Management of trihalomethanes in water by ZnO@kaolinite composite: integrated experimental and modeling studies

The issues of water scarcity and water quality are escalating worldwide due to various factors like population expansion, climate variations, and unsustainable practices of water consumption (Alomar et al. 2023). In today's era, where pollution has become unavoidable, the importance of access to high-quality drinking water cannot be overlooked (Srivastav & Kaur 2020). Byproducts from disinfection (DBPs) are commonly produced when complex organics, halides (As, Br, and Cl), and disinfectants react with one another in water.

Trihalomethanes (THMs) are one of the byproducts developed when chlorine, the predominant common water disinfectant is used in excess amounts, water that has been chlorinated reacts with the organic content to generate THMs, which are carcinogenic and mutagenic compounds. During the 1970s, the presence of THMs was initially discovered in drinking water. In 12 water purification plants across the United States, THMs constitute more than 13.7% of the halogenated DBPs, therefore being the most common DBPs found in chlorinated drinking water. The maximum permissible levels of total THMs in addition to five (haloacetic acids) HAAs in drinking water were set by the United States Environmental Protection Agency (US EPA) in 2006 at 80 and 60 μg L⁻¹, respectively, as stated in their annual average guidelines (Zhang et al. 2020). THMs level varied from 3 μg L⁻¹ in China to 439.2 μg L⁻¹ in Bangladesh, and from 0.5 μg L⁻¹ in Germany to 215 μg L⁻¹ in Russia, respectively. In 11 developed and 9 developing countries, data on THM from EU26, which encompasses approximately 75% of the population, is also available from 2005 to 2018 (Kothe et al. 2023). Over a period of 3 years, samples of drinking water from 23 Egyptian governorates totaling 1,667 were gathered. Total trihalomethane concentrations varied from 29.07 to 86.01 μg L⁻¹, consistently falling below the highest limit of contamination recommended by Egyptian standards (100 μg L⁻¹) (Mishaqa et al. 2022). With a standard deviation of 11.2, the population weighted average THM level was estimated to be 11.7 μg L⁻¹ (Evlampidou et al. 2020). The raw water might also include specific organic compounds, such as fulvic acid, hymatomelanic acid, algae material, and humic acid. When these compounds react with chlorine, they result in the formation of THMs. When the presence of halides and halogenated compounds exceeds the tolerable limits, these water contaminants can cause detrimental impacts not only on industrial establishments but also on aquatic organisms and human health (Oke et al. 2023). Exposure to THMs can induce different detrimental health effects, comprising induction of cancer birth defects, and other conditions that are mutagenic, teratogenic, carcinogenic, genotoxic, and cytotoxic diseases (Visvanathan et al. 1998). All these led to the intense focus of chemists, epidemiologists, environmentalists, biologists, and toxicologists worldwide.

THMs are composed of chloroform or trichloromethane (TCM), and brominated THMs like chlorodibromomethane (CDBM), bromodichloromethane (BDCM), or bromoform are the most prevalent types of single-carbon substituted halogens (CHX₃), where X can represent either bromine, chlorine, iodine, or fluorine, or a combination thereof (Vela et al. 2022). Due to THM's adverse effects on human health, several treatment technologies/methodologies, such as coagulation–flocculation, enhanced coagulation, adsorption, ozonation, ion exchange membranes, ultra/nanofiltration, and advanced oxidation methods that include using disinfectants other than chlorine or maximizing the amount of chlorine, along with treatment procedures that include removing precursor elements before adding chlorine using ion exchange resin or membrane filtering, processes have been developed. This study focuses on the adsorption technique owing to its stability and cost-effectiveness. Over the last decades, large numbers of inexpensive adsorbents have been suggested as possible alternatives for the efficient yet comparatively costly options. Various adsorbents have been investigated for eradication of natural organic matter including activated carbon derived from coal and wood (Karanfil et al. 1999), palm shell activated carbon (Pamidimukkala & Soni 2018), bentonite, rice husk (Menya et al. 2018), and commercial activated carbon (Iriarte-velasco & Jon 2008). However, the literature indicated that the reported adsorption capacities and removal efficiencies were minimal. The proper design of the adsorption processes determines the efficiency degree of the treatment techniques. The adsorption mechanism usually includes the chemical reaction among the metal ions or a cation and the functional groups on the adsorbent (Unuabonah & Adebowale 2007). Clay minerals are characterized by their heterogeneous and lamellar structure with a large number of advantages such as corrosive resistance, low cost, environmental friendly, present abundance, high thermal stability, and resistance to acids and bases (Cao et al. 2021). Kaolinite is considered as the most prevalent type of clay that may be encountered in soils and sediments (Ghosh & Bhattacharyya 2002). Also, semiconductors, like ZnO, TiO₂, MnO₂, and Fe₂O₃, have been evaluated to degrade organic contaminants, with TiO₂ and ZnO being the most commonly used semiconductors (Fardin Ehsan et al. 2023). The researchers developed ZnO@kaolinite nanocomposites by arranging ZnO nanoparticles onto the kaolinite rod's surface. The kaolinite rod's surface served as a platform for the congregation of ZnO nanoparticles, resulting in the formation of ZnO@kaolinite nanocomposites. The researchers developed highly photoactive nanocomposites with kaolinite and ZnO nanoparticles through a straightforward hydrothermal process. Different weights of ZnO nanoparticles (10, 30, and 50%) were incorporated into the nanocomposites (Kutláková et al. 2014). It is fascinating to realize that by complementing each other, they are able to overcome a significant number of their limitations (Hnamte & Pulikkal 2022). Moreover, comparative studies have been done on the removal of biochemical oxygen demand (BOD), chemical oxygen demand (COD), Fe(III), Cl, and Cr(VI) from tannery effluent using kaolin and kaolin/ZnO nanocomposites (Mustapha et al. 2020).

Hence, the main aim of the current research is to examine the ability of kaolinite and ZnO@kaolinite compounds to manage THMs in drinking water and identify the role of ZnO in the process efficiency. Numerous investigations, including SEM, X-ray diffractometer (XRD), Brunauer–Emmett–Teller (BET), and FTIR, were performed to describe these clay minerals before and after treatment. The adsorption behaviors of the minerals were assessed using various methods. Furthermore, the study optimizes the different operation parameters like contact time, sorbent dosage, solution pH, and THMs initial concentration. The mechanism of adsorption was analyzed using various isotherm and kinetic models.

The following section lists the chemicals, reagents, and methods used in this work.

Hydrochloric acid (HCl, 37%), sodium hydroxide (NaOH, ≥99%), sodium thiosulfate (≥99%), ethanol (99.9%), and hexane (≥99% purity or free of THMs), THMs: chloroform, BDCM, CBDM, and bromoform were procured from Sigma-Aldrich. Sodium chloride (NaCl, ≥99%, VWR PROLABO), methanol (HPLC-grade, Fischer Scientific), acetone (HPLC-grade, TEDIA Company), and reagent water (Type I) were also used in this study. Raw kaolin was purchased from El Nasr Pharmaceutical Chemical Company. All chemicals used were of analytical grade.

Stock and working solutions were stored at 4 °C. Aqueous calibration standards 5, 10, 50, 100, and 120 μg L⁻¹ were prepared by respectively diluting 5, 10, 50, 100, and 120 μL of the intermediate standard solution in 25 mL reagent water and then extracted and analyzed in the same manner as the unknowns over the concentration range containing at least three levels to confirm the linear response range of the system with a minimum correlation coefficient of 0.99 and to adjust each individual conc. THMs and/or calibration verification by middle-level standard (Standard Methods for the Examination of Water and Wastewater 2023).

The intermediate standard solution (25 μg mL⁻¹) is prepared from the stock solution in 5 mL methanol (1 μL = 0.025 μg); using a 100 μL syringe for injection of 62.5 μL from the stock standard solution into the filled area of the volumetric flask, it is then diluted with methanol to reach 5 mL (Standard Methods for the Examination of Water and Wastewater 2023).

A pH meter (Adwa, AD3000, Romania) with a reference electrode, magnetic stirrer (VELP SCIENTIFICA, Italy), drying oven (WON-105, Witteg, Germany), muffle furnace (Barnstead Thermolyne), electronic balance (Sartorius, Poland), cellulose nitrate filter paper (0.45 μm) (Sartorius Stedim Biotech), a multi-branch system with a filter holder (Sartorius Stedim Biotech), micro syringes (0.01–1 mL), measuring flasks (0.001, 0.005, 0.01, 0.025, 0.05, and 0.1 L), 40 mL amber glass bottles with Teflon screw caps, 1–2 mL disposable glass transfer pipettes (Pasteur), vials with crimp or screw caps for the autosampler, graduated cylinders (25 and 50 mL), bottle-top dispenser (2–10 mL capacity), and round, amber glass vials with trifluoroethanol (TFE)-lined tops (4 mL volume) for standard solution storage were used.

Various techniques were employed to characterize the prepared materials. The prepared materials' functional group variations were assessed using FTIR (Bruker, Vertex 70, Germany) before and after the adsorption procedure. The XRD (Panalytical Empyrean, 202964, Sweden) was used to get the diffraction patterns using the Cu-Kα irradiation source (wavelength λ = 1.5406 Å). It is also utilized to identify the material phases of the developed materials. FE-SEM, Zeiss, Sigma 500 VP (Germany) was employed to determine the elemental composition and the morphological structure of the prepared materials. To determine the surface characteristics, the Quantachrome Instrument (USA, Model TriStar II 2030 Version 3.02, Italy) based on the BET method was utilized.

Aqueous calibration standards 5, 10, 50, 100, and 120 μg L⁻¹ are prepared by diluting 5, 10, 50, 100, and 120 μL of the intermediate standard solution, respectively, in 25 mL reagent water and then extracted and analyzed in the same manner as the unknowns over the concentration range containing at least three levels to confirm the linear response range of the system (R²) with a minimum of 0.99 and to correct the concentration of each individual THM and/or calibration verification by the middle-level standard. The limit of detection was 1.70 μg L⁻¹ and the limit of quantification was 4 μg L⁻¹.

Using Agilent 6890N series gas chromatography supplemented with a GC-ECD (gas capture detector), 7683B THMs were identified using an autosampler, and the identification relied on retention time. Chemstation software was put on the equipment for reporting, data analysis, and operation. The column features a DB-5 fused silica capillary column with a diameter of 250 μm, a maximum length of 30 m × 0.32 mm, and a 0.25 μm film thickness. Nitrogen gas is used as the carrier gas for the GC-ECD. To achieve the optimal resolution across various boiling points, the column temperature was programmed in the following manner: it was initiated at 35 °C for 2 min, then raised at rate 10 °C min⁻¹ until reaching 70 °C, maintained for another 2 min, and finally continued to rise at rate 20 °C min⁻¹ until reaching 250 °C. Finally, it was kept for 5 min at a rate of 20 °C min⁻¹ to 250 °C. The oven had a temperature of 300 °C, a 20-min equilibrium time, a peak flow rate of 104.5 mL min⁻¹, and a pressure of 11.604 psi. The injection was performed at 250 °C for 25 min using a split/splitless mode injector.

The study investigated the adsorption of THM onto kaolinite and ZnO@kaolinite at various pH levels (1, 3, 5, 7, 10, and 11), while maintaining constant conditions for adsorbent dosage, contact time, and concentration. The effect of the sorbent dosage on the THMs adsorption onto kaolinite and ZnO@kaolinite was investigated at different adsorbent dosages (1, 5, 10, and 20 mg) while maintaining constant conditions for pH, contact time, and concentration. The effect of THMs initial concentration was studied by varying the THMs initial concentrations (50, 100, 200, 300, 400, 600, and 800 μg L⁻¹) while maintaining constant conditions for pH, adsorbent dosage, and contact time. The adsorption of THMs onto kaolinite and ZnO@kaolinite were investigated at varying contact times (5, 15, 30, 45, 60, and 120 min) while maintaining constant conditions for adsorbent dosage, pH, and initial concentration. The effect of ionic strength on the THM adsorption using kaolinite and ZnO@kaolinite was achieved with varied concentrations of NaCl (0.01, 0.05, 0.1, 0.03, and 0.5 M) while maintaining constant conditions for adsorbent dosage, pH, initial concentration, and contact time. The reusability of the sorbents was studied using the filtration approach, after that the adsorbents were permitted to settle and collected. Then ethanol was used to clean them (Hnamte & Pulikkal 2022). Moreover, green tea was used as a green solvent. Prior to being used in the next cycles, the resulting sample underwent initial drying within a stove adjusted at 70 °C for 2 h. The optimal conditions identified in batch adsorption studies were used to repeat the experiment.

The following sections discuss in detail the discussion of the characterization, adsorption, kinetic, and isotherm modeling results.

Table 3 reports a comparison with the previous studies for the adsorption of THMs onto different adsorbents.

The findings of this study suggest the potential of kaolinite and ZnO@kaolinite nanocomposite as effective sorbents for the elimination of THMs from drinking water. The analysis of the adsorbents involved the utilization of XRD, BET, FTIR, and SEM/EDX methods. The adsorption process was observed to be influenced by factors like pH, THMs initial concentrations, adsorbent dose, ionic strength, and contact time. Based on the variations in surface area, ZnO@kaolinite nanocomposite showed better adsorptive behavior under the applied conditions than kaolinite alone. The ideal conditions to remove THMs vary, and these variations have been connected to the types of pollutants present in drinking water. The Freundlich isotherm model is the best to explain the equilibrium adsorption data. According to adsorption kinetic data, the steps that determined the rate were both diffusion and IPD. The results indicate that kaolinite and ZnO@kaolinite nanocomposite are practically applicable for the adsorption of THMs from drinking water, indicating that adsorbents are an attractive choice for water management. Further studies should be conducted to investigate the scalability and cost-effectiveness of using kaolinite and ZnO@kaolinite nanocomposite for THMs removal in drinking water treatment plants. The effect of other factors such as temperature, particle size, and surface modification on the adsorption behavior of THMs onto kaolinite and ZnO@kaolinite nanocomposites should be explored. The use of other adsorbents such as ZnO, activated carbon, zeolites, and metal-organic frameworks should be investigated for comparison with kaolinite and ZnO@kaolinite nanocomposites.

The United States of America, via the US Agency for International Development (USAID), has graciously provided the financial support necessary for this publication. The authors alone are responsible for the information, which may not represent the views of USAID or the US government.

E.E.: Conceptualization, Methodology, Investigation, Data curation, Writing – original draft, Writing – review and editing. E.-S.I.M.: Supervision, Resources, Writing – review and editing. O.A.M.: Supervision, Writing – review and editing. N.S.: Methodology, Visualization, Investigation, Data curation, Writing – review and editing.

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

The authors declare there is no conflict.