3.1. Chracterizastion of the Carbon Materials
Table 1 presents the results of elemental analysis for the carbon materials obtained. A review of the data presented in
Table 1 reveals that sample C exhibits the lowest elemental carbon content, which also results in the highest oxygen content for the same material. In contrast, the percentages of hydrogen and nitrogen were 2.1 wt. % and 2.9 wt. %, respectively. Further analysis of the data presented in
Table 1 revealed that the activation of the precursor with carbon dioxide and the impregnation of sample C with urea resulted in a notable alteration of the carbon materials obtained. The NAC sample was characterised by its highest concentration of elemental carbon (86.9 wt. %). The carbon obtained by direct activation of the starting material with carbon dioxide exhibited a carbon content that was over 10 wt. % lower than that of NAC activated carbon. In the case of AC and NAC samples, an increase in the C
daf content was accompanied by an increase in the proportion of hydrogen and oxygen. Modification with urea allowed obtaining an adsorbent containing in its structure N
daf – 4.7 wt. %. Further analysis of the data revealed that the AC and NAC carbonaceous materials exhibited a low sulphur content, with a maximum of 0.1 wt. %. In contrast, the oxygen content of these samples was found to be between 4.8 wt. % and 5.3 wt. %. It is also pertinent to mention that each of the carbon materials obtained exhibits a markedly high ash content. The mineral substance in question has a weight percentage range of 7.8 wt. % to 12.1 wt. %. The high ash content of the obtained coal materials suggests that ash may be present within the pores of the carbon structure.
The results of the textural studies (
Table 2) indicate that the termochemical treatment of the residue following the extraction of the fungus
Inonotus obliquus did not facilitate the development of a porous structure effectively. This is demonstrated by the specific surface areas of the obtained activated carbons, which are 125 (sample C), 749 (sample AC) and 888 m
2/g (sample NAC), respectively. The most developed surface area was characterised by NAC adsorbent, which is the only one with a specific surface area greater than 400 m
2;/g.
The data presented in
Table 2 indicates a correlation between the specific surface area and the iodine numbers obtained for the carbon adsorbents produced. The tested adsorbents exhibited a porous structure comprising small mesopores, as evidenced by the average pore diameter values, which ranged from 4.15 to 7.32 nm. Furthermore, the nitrogen desorption adsorption isotherms, as illustrated in
Figure 1, demonstrated the presence of such pores. In accordance with the IUPAC classification, the isotherms depicted in
Figure 1 are indicative of type IV. A defining characteristic of the type IV isotherm is the presence of a clearly delineated hysteresis loop, which is associated with capillary condensation in the region of mesopores. The H4-type hysteresis loops visible in the isotherms around the p/p
0 pressure of approximately 0.4 suggest the condensation of nitrogen in the mesopores. This indicates that the tested coals have developed mesoporosity [
24].
SEM images of the adsorbent samples are presented in
Figure 2. Regarding char samples, the ash content may account for the brighter fragments observed.
The data presented in
Figure 3 indicates that the obtained adsorbents exhibit comparable acid-base properties. As evidenced by the presented data, the type and quantity of surface oxygen groups are contingent upon the variant of carbon sample production. Regardless of the preparation variant, all coals exhibited acidic and basic groups on their surface. The NAC sample exhibited the highest proportion of both groups. The carbon in question exhibited 2.0 mmol/g of acidic functional groups and 3.5 mmol/g of basic groups on its surface. Furthermore, the data revealed a prevalence of basic groups over acidic ones for all samples. The predominance of basic functional groups is also corroborated by the pH values presented in
Figure 3, which range from 7.8 to 8.5.
3.2. Physicochemical Properties and Adsorption Characteristics of the Membranes
Table 3 presents the results for the structural parameters and wetting angle values of the materials under investigation. The data in
Table 3 indicates that the char-enriched membrane exhibited the highest values for porosity (68.60 %) and equilibrium water content (88.21 %). The values for the MNAC membrane and the membrane without added char material were slightly lower. The lowest porosity and equilibrium water content values were determined for the MAC material. The analysis of the wetting angles revealed that the MAC material exhibited the most hydrophilic surface character, while the membrane without added carbon material exhibited the least hydrophilic character.
Table 4 shows the content of acid and basic oxygen groups on the surface of the extracted materials.
From the values obtained it can be seen that the MAC material had the highest number of acid functional groups (5.19 mmol/g). On the surface of the pure membrane, the number of acid groups was 3.69 mmol/g. For the other two membranes, however, the values were much lower at 2.57 mmol/g for MC and 2.80 mmol/g for MNAC. In the case of the alkaline groups, a completely opposite relationship was observed. On the surface of MC and MNAC, the highest content of such groups was found, 5.05 for the activated carbon membrane and 5.19 mmol/g for the nitrogen-enriched activated carbon modified membrane, respectively. A slightly lower amount of 4.60 mmol/g was found for the MAC material. In contrast, the least alkaline functional groups were present on the surface of the membrane without the addition of carbon materials and amounted to 1.66 mmol/g.
Figure 4 illustrates the flow rates determined for the materials under investigation. The highest flow values were observed for the M membrane, with a value of 24.25 L/m
2×h prior to filtration of copper ion solutions. A slightly lower value of 23.30 L/m
2×h was determined for the MNAC membrane, while the flow values for the MC and MAC membranes were 18.66 and 16.32 L/m
2×h, respectively. The flow values for the materials used to remove iron ions were similar to each other. The value for the M membrane was 22.48 L/m2h, for the MC material 20.05 L/m
2×h and for the MAC and MNAC materials 21.04 and 21.29 L/m
2×h, respectively. The highest flow rates for MC and MAC were 19.44 and 19.93 L/m
2×h, respectively, for the materials used in the phenol filtration processes. M and MNAC membranes exhibited slightly lower values for this parameter, with values of 16.73 L/m
2×h for M and 18.62 L/m
2×h for MNAC.
Figure 5 illustrates the efficacy of the materials tested in removing copper ions from aqueous solutions. All of the materials exhibited enhanced efficiency in removing copper ions from solutions with an initial concentration of 800 mg/L. The results indicate that copper ions, regardless of the initial solution concentration, are most effectively removed by the urea-impregnated carbon membrane. The material was found to be highly effective in removing copper ions from aqueous solutions, with 67 % of the ions removed from a solution with an initial concentration of 800 mg/L and 60 % from a solution with a higher concentration. In contrast, the use of the MAC material resulted in the removal of only 24 % of the ions from a solution with an initial concentration of 800 mg/L and 14% from a solution with an initial concentration of 1000 mg/L. In contrast, the use of MC membrane allowed the removal of 24 and 10 % of copper ions, respectively. The membrane without added carbon material was the least effective in removing copper ions from aqueous solutions, removing 21 % of the ions from a solution with a lower initial concentration and only 1 % from a solution with an initial concentration of 1000 mg/L.
A quantitative analysis of the data presented in
Figure 6 reveals that all the membranes tested exhibited enhanced efficiency in the removal of iron ions from aqueous solutions with an initial concentration of 20 mg/L. The values collected indicate that the membrane without added carbon material exhibited the highest efficiency in the removal of iron ions. The membrane was found to be highly effective in removing iron ions from aqueous solutions. It was observed that 64 % of the iron ions were removed from a solution with an initial concentration of 20 mg/L, while 58 % were removed from a solution with a concentration of 12 mg/L. In comparison, the results obtained for the other materials were considerably lower. The MC material demonstrated iron ion removal efficiencies of 26 % for the lower concentration solution and 39 % for the 20 mg/L solution. The use of MAC material resulted in the removal of 30 % of the ions from a solution with an initial concentration of 12 mg/L, while 35 % of the ions of this metal were removed with MNAC material. In contrast, the use of MAC material removed 34 % of the ions from a solution with an initial concentration of 20 mg/L, while 43 % of the ions of this metal were removed with MNAC material.
As with the removal of copper ions, the membranes tested demonstrated enhanced efficiency in the removal of phenol molecules from solutions with lower initial concentrations (
Figure 7). Regardless of the initial concentration of the solution, the membrane without added carbon materials exhibited the highest efficacy (89 %). The other materials exhibited significantly lower efficiency in the removal of phenol from aqueous solutions. The use of MC allowed for the removal of 44 % of phenol from a solution with an initial concentration of 15 mg/L and 27% from a solution with an initial concentration of 25 mg/L. In contrast, the use of MAC resulted in the removal of 61 % and 29 % of phenol molecules, respectively. In contrast, MNAC permitted the removal of 51 and 41 % of phenol molecules from solutions with initial concentrations of 15 and 25 mg/L, respectively.
Figure 8 illustrates the mean values of the renewal rate of the individual membranes following filtration of the solutions tested. The results demonstrate that the highest values for this parameter were observed for the MNAC material, with values of 91 % following filtration of copper ion solutions, 96% following filtration of iron ions and 89 % following filtration of phenol. The remaining materials exhibited slightly lower values. The FRR values for M were 82, 85 and 91 % after filtration of copper, iron and phenol solutions, respectively. In contrast, the FRR values for MC were 74 % after copper filtration, 86 % after iron filtration and 92 % after phenol filtration. Finally, the FRR values for MAC were 79, 86 and 91 % after filtration of copper, iron and phenol solutions, respectively. From the graph obtained, it can be seen that in most cases (for M, MC and MAC materials), the FRR values increase depending on the type of filtered solution. The lowest values were obtained after filtration of copper solutions, followed by increasing values after filtration of ferrous ion solutions, while the highest values were observed after phenol removal processes from aqueous solutions.
Table 5 presents the mean values of the individual resistances determined for the materials obtained following the filtration of copper ion solutions. The data indicates that the highest resistances (both for the membranes, pores and the resulting filter cake) are those of the MAC material, with a total filtration resistance of 25.26x10
13 L/m
2×h. The MC membrane exhibited slightly lower values, with an R
t of 22.66x10
13 L/m
2×h. In contrast, the M and MNAC materials exhibited lower resistance values. The total filtration resistances for these membranes are 16.27x10
13 L/m
2×h for M and 14.32x10
13 L/m
2×h for MNAC, respectively.
Table 6 presents the average resistance values determined following the iron ion filtration processes. The highest values for individual resistances and total filtration resistance were observed for the MC membrane, with R
t equal to 18.30x10
13 L/m
2×h. Slightly lower values were found for MAC, with the total filtration resistance equalling 17.50x10
13 L/m
2×h. The resistance values for M and MNAC are slightly lower than those determined for MC and MAC membranes. The total filtration resistance for M is 15.83x10
13 L/m
2×h and 15.51x10
13 L/m
2×h for MNAC.
The average resistance values determined following filtration of the phenol solutions are presented in
Table 7. The values obtained indicate that all materials exhibit comparable resistance values. The Rt value for M is 17.19x10
13 L/m
2×h, while for MC it is equal to 17.22x10
13 L/m
2×h. In contrast, the total filtration resistance value for MAC is 17.69x10
13 L/m
2×h, while that for MNAC is 18.61x10
13 L/m
2×h.