Roselle Colastre Lasagasab,
Chenju Liang*a,
Xuyen Thi Hong Luong
a and
Florencio Ballesteros Jrc
aDepartment of Environmental Engineering, National Chung Hsing University, 145 Xingda Road, South Dist., Taichung City 402202, Taiwan. E-mail: cliang@nchu.edu.tw; xuyenluong@yahoo.com; lasagas.roselle@gmail.com; Fax: +886-4-22856610; Tel: +886-4-22856610
bEnvironmental Engineering Graduate Program, University of the Philippines Diliman, Philippines
cDepartment of Chemical Engineering, University of the Philippines Diliman, Philippines. E-mail: fcballesteros@up.edu.ph
First published on 4th July 2025
Plant polyphenols, natural antioxidants, form complexes with iron minerals that enhance contaminant degradation via reductive processes. This study investigated the degradation of carbon tetrachloride (CT) using polyphenol–iron complexes synthesized from tree leaf extracts. Polyphenols were extracted from waste tree leaves, including Ficus microcarpa, Terminalia neotaliala, Haematoxylon campechianum, Ficus septica, Mangifera indica, and Ficus religiosa, with gallic acid identified as the predominant constituent. Among them, Terminalia neotaliala exhibited superior antioxidant capacity, reducing power, metal-chelating ability, and total phenolic content, making it the optimal choice for CT degradation experiments. Using the Taguchi method, optimal conditions for CT degradation were determined as pH 10, a leaf extract dose of 10 g L−1, and an Fe2+ concentration of 15 mM, with pH as the most influential factor. Under these conditions, CT degradation reached 99% in aqueous solution and 89% in field groundwater within 24 h. Detected intermediates included trichloromethane, dichloromethane, and chloromethane, with chloride ions as the final mineralization product. This study underscores the potential of tree leaf polyphenols, in combination with Fe2+, as a sustainable approach for groundwater remediation.
Polyphenols are a class of phytochemicals present in all vegetative parts of plants and can also be persistent in plant debris after decomposition. Their key functional properties include scavenging free radicals, donating hydrogen atoms or electrons, and chelating metal ions.6 Polyphenols are categorized according to the number of phenol rings and structural elements that link them.7 There are four main categories of polyphenols: phenolic acids (hydrobenzoic acids and hydroxycinnamic acids), flavonoids, stilbenes, and lignans.8 Ho et al.9 demonstrated that tree leaves possess significant antioxidant activities and polyphenolic constituents. Specifically, as an example, the leaves of Acer oliverianum were found to contain a total phenolic content (TPC) of 311.7 ± 7.7 mg Gallic Acid Equivalent (GAE) per g sample, with phenolic acids and flavonoids being the predominant phytochemicals.
Polyphenols can chelate ferrous ion (Fe2+), which is another potent reducing agent that can be found in iron minerals. In the study of Wang & Liang,10 the combination of tea polyphenol and Fe2+ created an elevated reducing potential and achieved complete degradation of organochlorine pesticide. In addition, the Fe2+ and green tea complex was demonstrated to be capable of degrading various halogenated compounds, while Fe2+ or green tea alone were found to be less effective. The ortho-dihydroxyl groups such as catechol and gallol groups are responsible of the metal chelating ability of polyphenols. The formation of iron complexes promotes the release of H+ and the recycle of Fe2+/Fe3+, leading to an increase in the total dissolved iron in the solution. This accelerated reduction of ferric to ferrous ions might induce enhanced degradation of organic contaminants.11–13 The polyphenol–iron complex is pH dependent.12
Polyphenols are widely recognized for their antioxidant properties; however, under specific conditions, such as variations in pH, the presence of oxygen, and metal ions, they can also exhibit pro-oxidant behavior, leading to the generation of reactive oxygen species (ROS).14 The coordination chemistry between polyphenols and iron varies with pH: under acidic conditions, a monocomplex typically forms, consisting of a single polyphenol ligand bound to Fe2+ or Fe3+. At neutral and alkaline pH, biscomplexes (two ligands) and triscomplexes (three ligands), respectively, become more favorable.15,16 In the presence of oxygen, polyphenol groups such as catechol or gallate can rapidly oxidize Fe2+ to Fe3+, resulting in the formation of stable Fe3+–polyphenol complexes. During this redox process, the polyphenol ligand donates an electron to reduce Fe3+ back to Fe2+ while being oxidized to a semiquinone radical (SQ˙). Under acidic conditions, this semiquinone can further oxidize to quinone, concurrently reducing Fe3+ to Fe2+ or molecular oxygen to the superoxide radical (O2˙−). The superoxide can spontaneously dismutate to hydrogen peroxide (H2O2), which in turn reacts with Fe2+ in a Fenton reaction to produce highly reactive hydroxyl radicals (˙OH). These radicals are capable of degrading a wide range of organic contaminants. However, the overall reaction efficiency can be limited by the slow reduction of Fe3+ back to Fe2+, which constrains the generation of ˙OH and, consequently, the degradation of organic compounds.17 At neutral pH, the formation of biscomplexes may enable an alternative pathway, in which Fe3+ reacts with H2O2 to form high-valent iron species such as Fe4+, a potent oxidant. In contrast, under alkaline conditions, triscomplexes are more stable and exhibit antioxidant behavior. Their fully occupied coordination sites inhibit interactions with H2O2, preventing ROS generation. These triscomplexes can also chelate metal ions, reducing their availability for redox cycling and free radical formation. Furthermore, triscomplexes may facilitate the reductive degradation of CT via a controlled electron transfer mechanism, sustaining the Fe3+/Fe2+ redox cycle while minimizing undesired ROS production.18
This study aimed to investigate the degradation of CT utilizing waste tree leaf extract (TLE) as a source of polyphenols to form complex with Fe2+ additive in aqueous solution. The specific objectives were to: (1) determine the composition and characteristics of TLE; (2) optimize leaf dose, pH, and iron dose using Taguchi orthogonal array for the reductive degradation of CT in aqueous solution; (3) investigate the degradation by-products and reaction mechanism; (4) assess the removal of CT in field groundwater solution using the optimal conditions.
The selection of tree species was guided by their availability on the campus of National Chung Hsing University (NCHU), their capacity to produce substantial leaf biomass, and their phytochemical profiles, particularly the abundance of polyphenols that are water-soluble. Water was selected as the extraction solvent due to its non-toxic nature and minimal risk of introducing additional contamination. The species evaluated for their phytochemical content and antioxidant potential included Ficus microcarpa (FM),19 Terminalia neotaliala (TN),20 Haematoxylon campechianum (HC),21 Mangifera indica (MI),22 Ficus septica (FS),23 and Ficus religiosa (FR).24 These trees are reported to be rich in polyphenols, particularly water-soluble phenolic acids, and possess notable antioxidant properties. MI, HC, and TN are deciduous species that shed all their leaves annually, producing significant biomass. FR is semi-deciduous, exhibiting partial and seasonal leaf drop. In contrast, FM and FS are non-deciduous, with FM having a broad, dense canopy and FS producing large leaves; both shed foliage gradually throughout the year. Groundwater was collected in a campus groundwater monitoring well (diameter of 4 in, a water depth of 15 m, and well screen positioned between 3.0 and 15.0 m) located at the Department of Environmental Engineering, NCHU. The deionized (DI) water used was generated using an Elga Micra Type II purification system.
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To determine which factor has significant influence on the response variable, the analysis of variance was used. The total variability of the S/N ratios was separated into contributions according to each parameter and error. The total sum of square (SST), degrees of freedom (DOF), sum of squares (SSA), variance (Ve), F-value, and contribution (%), were calculated using the eqn (2)–(7):27,28
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DOF = Ni − 1 | (3) |
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After the selection of optimal parameters, the final step is the confirmation test, which involves prediction and verification of results using the optimal parameters. The S/N ratio (η) at the optimal process parameters can be determined using eqn (8):27,28
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Fig. 1(c) shows the metal chelating ability expressed by the chelating effect. Among the six extracts, Ficus religiosa extract exhibits the highest chelating ability at approximately 74%, then Ficus septica with 71.5%. In the assay, iron tetrachloride was added to the plant extract sample followed by ferrozine. The phenolic compounds bind with certain amount of Fe2+, but the remaining Fe2+ reacts with ferrozine. In the presence of phenolic compounds, there is inhibition of ferrozine–Fe2+ complex formation due to binding of Fe2+ to phenolic structures, which leads to the decrease in absorbance value.30 The lower the absorbance value, the higher is the chelating ability. The last property is the TPC, which is expressed in mg GAE per g dry leaves. In this assay, Folin–Ciocalteu reagent was used to oxidize phenolic groups such as gallic acid to reduce heteropoly acid (phosphomolybdate–phosphotungstate) contained in Folin–Ciocalteu reagent into a molybdenum–tungsten complex, which is blue in color and can be measured at 765 nm. Sodium carbonate was added to the mixture because the combined phenolic can only be supported by Folin–Ciocalteu reagents in an alkaline environment so that protons dissociate in phenolic compounds into phenolic ions. As the concentration of phenolic ions increases, more heteropoly acid is reduced, resulting in a more intense blue color.31 Based on the result shown in Fig. 1(d), Terminalia neotaliala extract exhibits the highest TPC value of approximately 85 mg GAE per g dry leaves. The TPC values obtained from the six tree leaf extracts in this study ranged from 30 to 85 mg GAE per g (Fig. 1(d)), which fall within the reported range of 19–201 mg GAE per g for 18 indigenous tree species in Taiwan.32
The quantitative analysis of the four properties was used to determine the most suitable TLE for the reductive degradation of carbon tetrachloride. Based on the results, Terminalia neotaliala extract emerged as the optimal choice for CT degradation, as it consistently showed higher values in two of the screening criteria (i.e., antioxidant capacity and reducing power) and performed above average across all four properties.
The solubility and extractability of polyphenols from tree leaves are largely influenced by their structural characteristics. Phenolic acids, which contain hydroxyl and carboxyl functional groups, are generally water-soluble. In contrast, flavonoids, lignans, and stilbenes exhibit low water solubility due to their multiple aromatic rings, non-polar bonds, and relatively high molecular weights. To extract a broader range of polyphenols, polar organic solvents such as acetone, ethanol, and methanol are commonly employed. Among these, ethanol is widely regarded as a safe and effective solvent and is often used in combination with water in various proportions.33 Traditional extraction techniques like solvent-liquid extraction and Soxhlet extraction are widely used but come with limitations, including high solvent consumption, energy demand, and long processing times. More sustainable and efficient alternatives, such as microwave-assisted extraction and supercritical CO2 extraction, offer advantages including reduced solvent use, better energy efficiency, improved temperature control (which helps preserve polyphenol stability), and higher purity yields.34 In the context of this study, the extracted polyphenols are intended for subsurface remediation and would potentially be injected into groundwater. Therefore, water-soluble polyphenols are preferred to ensure compatibility and effective distribution in the aqueous subsurface environment. Nevertheless, exploring different solvent extraction methods remains valuable for isolating a wider variety of polyphenol compounds for potential applications beyond water-based systems.
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Fig. 2 HPLC/PDA chromatogram at 254 nm of polyphenolic constituents and tree leaves extracts. The inserted table summarizes the identified polyphenols present in the corresponding tree leaves. |
The 4-hydrobenzoic acid, which is observed in TN and FM, has hydroxy substituent positioned at C-4. Carboxyl group does not benefit radical scavenging because of its electron-withdrawing nature. But upon the carboxyl deprotonated, it becomes electron donating group.37 Vanillic acid has a methoxy group at C-3. Based on the result, vanillic acid was detected in TN and HC. According to Spiegel et al.,36 methylated compounds tend to exhibit lower antioxidant capacity than their non-methylated counterparts due to low active electron- and hydrogen-donating groups. Lastly, caffeic acid appeared to be present in the TN and FR. Caffeic acid is a derivative of cinnamic acid, featuring hydroxy groups at the 3 and 4 positions of the phenyl ring. Caffeic acid is reported to exhibit superior antioxidant capacity compared to other hydroxycinnamic acids, a property attributed to its enhanced radical stability achieved through extended π-electron delocalization and hydrogen bonding with the vicinal hydroxyl group formed following hydrogen atom transfer.38
Fig. 3 shows the FTIR spectra of the six tree leaves. The broad peak at 3300 cm−1 is related to the –OH group, attributed to the presence of hydroxyl groups in the molecules of tree leaf extract polyphenol.10,39 The peaks at 2925 and 2850 cm−1 correspond to vibrations of methyl (CH2),39,40 likely from substances such as vanillic acid. A peak at 1725 cm−1 is due to the bending vibration of carbonyl (CO) in the carboxyl group (–COOH), associated with compounds like 4-hydroxybenzoic acid. Additionally, a peak at 1605 cm−1, is related to the stretching of the carbonyl group such as in flavanols.39,41 Because flavanols are only slightly soluble in water, they may go undetected in plant extracts prepared exclusively with water. The peak at 1035 cm−1 is associated with C–O bonds in alcohol groups.39 Leaf extracts that exhibited higher Fe2+ chelation capacity, such as Terminalia neotaliala, showed stronger absorption bands corresponding to hydroxyl (–OH) and carbonyl (C
O) functional groups. These groups are known to play key roles in metal chelation due to their electron-donating ability. Specifically, phenolic –OH groups can coordinate with Fe2+ by donating lone pair electrons from the oxygen atom to the vacant d-orbitals of iron, forming stable chelate rings. Likewise, carbonyl groups, particularly in conjugated systems such as flavonoids, can act as bidentate ligands by coordinating through both carbonyl oxygen and adjacent hydroxyl groups, enhancing complex stability. This chelation mechanism has been documented in studies on polyphenol–metal interactions.42,43 The presence of these functional groups in higher density and accessibility likely contributes to the superior chelation activity observed in Terminalia neotaliala. This interpretation helps explain the differences in chelation efficiency across samples and is consistent with previous findings that polyphenolic structures with ortho-dihydroxy or carbonyl–hydroxyl motifs exhibit stronger metal binding affinity.
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Fig. 5 Concentration variations of CT and changes of pH and ORP after degradations at designated reaction conditions [CT]0 = 50 mg L−1. |
When the pH exceeds the pKa of polyphenols, such as gallic acid with pKa1 value of 4.24,48 the OH functional group dissociates, producing phenoxy radicals that release electrons to stabilize the structure. In addition, the oxidation of Fe2+ to Fe3+ releases electrons that cleave the C–Cl bond in CT, resulting in its degradation. At around neutral pH, the polyphenols present in TLE form complexes with Fe3+ particularly a biscomplex,49 in which two ligands (catecholate or gallate groups) of the polyphenols bind with iron. When a catecholate or gallate ligand binds to Fe3+, the polyphenol can reduce the iron to Fe2+, enabling its recycling within the system.13
Experiments (d), (e), and (f) were prepared at an initial buffered pH 9. At experiments (d) and (e), increasing leaf dose and iron dose were observed to increase removal of CT between 91% and 94%. When the iron dose was reduced while the leaf dose was increased as observed in experiment (f), the removal efficiency decreased to about 78%, suggesting that insufficient iron hindered the formation of stable polyphenol complexes. Negative ORP between −22 mV to −213 mV were also observed. On the other hand, experiments (g), (h), and (i) demonstrated relatively low final CT concentrations, achieving removal efficiencies between 92% and 96%. The low ORP values, ranging from −129 mV to −356 mV, indicated a highly reducing environment.
At alkaline condition, polyphenol and iron forms triscomplex, which has three ligand molecules in its coordination system. Triscomplex is stable and possess antioxidant capacity.12,49 Based on the results, CT removal was more effective under buffered alkaline conditions compared to experiments with an unbuffered neutral condition. Among all 9 experiments, experiments (g) and (i) achieved the highest removal efficiency at 95% and 96%, respectively, after 24 h. The Taguchi experimental design outcomes were analyzed to assess the contribution of each factor. The average CT removal was transformed into a signal-to-noise ratio using eqn (1), which served as the assessment index for evaluating the results.
As shown in Fig. 6 (data detailed in Table S4 (ESI)†), the optimal levels for the experimental factors in the reductive degradation of CT were identified as follows: pH at level 3 (S/N = 39.50), leaf dose at level 1 (S/N = 38.03), and iron dose at level 3 (S/N = 38.59). The trend suggests that increasing the pH and iron dose, while reducing the leaf dose, enhance CT degradation. Analysis of variance was employed to calculate the degrees of freedom, sum of squares, and variance for each factor, enabling the assessment of the relative importance of various parameters, including the F-value (degree of influence) and contribution percentage (statistical results tabulated in Fig. 6).
The calculated F-value can be compared to the critical value extracted from F-distribution table of 4.46 at 95% confidence interval (α = 0.05). The pH which F-value is 6.11 is found to be significant. The result indicates statistically significant difference in CT removal among pH values tested. Also based on the % contribution, pH has the highest influence on the CT removal accounting for 79%, followed by iron dose and then leaf dose. Eqn (8) is used to estimate the expected S/N ratio for a combination of factor levels identified as optimal based on the experimental data. By calculating the deviation of each optimal level's mean from the overall mean and summing them, this equation predicts the system's performance under optimal conditions. Accordingly, eqn (8) was applied to estimate the expected outcome of CT degradation under the identified optimal conditions. The corresponding predicted removal rate based on the calculated S/N ratio, is approximately 100%. This signifies that the Taguchi experimental design is effective in optimizing the parameters to achieve the best result.
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Fig. 7 Concentration variations of CT and its degradation byproducts at optimal experimental conditions. |
Fig. 8 shows the concentration profiles of Fe2+, total iron, and polyphenols during the reaction. Initially, Fe2+ was about 8000 mg L−1, but it dropped rapidly to 5.42 mg L−1 within 30 min and was nearly depleted by 24 h. This rapid depletion hindered the formation of polyphenol–iron complexes, thereby slowing CT degradation, as evidenced by the spikes in CT and its by-products around 24 h (Fig. 7). Total iron was measured to track Fe3+ levels, given that Fe3+ concentration can be inferred from the difference between total iron and Fe2+. Notably, total iron (predominantly Fe3+) continued to decrease after 24 h, along with a decline in polyphenol content. At this stage, polyphenols and their Fe3+ complexes became the key drivers of CT degradation. The polyphenol–Fe3+ complex can regenerate Fe2+, thereby supplying the electrons necessary for continued CT degradation.
When the pH exceeds the pKa of the polyphenols, the OH functional group dissociates, releasing H+ ions and subsequently emitting electrons (to stabilize the compound), which is transferred to CT. In addition, polyphenol forms coordination complex with Fe2+ but due to strong stabilization of Fe3+ by polyphenol ligands over Fe2+, the Fe2+–polyphenol complex undergoes rapid oxidation to produce Fe3+–polyphenol complex (triscomplex), releasing electrons in the process. The polyphenol in Fe3+–polyphenol complexes reduced Fe3+, therefore regenerating Fe2+. This Fe2+/Fe3+ cycle, which involves electron release, results in the degradation of CT.
The reductive degradation of carbon tetrachloride (CT) by electrons released from ascorbic acid5 or guava leaf extract,50 either alone or in combination with dissolved Fe2+ or iron minerals to enhance electron transfer, proceeds via hydrogenolysis and dichloroelimination pathways. In hydrogenolysis, the cleavage of a C–Cl bond in CT occurs through a single-electron transfer, resulting in the formation of trichloromethane (CHCl3). Alternatively, dichloroelimination may occur through a two-electron transfer, producing radical intermediates rather than CHCl3. Both pathways ultimately lead to the reductive dechlorination of CT, releasing four chloride ions. The rapid and substantial formation of CHCl3, followed by the appearance of CH2Cl2 and CH3Cl in the reaction system, suggests that hydrogenolysis may be the dominant degradation pathway. Accordingly, the degradation of CT using tea leaf extract in the presence of iron likely involves both hydrogenolysis and dichloroelimination mechanisms. The proposed degradation process involving polyphenol–iron complexes is illustrated in Fig. 9. In the hydrogenolysis pathway, CT accepts electrons, leading to C–Cl bond cleavage and substitution of chlorine with hydrogen to form CHCl3. The subsequent detection of intermediates such as CHCl2, CH2Cl2, and CH3Cl further supports the occurrence of sequential electron transfer steps. It is speculated that the final degradation product is methane (CH4), accompanied by the release of chloride ions. However, because qualitative analysis of intermediate compounds was not performed in this study, distinguishing between the hydrogenolysis and dichloroelimination pathways remains uncertain and warrants further investigation. During the reductive degradation of CT, a cascade of sequential hydrogenolysis reactions may occur, often resulting in the accumulation of partially dechlorinated intermediates such as chloroform, methylene chloride, and methyl chloride. These intermediates pose significant environmental and health risks. For example, methylene chloride is classified as a potential human carcinogen and has relatively high mobility in subsurface environments, increasing the likelihood of groundwater contamination.51 In some cases, concentrations of these intermediates may exceed regulatory limits, undermining the effectiveness of in situ remediation strategies. To mitigate the formation and persistence of these toxic by-products, one essential approach is to ensure complete reductive dechlorination. This involves optimizing polyphenol–iron complex electron donor availability, maintaining an appropriate redox environment, and ensuring sustained reaction conditions that promote full transformation of CT to non-toxic end-products such as methane and chloride ions. Additionally, a sequential treatment strategy could be employed. For example, an initial abiotic phase driven by polyphenol–iron complexes can be followed by a biotic phase utilizing specialized anaerobic microbial consortia capable of cometabolizing or reductively dechlorinating chloroform and methylene chloride.52,53 Overall, designing integrated treatment systems that combine abiotic and biotic mechanisms offers a more robust and sustainable strategy for minimizing the accumulation of toxic intermediates and achieving regulatory compliance in field-scale applications.
Property | Value |
---|---|
pH | 6.79 |
ORP (mV) | 101.63 |
DO (mg L−1) | 4.01 |
Total dissolved iron (mg L−1) | 0.28 |
Fe2+ (mg L−1) | 0.19 |
Fe3+ (mg L−1) | 0.09 |
Na+ (mg L−1) | 14.77 |
K+ (mg L−1) | 2.31 |
Ca+ (mg L−1) | 70.38 |
Mg+ (mg L−1) | 14.00 |
Cl− (mg L−1) | 14.33 |
F− (mg L−1) | 0.39 |
SO42− (mg L−1) | 103.31 |
Br− (mg L−1) | Not detected |
PO43− (mg L−1) | Not detected |
NO3− (mg L−1) | 0.14 |
NO2− (mg L−1) | Not detected |
Alkalinity (mg CaCO3 L−1) | 135 |
Hardness (mg CaCO3 L−1) | 234 |
Fig. 10 illustrates the CT removal, chloride ion generation, and the profiles of ORP and pH over various reaction times. The concentration of CT exhibited a rapid decrease within the first 30 min, consistent with previous observations. The removal efficiency of CT in groundwater after 24 h measured 89%, which is lower than the RO water solution experiment achieving 95% of CT removal. This effect may be attributed to the ionic composition of the groundwater (Table 1), which can influence the formation and reactivity of polyphenol–iron complexes critical for the reductive degradation of CT. The groundwater contained both Fe2+ and Fe3+, as well as Na+, K+, Ca2+, and Mg2+. While iron plays a beneficial role by forming reactive complexes with polyphenols, the non-redox-active cations (Na+, K+, Ca2+, Mg2+) can alter the solution's ionic strength, potentially affecting the interaction between reductants and contaminants. The identified anions, Cl−, F−, SO42−, and NO3−, may also impact the system. Both sulfate and nitrate are electron acceptors and may compete with CT for available electrons. Additionally, the groundwater's alkalinity implies the presence of carbonate and bicarbonate ions, which can precipitate iron and thereby potentially reduce the effectiveness of reductive processes. Chloride level rose to 57.86 mg L−1, in which background groundwater contains 14.33 mg L−1. The findings indicate that tree leaf polyphenols, when combined with iron, effectively degrade CT, highlighting their potential for practical applications. A preliminary test using actual groundwater to assess the reactivity of polyphenol–Fe complexes in degrading CT demonstrated successful reductive degradation, indicating that common groundwater constituents such as bicarbonate and sulfate did not inhibit complex formation or reactivity under these conditions. However, certain anions capable of competing for iron binding, such as phosphate, carbonate, and sulfate, may influence complexation dynamics to varying extents. A detailed investigation of these effects is recommended for future studies to optimize field applications.
While a pH of 10 condition enhances the reactivity of the polyphenol–iron complex, it presents several challenges for field remediation application. Maintaining elevated pH levels in situ requires frequent injection of soluble buffers, such as bicarbonates or carbonates, which may be rapidly consumed or transported beyond the target treatment zone.54 High pH conditions can also promote the precipitation of metals and minerals, potentially clogging aquifer pores, reducing permeability, and impairing reagent transport, which may result in uneven treatment distribution.55 Furthermore, elevated pH can inhibit native microbial communities, many of which thrive in the pH range of 6.5–8.5 and play a key role in natural attenuation of groundwater contaminants. Therefore, field applications must carefully consider the choice and dosage of buffering agents for pH control, the mobility and delivery method of reagents in the subsurface, and the compatibility of treatment conditions with indigenous microbial populations. In addition to commercial buffers, naturally occurring silicate minerals and carbonate rocks may offer long-term pH buffering. To improve reagent delivery, approaches such as recirculation wells or multi-point injection designs may be employed. Although polyphenol–iron complexes have shown no ecotoxicological effects,46 site-specific microbial assessments are essential to ensure treatment compatibility with the subsurface ecosystem.
The stability of polyphenols in extracts is a critical factor influencing the reproducibility and scalability of polyphenol-based remediation technologies. Their stability depends on both molecular structure and environmental conditions such as pH and temperature. Polyphenols with a greater number of hydroxyl groups are generally less stable due to their increased susceptibility to oxidation. For instance, compounds with a pyrogallol structure (e.g., gallic acid, tannic acid, epigallocatechin gallate) are more readily oxidized than those with a catechol structure (e.g., catechin, caffeic acid, epicatechin).56 Regarding pH, polyphenols tend to be more stable in acidic environments, whereas in alkaline conditions, they are prone to degradation, dimerization, and oxidation.57 Temperature also plays a crucial role, with lower storage temperatures helping to preserve polyphenol integrity. Salazar-Orbea et al.58 reported that storing polyphenols at temperatures between −20 °C and 4 °C can significantly slow their degradation, allowing preservation for up to 12 months. Given that different polyphenols respond variably to environmental factors, a comprehensive understanding of these influences is essential for assessing and optimizing stability.
The effect of leaf dose, iron dose, and pH in the degradation of CT in an aqueous solution were examined and optimized using Taguchi method employing an L9(33) orthogonal array. The optimal conditions identified were pH 10, a leaf dose of 10 g L−1, and an iron dose of 15 mM. Analysis of variance determined pH was identified to be the most significant factor influencing the CT removal. CT undergoes sequential reductive degradation by accepting electrons and gradually releasing chlorine ions. After 24 h, the removal efficiency of CT reached 99% in an aqueous solution and 89% in field groundwater. Intermediate byproducts detected via GC/MS included CHCl3, CH2Cl2, and CH3Cl. This study concluded that waste tree leaves from Terminalia neotaliala serve as a potential source of polyphenols capable of reacting with iron to form coordination complexes, facilitating the reductive degradation of CT.
Although pH 10 was found to be optimal, investigating CT removal under unbuffered neutral conditions using tree leaf polyphenol–iron is recommended, as it may offer a more feasible and natural approach. Additionally, factors such as extended reaction time and temperature can be explored to enhance the reaction efficiency under neutral pH condition. Additional research is required to evaluate how common groundwater ions affect CT removal. These ions may reduce electron availability and participate in coordination complex formation, potentially explaining the lower CT removal efficiency in groundwater compared to pure water. Moreover, alternative methods for extracting polyphenolic compounds from tree leaves should be developed, followed by qualitative and quantitative analyses of their composition. Certain polyphenols, such as flavonoids, have limited water solubility, meaning they may not have been fully captured using the extraction method employed in this study. Lastly, applying tree leaf extracts for the degradation of other pollutants is recommended, as this approach could contribute to agricultural waste recycling and offer innovative solutions for pollution remediation.
Future research should explore the broader applicability of the polyphenol–iron complex system to other classes of groundwater contaminants, particularly those containing carbon or nitrogen in positive oxidation states that are amenable to reductive transformation, such as chlorinated ethanes, ethenes, and nitroaromatic compounds. Investigations under a range of geochemical conditions, including varying pH, redox potential, and natural organic matter content, will be critical for assessing the system's robustness and adaptability. Given that this reductive system is dependent on the formation of polyphenol–iron complexes, pH buffering plays a key role in facilitating electron release. Pretreatment strategies to adjust subsurface pH, such as the application of alkaline soil conditioners like limestone, may be beneficial in enhancing alkaline buffering capacity. In cases where pH buffering is insufficient, the intrinsic reducing capability of polyphenols or their complexes may serve as electron donors, creating a favorable redox environment to support subsequent biotic or abiotic remediation processes. Finally, field-scale validation and long-term performance evaluations are essential to advance the practical implementation of this sustainable remediation approach.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5ra01391g |
This journal is © The Royal Society of Chemistry 2025 |