Removal of phenolic compounds from olive mill wastewater (OMW) by tailoring the surface of activated carbon under acidic and basic conditions Open Access

BDC:

benzene-1,4-dicarboxylic acid

BET:

Brunauer–Emmett–Teller

COD:

chemical oxygen demand

PES:

polyethersulfone

SEM:

scanning electron microscopy

TDS:

total dissolved solids

TEM:

transmission electron microscopy

TPC:

total phenolic compounds

TSS:

total suspended solids

XRD:

X-ray diffraction

The Mediterranean basin and the Middle East are considered as the major producers of olive oil worldwide because they have about 98% of olive trees in the world (Tsagaraki et al. 2007). In Jordan, olive cultivation is vital for the national economy, where the total area planted with olive trees is estimated to be 1.3 million acres, which is 72% of the total cultivated area with fruit trees. Based on the annual report of the Jordanian Ministry of Agriculture, Jordan produced around 26.000 tons of olive oil in 2016. However, the growth in olive cultivation is associated with the production of huge quantities of dark aqueous waste called ‘olive mill wastewater’ (OMW). There are about 130 olive mills in Jordan, which produced over 200,000 m³ of OMW in 2007 (Bani Salamah et al. 2015).

The high polluting power of OMW is related to the presence of many organic compounds such as polyalcohol, phenolic compounds (PCs), lipids, sugars, tannins, lignin, and pectin. The existence of PCs is associated with their phytotoxicity, inhibition of seed germination, and antibacterial property that handles the non-biodegradable nature of OMW. Therefore, improper disposal of OMW threatens both soil and water quality, particularly in arid regions like Jordan, where water resources are already scarce. Developing effective, sustainable, and affordable treatment methods for OMW is critical to mitigate its environmental impact. Furthermore, it is not possible to use OMW for further applications such as irrigation or fertilizer applications despite the presence of a large number of polysaccharides, proteins, mineral salts, and humic acids in OMW, which are useful for agriculture (Niaounakis & Halvadakis 2004). In the absence of an effective treatment system, wastewater is disposed of in valleys and mixed with rainwater, creating an extra load for wastewater treatments.

Therefore, various methods have been developed to reduce the organic load (especially phenols) significantly from OMW, including electrochemical oxidation (Abdelwahab & Nassef 2013), physical processes (Jerman Klen & Mozetič Vodopivec 2011), chemical treatments (Amor et al. 2019), filtration (Garcia-Segura et al. 2016), solvent extraction (Pelendridou et al. 2014), and bioremediation (Ran et al. 2019). However, these approaches often have significant drawbacks, such as high energy requirements, the use of chemicals, and the generation of secondary pollutants during treatment (Paraskeva & Diamadopoulos 2006). Additionally, these methods may degrade PCs, preventing their recovery. Consequently, there is a pressing need for alternative methods that enable both the removal and recovery of PCs from OMW (Turco & Malitesta 2020).

Adsorption techniques present a promising alternative in this context. A variety of adsorbents, such as granular-activated carbon (GAC; Al Bawab et al. 2018a, b), agricultural wastes (Achak et al. 2009), zeolites (Aly et al. 2014), and amberlite (Frascari et al. 2016), have been explored for this purpose. However, many of these materials are in powdered form, necessitating the use of large centrifuges or filtration systems for their handling during treatment. Alternatively, powdered adsorbents can be packed into columns for continuous-flow separation, but this approach may prove costly due to the seasonal production of OMW and the need to transport it from numerous olive mills to central treatment facilities (Takahashi et al. 2019). These challenges highlight the importance of developing efficient, cost-effective, and practical adsorption systems for managing OMW.

Adsorption is one of the useful physicochemical processes, which involves the attachment of dissolved substances from wastewater to the surface of a solid substance. The colouring agents, inhibiting compounds, and non-biodegradable pollutants can be removed from OMW by the adsorption method. Activated carbon (AC) is known as the most common adsorbent with high adsorption ability. The adsorption ability of AC has been investigated intensively by several researchers in the literature. Galiatsatou et al. (2002) used AC for reducing the amount of phenol content in OMW and found that the adsorption of PCs is affected by the porosity of carbon and the adsorption capacity of synthesized AC material changes accordingly with the porosity of material. The same result was confirmed by Salame & Bando (2003) who reported that the uptake of phenol by AC in the acidic medium depends on the surface chemistry of carbon and its porosity. Atieh (2014) concluded that AC showed high capacity on 65% phenol removal from wastewater at pH neutral. He also investigated the effect of various parameters such as pH of the solution, agitation speed, contact time, and adsorbent dosage on the PCs' removal from wastewater of AC materials.

In Jordan, like in some Mediterranean regions, researchers tried to address this problem by researching to find a solution. Our research group has done many investigations on this topic during the past 15 years. For instance, Odeh et al. (2013) used a new type of surfactant, sodium polypropylene oxide sulphate (branched hydrocarbon chain) (propoxyle group)-(sulphate) to enhance OMW remediation using a modelled sample of OMW. Then, a review was published by Al Bawab et al. (2017) that summarized many conducted studies in the Mediterranean region, which investigated different OMW treatment methods, and many chemical, physical, and biological methods have been listed for OMW treatment. Meanwhile, Al Bawab et al. (2017) used a modified extended surfactant (sodium polypropylene oxide sulphate combined with cationic hydrotropes tetrabutylammonium bromide) and the recovery of phenols using different combinations reached up to 99.8%. Following that, cost-effective media of two types of granular carbon were prepared (Al Bawab et al. 2018a, b), in which the percentage removal of PCs from the OMW samples reached around 97% by using oxidized GAC impregnated with Span 20 soaked for 15 days. Following this, in the field of nanotechnology, an investigation was done for the possibility of using coupling magnetite and goethite nanoparticles with sorbent materials for OMW remediation (Odeh et al. 2022). In parallel with these studies, Abu-Dalo et al. (2021) also treated the real OMW samples by using AC, which is prepared from olive cake waste and functionalized with Cu/Cu₂O/CuO that offers a cost-effective treatment solution, and percent uptake of TPC was (85%), COD (42%), TSS (89%), and TDS (88%) by the adsorbent product. On another hand, Abu-Dalo et al. (2022a, b) investigated that volcanic tuff-magnetite nanoparticles coupled with coagulation–flocculation methods were successfully implemented as a treatment approach for OMW real samples, with the maximum COD removal of 76% at pH 8. In addition, the volcanic tuff activated by calcination and coupled with 0.5% by weight of magnetite nanoparticles at pH 10 resulted in structures that provided 73 and 70% removal for both TPC and COD, respectively.

In 2022, Al Bawab et al. (2022) investigated a new approach by using photocatalyst media to treat OMW. In this study, novel nano-photocatalysts were prepared and their photocatalytic degradation for total PCs was tested. XRD and TEM analyses confirmed the nanoscale structure of all prepared and doped materials (5.9–17.8 nm). The most effective photocatalyst for phenol degradation was Mn₂Zr₂O₇. Studying the pH effect on the degradation process confirms that the photocatalyst system performed most effectively in the basic medium. The removal of TPCs reached up to 70% by using the basic medium up to a pH of 10 and a long exposure time of up to 1 day. Preparing an adsorbent – Mn₂Zr₂O₇ composites – and increasing the LED lamp intensity can provide a promising OMW treatment approach.

In 2023, a composite of copper-based metal–organic frameworks and granular-activated carbon (GAC/Cu (BDC) MOF) by a simple hydrothermal method was used as another approach. At optimum conditions, the GAC/Cu (BDC) MOF composite can remove about 91% of PCs from OMW (Abu-Dalo et al. 2023). It is worth mentioning that Abu-Dalo et al. (2022a, b) developed and characterized a polymer membrane(s) impregnated with carbon nanotubes (CNTs). The prepared membranes (PES/CNTs) with 0.5 wt. % CNTs showed the highest total phenol removal of 74%.

Numerous studies have explored various treatment technologies for OMW, including biological, chemical, and physical approaches. Among these, adsorption using AC is widely recognized for its high efficiency in removing organic pollutants, including PCs, due to its large surface area and porous structure. However, most research has focused on unmodified AC to enhance its affinity for PCs under varying pH conditions or other adsorbents without tailoring surface properties to optimize adsorption under specific conditions. Additionally, many studies neglect the long-term stability of adsorbed PCs and desorption potential, limiting the practical applicability of the proposed solutions. The high cost and limited scalability of the existing treatment methods hinder their widespread applications in regions with resource constraints.

The present research makes several contributions considering the environmental issues that have brought about the removal of PCs from OMW using modified GAC. The main contributions include: (i) the introduction of a dual approach to modify GAC surfaces, enhancing their adsorption capabilities for PCs across a wide pH range, (ii) evaluation of the effects of surfactant types (such as Span 80, Span 20, Brij 93, Cetyltrimethylammonium bromide (CTAB), and Alfoterra L167-4S), concentrations, and pH levels on adsorption efficiency, and (iii) an assessment of the desorption behaviour of PCs from GAC over extended periods, providing insights into adsorption stability and reusability, and (iv) the introduction of a scalable, low-cost treatment approach suitable for resource-limited regions such as Jordan. Furthermore, it addresses the identified gaps by tailoring the surface properties of GAC through oxidative and reductive treatments, optimizing its performance for PC removal under acidic and basic conditions.

By addressing these challenges and presenting a robust adsorption method, this research contributes to the development of sustainable and practical solutions for managing OMW, offering significant environmental and economic benefits.

Raw OMW was obtained from the sink of a local mill in Irbid City during the harvest season and filtered to remove suspended solids. The wastewater was then stabilized with concentrated hydrochloric acid, adjusting the pH to approximately 3.0. This step was undertaken to prevent the degradation of PCs caused by enzymatic and non-enzymatic oxidative reactions, enhance the precipitation of suspended solids, and reduce the organic load (Kontos et al. 2015). The treated samples were stored at 4 °C until further use.

The analytical grade materials required for this research were all purchased from Sigma Aldrich without any purification: GAC (extra pure), nitric acid (HNO₃, 70%, ACS specification), and ammonia solution (NH₄OH, extra pure, AR Grade 30%). The following chemicals and materials were used without further purification: Span 80 (non-ionic surfactant); CTAB (cationic surfactant) was obtained from Techno Pharmchem, India; ALFOTERRA L167-4S (alcohol propoxy sulphate, sodium salt aqueous solution, an anionic surfactant) was obtained from Sasol, North America Inc.; 99% Phenol was obtained from Across Organics, New Jersey, USA; and 98% water used in this study was deionized and distilled.

The modified GAC samples have been characterized by a Quanta FEG 450 Scanning Electron Microscope (SEM), an EuroEA Elemental Analyzer, and a NOVA-2200 BET instrument. A UV–VIS–NIR Spectrophotometer (The Cary 5000) has been used for batch adsorption and desorption experiments to determine the concentration of total PCs.

Oxidation was conducted by the addition of 50 g of commercial GAC to 125 mL of 20% HNO₃ and mixing the solution on a magnetic stirrer at 90–95 °C for 8 h. Oxidized GAC was washed with deionized H₂O until the effluent reached a pH of 3.0 and was filtered. GAC has been dried in an oven at 90 °C for 2 days and kept in a desiccator. The method was adapted from earlier investigations (Abu-Dalo et al. 2013).

Each 4 g of GAC was soaked in 150 mL of 10 wt.% of ammonia solution for 48 h at room temperature. Earlier investigations were undertaken to establish the equilibrium time and the ratio of GAC to OMW. The chosen ratio has proven to be practical for mixing GAC with OMW, ensuring efficient dispersion and helping interaction among the surfactant, GAC, and OMW constituents. The reduced GAC was separated by filtration from the liquid phase and dried at 105 °C for 24 h and kept in a closed container. The method was adapted from the published method by Shaarani & Hameed (2011).

In the batch adsorption process, OMW samples were treated under various experimental conditions to determine the optimal parameters, including different GAC:OMW ratios, surfactant types, surfactant concentrations, and pH levels. The GAC-to-OMW ratio was maintained at a constant 1:20 by weight. A fixed adsorbent dose/ratio was used to standardize the experiments and provide a basis for scalability. To investigate the influence of surfactants on phenolic adsorption, experiments were performed using non-ionic (Span 80), cationic (CTAB), and anionic (L167-4S) surfactants at concentrations below, equal to, and above their critical micelle concentration (CMC). The concentration range was selected based on literature for typical surfactant use in wastewater treatment. The CTAB surfactants were dissolved in either penta-2-ol or deionized water. The percentage removal of PCs from OMW was assessed at pH levels of 4, 6, and 9, with the pH adjusted using 0.1 M NaOH before adsorption. The pH range (4–9) was chosen to encompass the typical pH variability in OMW and to evaluate the adsorbent's performance under acidic, neutral, and basic conditions. The extreme pH values test the resilience of the modified GAC and its surface functionality. The batch adsorption process was conducted for 48 h, identified as the optimal duration for adsorption (Odeh et al. 2013; Al Bawab et al. 2017), after which the samples were collected for analysis. The contact time of 48 h was chosen based on preliminary tests, indicating significant adsorption activity within this time frame, and to ensure sufficient time for equilibrium to be reached while assessing adsorption kinetics. The experiment was conducted at room temperature (∼25 °C), reflecting the practical treatment conditions suitable for resource-limited settings and eliminating the need for additional energy input. Additionally, it assesses adsorption under ambient conditions typical of olive mill environments.

The range of operational parameters was guided by previous studies on phenolic adsorption and OMW treatment, ensuring both comparability and relevance. Preliminary tests were conducted to identify effective ranges for pH, contact time, and surfactant concentration, optimizing adsorption performance while minimizing resource consumption. The experimental design prioritized conditions practical for large-scale implementation in olive-producing regions.

To evaluate the potential for the desorption of PCs from GAC, samples after the batch adsorption experiment were filtered to separate GAC from OMW. GAC was dried at room temperature for 1 day. This dried GAC was added to deionized water with continuous shaking by using a shaker. Samples were withdrawn after a week, 2 weeks, 3 weeks, and a month and evaluated for PCs.

OMW was treated with different surfactant concentrations (0.2 × 10⁻⁵, 1.7 × 10⁻⁵, and 13.6 × 10⁻⁵ M for Span 80; while 0.1 × 10⁻³, 0.86 × 10³, and 6.88 × 10⁻³ M for CTAB; finally, 0.002 × 10⁻³, 0.019 × 10⁻³, and 0.19 × 10⁻³ M for Alfoterra (L167-4S)). The surfactant concentrations were varied, above, below, and equal to the CMC. The CMC was determined using the method, as described by Dominguez et al. (1997).

The extraction of PCs from OMW by liquid–liquid extraction is crucial before UV–Vis analysis to eliminate the interfering matrix substances. OMW samples were acidified with concentrated HCl until a pH of 2. According to the literature studies, the best recovery of monomeric PCs from OMW occurs at pH 2 (Niaounakis & Halvadakis 2006). The solution was washed with hexane for the removal of lipids, in which each 5 mL of OMW was mixed with 8 mL of hexane and the mixture was shaken by vortex, followed by centrifugation for 5 min at 3,000 rpm. The phases were separated by a separatory funnel. Hexane washing was repeated two times for each sample. The PCs were extracted from washed OMW with ethyl acetate by mixing and shaking the OMW samples with 5 mL of ethyl acetate. The phases were separated by a separatory funnel, and the extraction was repeated three times for each sample. Ethyl acetate was evaporated by using a rotary evaporator. The dry residues were dissolved in 2 mL of methanol to be ready for the characterization and quantification of PCs with a method mentioned in the literature (De Marco et al. 2007).

The initial concentration of PCs was determined using spectrophotometric analysis with the Folin-Ciocalteu reagent. For the assessment of the total phenol content, 1 mL of the extracted sample or standard gallic acid solution (10, 20, 40, 60, 80, and 100 μg/mL) was transferred into test tubes, followed by the addition of 5 mL of deionized water and 0.5 mL of the Folin-Ciocalteu reagent. The mixture was thoroughly mixed, and after 5 min, 1.5 mL of 20% sodium carbonate solution was added. The final volume was adjusted to 10 mL with deionized water. The samples were then incubated at room temperature for 2 h. After incubation, absorbance was recorded at 750 nm using a UV–Vis spectrophotometer. This method was adapted from Kamtekar et al. (2014).

The adsorption process of PCs via GAC depends on the concentration of carbon atoms and the density of the surface functional groups. Commercial GAC has been modified by oxidation and reduction. Tables 1 and 2 summarize the characteristics of commercial, reduced, and oxidized GAC (by using an EuroEA Elemental Analyzer and a NOVA-2200 BET instrument). The percentages of carbon and oxygen in oxidized GAC were found as 55 and 45%, respectively, and they are compared with commercial GAC, which has 87 and 13% carbon and oxygen, respectively, while the base treatment of GAC with ammonia reduced the oxygen to 6% and increased the carbon and nitrogen to 92 and 1%, respectively (Table 1).

According to the BET results, the reduction of GAC improves the surface porosity of adsorbent materials, while the oxidation of GAC has a negative effect on porosity.

Oxidized GAC (c) also shows a porous structure but with more irregular and uneven surface features compared to reduced GAC. The surface appears rugged, with some larger openings, possibly resulting from oxidation-induced structural changes. This rough morphology might also facilitate adsorption but could vary in efficiency depending on the specific characteristics of the PCs.

Overall, reduced GAC (b) stands out as the most optimized surface for adsorption, with its organized porosity and increased availability of the adsorption sites. Therefore, reduced GAC has the highest capacity for the adsorption of PCs from OMW than commercial and oxidized GAC.

These observations align with the hypothesis that surface modification enhances the adsorptive properties of GAC, making it more effective in treating phenolic-rich OMW.

The oxidation process may also decrease both the surface area and total pore volume of GAC as confirmed by BET analysis; oxidized GAC is, therefore, less capable of removing PCs. On the other hand, the reduction process increases the specific surface area of AC, which leads to an increase in the adsorption of PCs by GAC.

At pH 9, additional OH⁻ ions were introduced to the solution because of the basic medium. According to the surface chemistry of reduced GAC, which was used in this part of the study, OH⁻ cannot compete with phenol on the presented adsorption sites. Pyrone, chromene, and quinone are the basic oxygen groups, which exist on the surface of GAC, in addition to the N-containing groups that facilitate the adsorption of PCs by the formation of H-bonding. As the pH increases, the precipitation of PCs also increases, which leads to a higher PC adsorption, considering that the carbon amount of reduced GAC was 92%. These results agree with the results obtained by Ma et al. (2013). Based on the experimental results, pH 9 was selected to complete the subsequent experiments.

The removal percentages of PCs via the modification of GAC with Span 80, CTAB, and Alfoterra L167-4S were investigated over a range of surfactant concentrations at CMC and concentrations both above and below the CMC. The removal percentage of PCs by GAC without the addition of the surfactants was 81% for reduced GAC at pH 9. The removal percentages of GAC by using Span 80 at 0.2 × 10⁻⁵, 1.7 × 10⁻⁵, and 13.6 × 10⁻⁵M were recorded as 81, 82, and 81%, respectively. The results were similar for CTAB, in which the removal percentages at 0.1 × 10⁻³, 0.86 × 10⁻³, and 6.88 × 10⁻³M were 82, 81, and 79%, respectively. For the concentrations of 0.002 × 10⁻³, 0.019 × 10⁻³, and 0.19 × 10⁻³M of Alfoterra L167-4S that were prepared, the removal percentages were 85, 88, and 85%, respectively. The removal percentage of PCs from the control sample, which contain OMW treated only with reduced GAC, was 88%.

Table 5 demonstrates that Span 80, CTAB, and Alfoterra L167-4S are ineffective in the reduction of the phenolic content in OMW, where the existence of those surfactants did not enhance GAC capacity to adsorb the PCs, keeping in mind that the surfactants were dissolved in the OMW solution.

The proposed surfactants were expected to enhance the removal of PCs from OMW because they have a significant influence on the interaction between the surface functional groups of GAC and OMW. However, the UV results showed that there is no difference in the removal of PCs by using Span 80, CTAB, and Alfoterra L167-4S even at three different concentrations. On the other hand, surfactants did not enhance the capacity of reduced GAC for PC removal. Considering the experimental findings that indicate surfactants do not influence phenolic removal. Thus, Span 80 was singled out to conduct subsequent experiments aimed at assessing the impact of pH on removal efficacy.

According to the results, the utilization of some selected surfactants at chosen conditions did not show enhancement in PC adsorption at room temperature and basic conditions. These results indicate that the presence of surfactants with GAC inhibits the working of surfactants as solubilization agents (Odeh et al. 2013; Al Bawab et al. 2017), which proposed that the presence and function of GAC are stronger than the surfactants. On the other hand, they cannot work together to increase the removal of surfactants, so, there is no need to add the surfactants in further investigations unless these surfactants can be added alone, keeping in mind that they were dissolved in the OMW solution.

Desorption studies were carried out to clarify the adsorption mechanism of PCs on GAC. After 7 and 32 days, the desorption percentages were recorded as 1 and 2%, respectively (Table 6). Low desorption indicates that PCs are strongly attached to the GAC surface through the functional groups and strongly adsorbed by GAC surface pores (Carmona et al. 2013).

Evaluating the desorption behaviour adds a novel dimension to the research, ensuring the practical sustainability of the modified adsorbent. By leveraging the modifying GAC features, this study bridges the gaps in existing research and demonstrates a novel pathway for improving OMW treatment efficiency and sustainability.

In summary, unlike prior studies that predominantly focused on unmodified or singly modified adsorbents, this work introduces a dual modification process for GAC, enhancing its adsorption efficiency under varied pH conditions. This approach, coupled with an evaluation of long-term adsorption stability and cost considerations, establishes a novel and scalable treatment strategy for OMW. This study provides actionable insights for scaling up the OMW treatment in regions like Jordan, addressing environmental challenges and resource limitations with a practical, cost-efficient solution. The lack of significant improvement in phenolic removal efficiency with surfactants may indicate competition for the adsorption sites or saturation effects under the tested conditions. Further studies could explore alternative surfactant types or concentrations to validate these findings.

Future research can explore the scalability of this treatment method in decentralized OMW processing units, particularly in olive-producing regions, and assess the feasibility of regenerating modified GAC for repeated use. The adsorption capacity was not calculated or reported in this study; this information will be included in future analyses, as it serves as a standard measure to compare the performance of adsorbents across studies.

This methodology provided a comprehensive understanding of how tailored GAC functions under varying environmental and operational conditions for OMW treatment. The surface of GAC was tailored through oxidation and reduction, and the PC adsorption capacity of materials was investigated by using different pH and surfactants. According to the results, reduced GAC showed a considerably high PC removal capacity from OMW at basic conditions. The results depict that the removal of PCs via GAC is affected by the physical nature of AC such as the surface functional groups and the pore structure. The surface groups, pore size, and pore distribution are strongly influenced by the post-treatment processes of GAC like oxidation and reduction. The reduction of GAC by ammonia increased the adsorption capacity of GAC for PCs due to the increase of π-electron density, surface area, and total pore volume in the graphene layers. Moreover, the removal of PCs from OMW increased as the pH increased from 4 to 9 due to the changes in the surface functional groups under the effect of pH. Surprisingly, the presence of surfactants dissolved in the OMW solution did not enhance the adsorption of PCs on the surface of GAC even at different concentrations. The highest PC removal capacity was recorded as 88% for the reduced type of GAC at pH 9, and this result is comparable to the results given in the literature of AC adsorption studies. This study demonstrates a practical and sustainable approach for the modification of GAC on PC removal while controlling some paraments.

The authors acknowledge the support provided by the Department of Chemistry at the Jordan University of Science and Technology, Irbid, Jordan, for carrying out the research work.

This work was funded by the Deanship of Research at the Jordan University of Science and Technology (Grant Number 421/2016). The authors would also like to acknowledge the Middle East Desalination Research Center (MEDRC) for financing the project.

M.A.-D. and A.A.B. contributed to conceptualization, supervision, and administration. M.A.-D., A.A.B, and H.K. contributed to the methodology. H. K. N.A.A.-R., and F.O. contributed to validation. H.K. and F.O. contributed to the formal analysis. H.K. contributed to the investigation. A.A.B., M.A.-D., and F.O. contributed to data curation. H.K, M.A.-D., and A.A.B. contributed to writing the original draft. M.A-D., A.A.B., and N.A.A.-R. contributed to resources. All authors contributed to writing, review, and editing.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.