Oxidize AC by HNO3 with concentration in the range of 3 – 14 M for 2, 4 and 6 h at 100oC. The portion of AC and HNO3 solution was 1:3. In order to avoid loss of HNO3 during oxidation process, the oxidation device was equipped with back flow system. The oxidation temperature was kept at 100 ± 5oC by thermal control system. Step 4. After oxidation, the liquid phase was decanted out. The solid phase was washed by distilled water until pH of effluence meets constant pH value (about 4 – 5). Step 5. Dry oxidized activated carbon at 105oC for 4 h (the sample was signed as OAC) Step 6. Soak a part of OAC in NaOH 0.5M for 24 h at room temperature to neutralize acidic groups on OAC surface. The volume portion of OAC and NaOH solution was 1:10. Step 7. After neutralization, the liquid phase was decanted out. The solid phase was washed out by distilled water until pH was constant (about 7 – 8). Step 8: Dry neutralized OAC at 105oC for 24 h (the sample was signed as OACNa) 2.3.1.2. Oxidation of activated carbon by KMnO4 solution The oxidation process was carried out at room temperature by KMnO4 0.01M solution in H2SO4 6M as follow: Step 1. KMnO4 solution was dripped into accurate quantity of AC in erlenmeyer flask and continuously stirred at 150 r/min until solution appeared violet color stable for 5 min. Note the spent volume of KMnO4 for further calculation. Step 2. Pour liquid phase out and wash out the solid phase by distilled water. Step 3. Dry solid portion at 105oC for 24 h (the product was signed as OACKMnO4). 2.3.1.3. Oxidation of activated carbon by K2Cr2O7 solution The oxidation process was carried out at room temperature by K2Cr2O7 0.1M solution in H2SO4 1.5M. The experiment steps were the same as oxidation by KMnO4. Except the equilibrium point was determined by dimethylcarbaside indicator. The product was signed as OACK2Cr2O7. 2.3.2. Investigation of adsorption/ion exchange in static model Investigation of adsorption/ion exchange in static condition was carried out with constant portion of solid phase and liquid phase at the same temperature and pressure (atmospheric pressure). By the way of the change of pH solution, contact time of the phases and initial ions concentration to determinate the optimal pH, the time for equilibrium establishment and isotherm adsorption/ion exchange curves. From isotherm curves, the maximum adsorption/ion exchange of investigation ion was determined. 2.3.3. Investigation of adsorption/ion exchange in dynamic model Investigation of adsorption/ion exchange in dynamic condition was carried out on the PVC transparent column with internal diameter of 13 mm and height of investigation material about 50 - 60 mm. Adsorbate solution with optimal pH and ions concentration was flowed though the column with designed rate. Ions concentration of effluent fractions was determined. All important factors were determined. 2.3.4. Investigation of regeneration possibility of the used material. Investigation of regeneration of OACNa material after ammonia adsorption was carried out by HCl solution with concentration of 1; 2 and 3 M; then neutralization by NaOH with concentration of 0.3; 0.5; 0.7 and 1.0 M. Investigation of regeneration of OAC-Mn and OAC-Fe material after arsenic adsorption was carried out by NaOH solution with concentration of 0.1 – 0.5 M. 2.3.5. Attaching of Mn2+ and Fe3+ on OACNa a. Attaching of Mn2+ The experiment was carried out by the way of soaking of OACNa into MnSO4 solution of 3; 5; 7 and 10%. The portion of OACNa and MnSO4 solution was 1/20. pH of the mixture was adjusted to 8 by NaOH 0.1 M solution and continuously stirred at 150 r/min for 24 h. The liquid phase was then decanted out. The solid phase was washed out by distilled water; then dried at 105oC for 24 h. The material was signed as OAC-Mn. b. Attaching of Fe3+ The initial step of linking of Fe3+ on OACNa was carried out by the same way as for Mn2+; but the pH of the mixture was adjusted to 3 by HCl 0.1 M and concentrations of Fe3+ were 3; 5; and 8.1%. The solid phase was neutralized by NaOH 0.1 M until pH of the effluent solution meet 9; then washed by distilled water until clean of Cl- ions and dried at 70oC for 24h. 2.3.6. The methods for surface characters determination The SEM, EDX, IR, BRT methods, Boehm titration and pHPZC test were used for determination of the material surface characters. 2.5. Analytical methods 2.6. Methods for calculation Chapter 3. RESULTS AND DISCUSSION 3.1. The results of AC oxidation by different oxidative reagents 3.1.1. The materials achieved by oxidation and surface treatment 36 materials were achieved after oxidation of AC by HNO3 and surface treatment by NaOH, and 4 materials after oxidation of AC by KMnO4 and K2Cr2O7 (ACKMnO4, ACKMnO4-Na, ACK2Cr2O7 và ACK2Cr2O7-Na) 3.1.2. The surface characters of the materials before and after oxidation 3.1.2.1. The morphology and elemental composition of the AC and OAC surface The SEM image of original AC showed that is has fibrillary form with dimension in the range of nanometers, porous and fold each on the other. In the SEM image of OAC, there is no remarkable difference in comparison with the SEM image of AC. EDX analysis showed that, the percentage of carbon in OAC is reduced in comparison with original AC; but that of oxygen is increased and appeared Na component in OACNa. This phenomenon demonstrate that carbon in AC was oxidized and formed more oxygen containing functional groups and Na atoms were bound (see also at table 3.2). Table 3.2. The composition of the AC and OACNa Material % mass % element C O Na C O Na AC 87,46 12,54 0 90,28 9,72 0 OAC10-4Na 82,72 15,21 2,07 86,87 11,99 1,14 3.1.2.2. FTIR analysis FTIR also showed the fact that oxidation by strong oxidative reagents (HNO3, KMnO4, K2Cr2O7) made appearance oxygen containing functional groups on the AC surface. The peak at wave number of 3448 cm-1 showed presence of O-H bond in OH group. At wave number of 2931 cm-1 appeared peak of C-H bond in aromatic rings. On OAC surface, there appeared a sorption peak at 1727 cm-1, due to double binding of C=O in carboxylic group. At wave number of 1627 cm-1 are bonds of C=C in the aromatic rings and bonds of C=O in ketone, lactone and aldehyde groups. At wave number 1380 cm-1 appeared peak of C-O bonds in phenol and alcohol groups. Particularly, when AC oxidized by HNO3, appeared peak at 1126 cm-1, it could be the bonds of C-N in aromatic ring and N-O in long chains [60[, [107], [108]. For OACNa, all the peaks were appeared in the same wave number corresponding to bonds on the OAC, but the bonds of O-H and C=O in OACNa have intensity lower than on OAC. The reason could be effect of Na binding with oxygen containing groups. 3.1.2.3. Specific surface area analysis The original AC has average BET specific surface area (SBET) 785 m2/g. After oxidation, the SBET and total capillary volume (Vtot) of the materials in generally were decreased in comparison with AC, however, OAC oxidized by HNO3 has most decrease. The decrease of SBET and Vtot of OAC10-4, OACKMnO4 and OACK2Cr2O7 were 3,0; 9,1; 17,5% and 7,25; 8,77; 18,87 % respectively. 3.1.2.4. The results of Boehm titration and pHpzc determination - Oxidation by HNO3 Results of acidic and basic group concentration on material surface determined by Boehm titration are listed in table 3.6. Table 3.6. Boehm titration results Material Acidic group concentration (mmol/g) Basic group conc. (mmol/g) Cacboxylic Lactone Phenol Total AC 0 0.15 0.35 0.5 0.68 OAC3-4 0.41 0.28 0.33 1.02 0.67 OAC3-4Na 0.1 0.1 0.3 0.5 1.05 OAC3-6 0.87 0.64 0.6 2.11 0.62 OAC3-6Na 0.15 0.21 0.6 0.96 1.17 OAC6-4 1.6 0.76 0.55 3.55 0.65 OAC6-4Na 0.3 0.15 0.4 0.85 1.0 OAC8-4 2.01 0.8 0.62 3.11 0.52 OAC8-4Na 0.6 0.25 0.6 1.45 1.2 OAC10-2 2.3 0.75 0.86 3.91 0.58 OAC10-2Na 0.55 0.15 0.55 1.25 1.22 OAC10-4 2.5 1.1 1.15 4.75 0.55 OAC10-4Na 0.5 0.3 0.5 1.3 1.05 OAC10-6 2.35 0.85 1.1 4.3 0.6 OAC10-6Na 0.6 0.2 0.6 1.4 1.15 OAC14-4 2.43 0.95 0.1 4.38 0.51 OAC14-4Na 0.75 0.3 0.7 1.75 1.1 There was the trend of increase of acidic groups when increase HNO3 concentration. The acidic group of OAC oxidized by HNO3 10 M for 4 h was 4.75 mmol/g, while oxidized by HNO3 3 M for the same time was only 1.02 mmol/g. When OAC was treated by NaOH, the acidic groups decreased significantly. The total basic groups almost ware not been effected by oxidative reagents. - Oxidation by KMnO4 và K2Cr2O7 AC oxidized by KMnO4 has acidic group highest (5.55 mmol/g); while AC oxidized by K2Cr2O7 reached the value of 2.95 mmol/g only. Although ACKMnO4 has highest acidic group (in which 2.2 mmol/g carboxylic group), but OAC10-4 has highest carboxylic groups (2.5 mmol/g). The total basic groups were almost uninfluenced by oxidative reagents. 3.1.2.5. Quantification of reductive capacity of the AC by KMnO4 and K2Cr2O7 The electron exchange capacity of AC determined by titration with KMnO4 and K2Cr2O7 were 8.9 and 3.0 mmol/g respectively. Based on the results of this survey, there appeared idea that AC could be used for treatment of oxidative components in water environment. 3.2. The NH4+ ion exchange capacity of OAC oxidized by HNO3, KMnO4 and K2Cr2O7 3.2.1. The NH4+ ion exchange capacity of OAC oxidized by HNO3 From investigation results, the material oxidized by HNO3 10 M for 4 h has highest NH4+ ion exchange capacity. 3.2.1.1. The NH4+ ion exchange capacity of OAC10-4 and OAC10-4Na The NH4+ ion exchange capacity of OAC10-4 and OAC10-4Na was investigated. The results were demonstrated in table 3.9. The ion exchange process was according to Langmuir adsorption isotherm (R2 = 0.979). The maximum adsorption capacity of ammonium ion on OAC10-4Na was significantly higher than that of OAC10-4 (20.41 mg/g and 1.53 mg/g respectively). This phenomenon could be explained by different state of acidic groups on the material surface. Before neutralization, almost acidic groups exist in the weak and very weak acidic form; the H+ ion did not dissociate, those resulting to limit ion exchange of ammonium with materials surface. After neutralization, almost acidic groups transformed into Na salts. Ion Na+ is easy to dissociate in water solution, therefore the NH4+ exchange capacity increased obviously. Table 3.9. The parameters of Langmuir isotherm adsorption function for NH4+ on OAC10-4 and OAC10-4Na Material Langmuir Isotherm adsorp. equation qm KL (L/mg) RL R2 mg/g mmol/g OAC10-4 y = 0,655x + 30,70 1,53 0,11 0,021 0,133 0,994 OAC10-4Na y = 0,049x + 1,659 20,41 1,46 0,030 0,099 0,981 3.2.2. The NH4+ ion exchange capacity of OAC oxidized by KMnO4 The NH4+ ion exchange with OAC oxidized by KMnO4 also according to Langmuir adsorption isotherm (see figure 3.9) with R2 at 0.996 for ACKMnO4 and 0.994 for ACKMnO4-Na. Ion exchange capacity of ACKMnO4 was 3.88 mg/g and that of ACKMnO4-Na was 14.29 mg/g. The ion exchange capacity of material after neutralization was higher than before. This phenomenon has the same reason mentioned above for OAC oxidized by HNO3; but the increased value of ACKMnO4-Na was lower in comparison with that of OAC10-4Na. It possibly means: oxidation by KMnO4 in strong acidic solution, the porous system was deeply changed than that by HNO3 and total carboxylic group on ACKMnO4 lower than that on OAC10-4Na. Figure 3.9. The adsorption isotherm curves of NH4+ on AC oxidized by KMnO4 3.2.3. The NH4+ ion exchange capacity of OAC oxidized by K2Cr2O7 The ion exchange capacity of NH4+ on AC oxidized by K2Cr2O7 was 7.30 mg/g, higher than AC oxidized by KMnO4 but lower than AC oxidized by HNO3. 3.3. The ion exchange ability in static model of ammonium and 2 and 3 valence ions on AC oxidized by HNO3 3.3.1. The contact time a. For ammonium Adsorption capacity of ammonium on AC original or OACHNO3 was very low and the contact time reaching equilibrium state was about 40 min. However, on the OACHNO3 neutralized by NaOH, the adsorption capacity was very high and the time for reaching equilibrium increased to 60 min. b. For Ca2+ y = 0.260x + 14.09 R² = 0.996 y = 0.070x + 3.397 R² = 0.994 0 20 40 60 80 100 0 50 100 150 200 250 300 C e/ q e (g /l ) Ce (mg/l) ACKMnO4 ACKMnO4-Na The time reaching adsorption equilibrium for Ca2+ on AC, AC oxidized by HNO3 and AC oxidized by HNO3 then neutralized by NaOH were 50, 50 and 70 min respectively. c. For Cr3+ The situation for Cr3+ was the almost same as for Ca2+ (on AC, AC oxidized by HNO3 and AC oxidized by HNO3 then neutralized by NaOH were 40, 50 and 70 min respectively). 3.3.2. The influence of pH on adsorption/ion exchange process a, For Ammonium The pH range surveyed for pH influence on NH4+ was from 3 to 9, because pH higher than 9, NH4+ could be lost due to NH3 evaporation. The result showed that, the influence on AC original was not clear and on the OAC, the optimal pH was in the range from 5 to 7 b. For Ca2+ For Ca2+ the optimum pH was 6 – 7. c. For Cr3+ For Cr3+, the optimum pH was 6 – 7 and best at 6. 3.3.3. The influence of initial concentration of ion adsorbate on adsorption capacity a. For NH4+ ion Results of the investigation were demonstrated according to Langmuir and Freundlich adsorption isotherm curves on figure 3.17 (for Lngmuir) and figure 3.18 (for Freundlich). The adsorption capacity was in the order: OAC10-4Na > OAC8-4Na > OAC6-4Na > OAC10-4 > OAC6-4 > AC. The adsorption capacity of OAC10-4Na was highest (even higher than OAC14-4Na) and neutralized OACs always have adsorption capacity higher than OACs in acidic form. Figure 3.17. The Langmuir adsorption isotherm curve of NH4+ NH4+ The adsorption isotherms seem according to both Langmuir and Freundlich adsorption isotherm equations. This case possibly showed a complicated process including adsorption and ion exchange. At the survey pH (6 – 7), stronger acidic function groups could be dissociated to create negative charged centers. These centers play the role as adsorption centers to attract NH4+ according to adsorption mechanism. The weaker acidic function groups were undissociated, so NH4+ linking with these groups according to ion exchange mechanism. On the electron affinity groups, the attraction of NH4+ was more complicated mechanism. For this reason, in our thesis, the phrase “adsorption/ion exchange” or only “adsorption” was used. Figure 3.18. The Freundlich adsorption isotherm curve of NH4+ b. For Ca2+ The influence of initial Ca2+ concentration on equilibrium adsorption capacity was illustrated at figure 3.19 and 3.20. The situation in this case was similar to ammonium case. There was a similar trend of increasing adsorption capacity in the order of OAC10-4Na > OAC8-4Na > OAC6-4Na > OAC10-4 > OAC6-4 > AC. The maximum adsorption capacity belongs to OAC10-4Na and it was 43.6 mg/g or 1.09 mmol/g. Figure 3.19. The Langmuir adsorption isotherm curve of Ca2+ NH4+ Ca2+ Figure 3.20. The Freudlich adsorption isotherm curve of Ca2+ c. For Cr3+ The Langmuir adsorption isotherm curve of Cr3+ was illustrated at figure 3.21. Figure 3.21. The Langmuir adsorption isotherm curve of Cr3+ The maximum equilibrium adsorption of Cr3+ calculated from Langmuir equation was 38.46 mg/g and it also belonged to OAC10-4Na. The adsorption/ion exchange of ion Cr3+ was very complicated. There was recognition that, the oxidation of activated carbon significantly increased adsorption capacity of anions in water solution, even hexavalence ion of chromate [34]. 3.4. The adsorption/ion exchange ability of NH4+ on OACs in dynamic model 3.4.1. The influence of initial concentration In the same condition of column height and flow rate, increasing of initial concentration of ammonium, the break through time decreased, but dynamic equilibrium adsorption capacity increased. In the case of initial ammonium concentration was 10; 20 and 50 mg/L, the break through time was 1,560; 1020 and 660 min, and dynamic equilibrium adsorption capacity was 10.9; 12.3 and 14.1 mgN/g respectively. 3.4.2. The influence of flow rate Increasing of flow rate (1.52; 3.03 and 4.55 ml/min/cm2), the break through time decreased corresponding to 1,560; 780; and 600 min, and the dynamic equilibrium adsorption capacity decreased corresponding to 10.98; 10.56 and 8.11 mg/g. The results showed that, if need be increase adsorption capacity in this case, the flow rate must be decreased. But according to adsorption theory, the flow rate could be decreased maximum to the rate equal to diffusion rate. Because when flow rate was lower than diffusion rate, the adsorption capacity could be decreased. These results were also affirmed in research of Karadag et al. (2008) and Simon Kizito et al. (2016) [123], [124]. 3.4.3. The influence of adsorption column height Evidently, the adsorption column height increased, the break through time also increased and dynamic equilibrium adsorption capacity, in general, unchanged. However, in real experiments, the most effect adsorption occurs only at the certain ratio between column diameter and column height. 3.4.4. The adsorption kinetics Based on the research results, the adsorption kinetics was established according to several models. The Bohart – Adam and Thomas models were presented in table 3.18 when the initial concentration, flow rate and adsorption column height changed. Table 3.18. The adsorption parameters for NH4+ according to Bohart - Adam and Thomas kinetic adsorption equation Parameter Thomas model Bohart – Adam model KT (ml/min/mg) q0 (mg/g) R2 KAB (l/min/mg) N0 (mg/l) R2 Initial concentration (mg/l) 10 4x10-4 13 0,979 3x10-4 25073 0,969 20 2,5x10-4 15 0,993 1,5x10-4 35509 0,934 50 1,6x10-4 15 0,994 0,6x10-4 52712 0,92 Flow rate (ml/min/cm2) 1,52 4x10-4 13 0,979 3x10-4 25073 0,969 3,03 6x10-4 12 0,973 2,7x10-4 26347 0,978 4,55 14x10-4 8,8 0,994 2x10-4 38906 0,978 Column height (cm) 4,44 6x10-4 12 0,979 2,9x10-4 39540 0,924 6,67 4x10-4 18,4 0,985 3x10-4 87800 0,936 11,11 2x10-4 22,2 0,975 3,6x10-4 214345 0,965 3.5. Regeneration of used OAC after exchange with NH4+ 3.5.1. The generation at static model As previous discussion, the sorption of NH4+ on OAC10-Na was predominantly belonging to ion exchange mechanism; therefore, HCl was chosen as regenerative reagent and NaOH was for surface structure recovering reagent [124]. HCl is strong acid, so HCl solution contacts with NH4+ exchanged material, ion H+ easily takes place ion ammonium at adsorption centers on the material surface. In static model, the experiment was carried out with HCl concentration in the range from 0.5 to 2.0 M and Na+ surface of the material was recovered by NaOH solution 0.5M. The results showed that, after 3 regeneration periods, the maximum adsorption capacity was unconsidered reduced but always reached more than 90% in comparison with initial period. 3.5.2. The regeneration at dynamic model For dynamic model, the experiment was carried out in the column with the same dimension as used for adsorption process. For OAC10-Na, the results of 3 regeneration periods the adsorption capacity still reached values 93.2; 92.3 and 84.4 % in comparison with initial period. 3.6. Attaching of metal ion on OAC and application for As treatment in water The original AC has hydrophobic and non polar surface, therefore, the charged elements, particularly, metals ions are very limited to attach on. OAC is unlike original AC, on OAC surface appeared acidic function groups. For that very reason, the dissociation of H+ or Na+ (salt form) create negative centers to easy attach cations in the solution. In the case of electron affinity centers, the attractive ability of positive elements is increased. In this investigation, ion Fe3+ and Mn2+ were chosen for the arsenic treatment goal. The material was created by the way, that after attaching Fe3+ and Mn2+ onto OAC, these ions were hydrolyzed to change into FeOOH and MnO2 forms (signed as OAC-Fe and OAC-Mn). The last forms were used for further investigations. 3.6.1. The character of OAC surface attached Mn, Fe When Fe or Mn was attached on the surface of OAC, the SEM image was clearly changed. There were appeared micro crystals of MnO2 and FeOOH on the wall of the capillary and on the edge of the structure. The MnO2 crystals have clearly needle shape while FeOOH crystals have barbed spherical shape. The content of Fe and Mn in original AC is very low and depends on source and nature of the initial material. After attaching, the content of Fe and Mn is significantly increased and depends on concentration of the metals in attaching solutions. Although the metals concentration in attaching solution increased, the content of the metals in AC also increased; but the content of the metals in AC only reached to certain value, then was almost constant or lightly decreased even concentration of the metals in the attaching solution further increased. 3.6.2. The As adsorption ability of the OAC-Fe and OAC-Mn 3.6.2.1. The influence of pH on As(III) and As(V) adsorption capacity The investigation results illustrated in figure 3.32 showed that, the highest adsorption capacity of As(V) was in the range of pH from 3 to 6 and began to decrease at pH from 6 to 7. Particularly, when pH higher than 7, the adsorption of As(V) decreased sharply. This scene occurs on both OAC-Fe and OAC-Mn; but the adsorption capacity of As(V) on OAC-Fe is significantly higher than on OAC-Mn. The pHPZC of OAC-Mn and OAC-Fe were determined 7.3 and 6.5 respectively to contribute to explanation of the variation of adsorption capacity of anion arsenate according to pH variation. Figure 3.32. Influence of pH on adsorption of As(III) and As(V) For ion As(III), the adsorption capacity reached highest value at pH in the range from 6 to 10 and decreased when pH was higher than11. This phenomenon was similar for both OAC-Fe and OAC-Mn. The highest adsorption capacity of As(III) moved to higher pH area because of arsenious acid is more weaker than arsenic acid. At pH < 6, almost arsenic exists in the non dissociated arsenious acid molecule and at pH > 10, anion 0 0.3 0.6 0.9 1.2 1.5 2 4 6 8 10 12 14 D u n g l ư ợ n g h ấ p p h ụ (m g /g ) pH OAC-Mn7 AsIII OAC-Fe5 AsIII OAC-Mn7 AsV OAC-Fe5 AsV hydroarsnite and arsenite were competed with anion OH- in solution. Those explained the adsorption capacity variation of anion As(III) according to pH solution. On the other way, different than As(V), the adsorption capacity of As(III) on OAC-Mn was higher than that on OAC- Fe. 3.6.2.2. Influence of initial As(III) concentration and attaching concentration of Fe and Mn on As(III) adsorption capacity The adsorption of As(III) on or