The structure of wood cells offers a natural level of hierarchical organization, stimulating the applications for functional materials. Wood is porous material comprised of cellulose, hemicellulose, and lignin, with plenty of hydroxyl groups. Many treatments on lignocellulosic materials have focused on modifying these hydroxyl groups to bring target functionalities (Beims et al. 2022). Delignification of a wood is a widely utilized strategy to increase its porosity and make the hydroxyl groups more accessible within the wood structure (Fig. 3), and in turn to promote functionalization efficiency and, consequently, adsorption capacity if applying it as a bio-adsorbent. After delignification, wood porosity increased from 70.7 to 80.0%. However, lignin also carries some hydroxyl groups, so removal of lignin could impact the available sites for functionalization reactions. This study aimed to produce wood-based bio-adsorbents from natural wood by delignification and functionalization of the de-lignified wood and examine their adsorption capacity for heavy metal ions in water.
3.1 Wood functionalization
In this work, the natural wood (NW) and delignified wood (DW) were functionalized by esterification of their hydroxyl groups using maleic anhydride (MA) and citric acid (CA) as the functionalization agents in two methods: dry/wet processes. Esterification has been widely employed in the preparation of bio-adsorbents. Hydroxyl groups in wood cell walls react with the carboxylic acid or anhydride resulting in grafting with a higher -COOH content, promoting the chelation of heavy metals. For both functionalization methods employed in this study, wood samples after impregnation of solvent and functionalization agent were initially submitted to a vacuum pre-treatment step. The purpose of this step was to swell the wood cells to facilitate reactant infiltration. DMF is a widely used solvent for esterification reaction as it offers an additional benefit to wood functionalization due to its high capacity for swelling wood fibers (Cooper 1996). The functionalization agent and solvent could infiltrate into the wood cell under vacuum, which would facilitate the esterification reaction.
The esterification of wood hydroxyl groups generates water as a by-product. As it is well-known, water should be removed from the reaction system to shift the equilibrium to the product's side. As such, a Dean-Stark apparatus was employed in the wet process method to remove water from the reaction mixture as the reaction progressed. In the dry process, however, this is not an issue as the wood sample was not immersed in the solution, and the reaction temperature was kept above the water boiling point.
Figure 4a illustrates the FT-IR spectra of the NW and DW wood samples and their esterified products in both dry process (DPR) and wet process (WPR) with a functionalization agent of MA or CA. Comparing the FT-IR spectra of the NW and DW wood samples before and after esterification (Fig. 3a), one can observe that the carbonyl (C = O) peak at 1730 cm− 1 increased after the esterification functionalization in both wet and dry processes, which can evidence the successful esterification reaction.
Figure 4b shows the COOH graphing density on NW and DW under different esterification methods. One can observe in Fig. 7-3b that the wet process method outperformed the dry process, resulting in functionalized wood samples with a higher COOH graphing density, although the dry process has the intrinsic benefit of reducing solvent requirements. The wet process allows better contact between the solvent and the functionalization agent and hydroxyl groups in the wood cell, resulting in a greater carboxyl grafting extent for either NW or DW. From Fig. 4b, overall, esterification of NW or DW with MA resulted in higher COOH graphing density in the samples than esterification with CA. For instance, with the wet process, carboxyl grafting of NW with MA resulted in COOH graphing density, nearly twice that from carboxyl grafting of NW with CA. However, with the wet process, carboxyl grafting of DW with CA outperformed carboxyl grafting of DW with MA in terms of the COOH graphing density of the functionalized wood samples. A possible reason could be that the high porosity of DW facilitated CA infiltration in the cell wall, and hence promoted the esterification reaction.
3.2 Adsorption of Cu2+ ions
Cu2+ adsorption tests were conducted with selected functionalized wood samples as bio-adsorbents. Only samples of NW and DW functionalized with MA or CA by the wet process were tested owing to their higher COOH group grafting. In addition, the NW and DW wood samples without functionalization and the DP-DW-MA sample obtained from the dry process functionalization of DW with MA were also tested for comparison. Figure 5 illustrates photos of the NW and DW sample without functionalization before and after Cu2+ adsorption experiments. Both NW and DW samples showed a light bluish color after the adsorption tests, which indicates the adsorption of Cu2+ ions to these materials.
Figure 6 shows the Cu2+ adsorption capacity of various wood-based bio-adsorbents. Cu2+ adsorption occurs through the chelation of copper ions by carboxyl groups or ion exchange with the COOH group and OH groups in the bio-adsorbents. As wood naturally has OH group available from cellulose and lignin, some adsorption capacity was expected from the natural wood sample NW (Hou et al. 2005). Delignification of the wood removes most part of the lignin (DW lignin content is around 4 wt.%, compared with 28.4 wt.% lignin content in the NW), and, consequently, reduces OH group content in DW, as evidenced by the slightly reduced Cu2+ adsorption capacity to 0.03 mmol/g for DW when compared with 0.04 mmol/g for NW (Fig. 6).
Wood functionalization by esterification with MA or CA showed marked increases in the Cu2+ adsorption capacity of all functionalized wood samples (Fig. 6). As expected, the Cu2+ adsorption capacity was higher for the adsorbents with greater COOH group content. Compared with wood-based bio-adsorbents prepared with wet process, the DPR-DW-MA adsorbent prepared with the dry process has much lower adsorption capacity, likely because the dry process is less effective for grafting COOH group in the wood sample, resulting in a lower copper adsorption capacity than those of samples prepared with the wet process. Wood delignification appeared to show insignificant effects on the Cu2+ adsorption capacity for wood samples functionalized with MA. The Cu2+ adsorption capacities of both WPR-NW-MA and WPR-DW-MA are similar or have minimal difference within the standard deviation.
In contrast, wood delignification showed significant effects on the Cu2+ adsorption capacity for wood samples functionalized with CA. The DW sample functionalized with CA led to superior Cu2+ adsorption capacity (0.22 mmol/g), being the highest amount all wood-based bio-adsorbents tested. Such high Cu2+ adsorption capacity is a result of the highest COOH group content in WPR-DW-CA. As evidenced in Fig. 7, the Cu2+ adsorption capacity for all wood-based bio-adsorbents tested follows an almost linear correlation with COOH group grafting in these bio-adsorbents.
It was hypothesized that Cu2+ adsorption would occur faster on functionalized de-lignified wood DW than that on the functionalized natural wood NW owing to the improved porosity and water absorption properties of DW. To prove this hypothesis, fresh samples of WPR-DW-CA and WPR-NW-CA were tested for Cu2+ adsorption kinetics, by collecting water samples at various time intervals during the adsorption test for a total time period of 1440 min (24 h). The Cu2+ adsorption vs. adsorption time on these two samples is shown in Fig. 8. Apparently, the Cu2+ adsorption on the WPR-DW-CA is magnitude higher than that on the WP-NW-CA in the entire period. Moreover, the Cu2+ adsorption attained equilibrium in about 60 min on the WPR-DW-CA sample, only half of that (120 min) on the WPR-NW-CA. Thus, this result proved that Cu2+ adsorption occurred faster on CA functionalized de-lignified wood DW than that on the functionalized natural wood NW owing to the improved porosity and water absorption properties of DW.
The recyclability of the wood-based bio-adsorbent materials was investigated by regenerating the used WPR-NW-CA and WPR-DW-CA through immersing them in acetic acid solution (2 M) for 24h followed by washing with distilled water and drying overnight at 105 °C. New adsorption tests were conducted to test the Cu2+ adsorption capacity of the regenerated bio-adsorbent samples after each regeneration cycle (Fig. 9). Although WPR-NW-CA showed significant loss in the Cu2+ adsorption capacity after each cycle, the bio-adsorbent prepared from delignified wood (WPR-DW-CA) showed less than 10% reduction in adsorption capacity after three regeneration cycles, suggesting good reusability of the wood-based bio-adsorbent materials.
Additionally, activated carbon was tested for copper adsorption at the same experimental conditions as a baseline to compare wood-based materials’ efficiency to a well-established adsorbent material. Activated carbon adsorption capacity was 0.421 ± 0.05 mmol/g, almost double the WRP-DW-CA (0.211 ± 0.02 mmol/g). It is well known that its high sorption capacity makes activated carbon an excellent adsorbent. However, high-cost, low selectivity, and regeneration are common concerns about its use (Deliyanni et al. 2015). Wood-based adsorbents, on the other hand, are expected to be a low-cost source material, with a simple yet effective regeneration step by their immersion in acetic acid solution, as demonstrated in Fig. 9. Nonetheless, functionalization strategies need further improvement for wood-based materials to achieve capacities such as activated carbon.