This research investigated the reduction of dissolved organic matter (DOM) fractions and their trihalomethane formation potentials (THMFPs) by powder activated carbon (PAC) adsorption. Leachate contaminated groundwater around an inactive open-dumping landfill was selected as the raw water. The PAC adsorption reaction was proven to be the pseudo second order kinetic reaction and the Freundlich isotherm. The dissolved organic carbon (DOC), dissolved organic nitrogen (DON), and THMFP removals were 55%, 57%, and 73%, respectively. The hydrophobic (HPO) fraction exhibited a higher THMFP compared to the hydrophilic (HPI) fraction. The results of DOM fractionation show that the use of PAC adsorption produced an efficient 87% reduction of the HPO fraction, which is characterized as having a high reactivity toward THMFP.
Groundwater is a major natural water source that is used for human consumption. Households in Thailand used up to 18.6 percent of groundwater for various household purposes and 6.7 percent of it for drinking [ 1 ]. In the past, groundwater contamination was not as severe. Groundwater was easily treated using conventional processes such as coagulation, flocculation, sedimentation, and boiling. At present, groundwater has been contaminated by human activities, agriculture, and the livestock industry as well as leachate from open-dumping landfills. As a result, the conventional processes are unable to treat contaminants in groundwater successfully.
From 1958 to 1989, Chiang Mai Province in Thailand utilized an open-dumping landfill in Tambon Mae Hia, located in Muang District, to solve the problem of waste management in the Chiang Mai Municipality. Surrounding communities of the landfill site suffered from its odor and leachate [ 2 ]. It led to resistance from the surrounding communities. Finally, the open-dumping site was closed. However, the surrounding communities continue to be affected by the leachate contaminated groundwater. Common compounds present in the leachate include natural organic matters (NOMs), phenol, ammonia, phosphate, sulfide salts, inorganic salts, toxins, and heavy metals [ 3 ]. When the groundwater was contaminated by the leachate, the groundwater quality degraded and could not be used for consumption. Furthermore, conventional treatment processes were unable to produce adequate clean and safe water. Advance treatment processes such as adsorption and membrane filtration have, therefore, been introduced to supplement the current disinfection process.
NOMs are complex compounds present in water. Parameters representing NOMs in water include the total organic carbon (TOC), total organic nitrogen (TON), absorption of ultraviolet light at a wavelength of 254 nm (UV-254), and fluorescence excitation emission matrix (FEEM). Huo et al. [ 4 ] conducted a study of dissolved organic matters (DOMs), a dissolved form of NOMs, in leachate from landfills at different ages. The study shows that the leachate from young landfills contains high protein compounds, while the leachate from moderate and old landfills contains mainly humic acid and fuvic acid compounds. Additionally, it was found that aromatic and aliphatic compounds in leachate increased with the age of the landfill.
The disinfection process is used in water treatment to prevent the spread of diseases in water. Common disinfectants include chlorine, chloramines, and chlorine dioxide. Disinfectants can inhibit harmful microorganisms effectively; however, these disinfectants could react with NOMs in water and form disinfection by-products (DBPs). DBPs, in the form of trihalomethanes (THMs), were first detected in 1974 when chlorine reacted with NOMs in water during a disinfection process [ 5 ]. More than 600 species of DBPs have been found and reported. Some of them detected in drinking water have been identified as carcinogenic compounds [ 6 ]. Major DBPs, aside from THMs, include haloacetic acids (HAA) and haloacetonitrile (HANs) compounds. Zhang et al. [ 7 ] studied the relationship between DOMs and DBPs. Their study found that the characteristics of DOMs affected the chances of the formation of DBPs. Organic acids served as precursors to the formation of trihalomethanes (THMs). Jiarsirikul [ 8 ] tested the relationship between the trihalomethane formation potential (THMFP) and TOC in water from shallow wells nearby an open dumping landfill. It was found that the THMFP had a good correlation with the TOC concentration as well as chlorine demand and UV-254. Xue et al. [ 9 ] showed the effect of changes in the dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) concentrations on THM and HAN formations. Their results show a positive correlation between DOC and THMs, but no correlation between DON and HANs.
The control and reduction of DBP formation can be carried out by, for example, removing DBP precursors, changing the disinfectant, and/or removing DBPs directly after the disinfection process. DOC and DON are common DBP precursors, and a method could be solely selected or a combination of methods could be used together [ 10 ]. The reduction of DBP precursors prior to the disinfection process has been an effective method in reducing the DBP formation potential. Adsorption, ion exchange, and the membrane filtration have also been recommended [ 11 ]. The adsorption process using activated carbon as an adsorbent is highly recognized for its ability to remove DOMs [ 12 ]. Many advantages of its use have been reported such as its high DOM removal efficiency, ability to remove a wide range contaminant concentrations, and easy application. Furthermore, the adsorption process can be applied in various ways; for example, activated carbon has been applied to reduce a residual aluminium-dye complex and dolomite has been used for fluoride removal [ 13 - 14 ].
This research studies the effectiveness of activated carbon on DBP precursor removal from leachate contaminated groundwater nearby the closed open-dumping landfill in Tambon Mae Hia, Muang District, Chiang Mai, Thailand.
2. Materials and Methods
This research was conducted at the laboratory of the Department of Environmental Engineering, Faculty of Engineering, Chiang Mai University. Details of the research are listed as follows.
2.1. Water Sample
Groundwater from a shallow-well around an inactivated open-dumping landfill in Tambon Mae Hia, Muang District, Chiang Mai, Thailand was collected in a clean plastic container. The water sample was analyzed for pH (HORIBA, pH METER F-21), temperature, alkalinity (titration method, Standard Method 2320B) [ 15 ], conductivity (SCHOTT, handy lab LF1), dissolved oxygen (WTW, Oxi 3205), and turbidity (WTW, Turb 430T). The DBP precursor, namely DOC, was measured by a TOC analyzer (O.I. analytical 1010). Total Kjeldahl Nitrogen (TKN) and ammonia nitrogen (NH 3 -N) were measured by 4500-NH 3 F, the phenate method [ 15 ]. DON was determined by subtracting NH 3 -N from TKN. For the DOC and DON measurements, the water sample was filtered through a 0.7 µm GF/F filter and 0.45 μm nylon membrane filter, respectively, prior to the measurements. In order to avoid the organic matter contaminant from the GF/F filters, the filters were combusted at 550 ๐C prior to use. The nylon membrane filters were rinsed with 100 ml of pure water following by 50 ml of water sample prior to use. The filtered leachate contaminated groundwater was further measured for UV-254 by using a UV/VIS spectrophotometer (Perkin-Elmer, Model Lambda 25) and FEEM by a spectrofluorometer (JASCO, FP-6200). The filtered leachate contaminated groundwater was kept at 4 ๐C until analysis and experimental use.
2.2. Activated Carbon
Powder activated carbon (PAC) was used in this research. The surface morphology of the PAC was observed by scanning electron microscopy (SEM). The surface properties of the PAC were analyzed using the Brauner Emmet Teller (BET) method [ 16 ]. Before using the PAC in this research, the PAC was washed and soaked in pure water for one week to remove contaminants. Then, the PAC was dried at 105 °C for 48 hours in an oven. After that, it was put in a desiccator and ready for use.
2.3. Adsorption Kinetics and Isotherms
The filtered leachate contaminated groundwater was used to study the adsorption kinetics and isotherms. The adsorption kinetics was analyzed by varying the adsorption time from 0 to 24 hours. The initial DOC concentration in the filtered leachate contaminated groundwater was around 6.0 mg/L with a PAC content of 1,000 mg/L, at an approximate pH of 7.0. Then, the filtered leachate contaminated groundwater mixed with the PAC was shaken at 200 rpm at room temperature. Afterwards, they were filtered through a nylon syringe filter with a pore size of 0.45 µm to separate the solutions. The solutions were further analyzed for their remaining DOC concentrations using a TOC analyzer. The adsorption isotherms were studied by varying the PAC dosages: 100, 500, and 1,000 mg/L, respectively. The initial DOC concentration in the filtered leachate contaminated groundwater was around 6.0 mg/L, with an approximate pH of 7.0. The PAC and filtered leachate contaminated groundwater mixtures were shaken at 200 rpm at room temperature. In addition, the equilibrium time of adsorption was based on the kinetic study results. After the equilibrium time, the samples were filtered through a nylon syringe filter with a pore size of 0.45 mm to separate the solutions. The solutions were further analyzed for their remaining DOC concentrations.
2.4. DBP Precursor Removal by PAC
The filtered leachate contaminated groundwater was used to study the removal of DBP precursors by PAC. The PAC dosage was set at 1,000 mg/L. As mentioned above, the equilibrium time of adsorption was based on the kinetic study results. The DOC and DON concentrations, as well as UV-254 and FEEM, were measured to determine the efficiency of PAC on DBP precursor removal before and after the PAC adsorption tests.
2.5. Formation Potential of DBPs
The filtered leachate contaminated groundwater both before and after the PAC adsorption tests were examined for their THMFP and haloacetonitrile formation potential (HANFP) following Standard Method 5710 B., 4500-Cl B., 6232 B [ 15 ]. The THMs were analyzed for chloroform, bromodichloromethane, bromochloromethane and bromo-form, while HANs were analyzed for monochloro acetonitrile, dichloroacetonitrile, and trichloroacetonitrile. The THMFP and HANFP analyses consisted of three steps: (1) a free chlorine residual measurement, (2) liquid-liquid extraction, and (3) sample analysis. For the free chlorine residual measurement, the residual chlorine was measured until its concentration was within the range of 3-5 mg/L by a portable spectrophotometer (HACH, DR/890 colorimeter). Then, it could proceed to the next step. For the liquid-liquid extraction, pentene was used as the solvent. The extracted solution was further taken out and kept in a refrigerator under 4 °C. Finally, the extracted solution was collected and analyzed by gas chromatography, GC-ECD (Hewlett Packard, HP 6890 GC) with an RTX624 column.
2.6. DBP Precursor Fractionation
DOMs in the filtered leachate contaminated groundwater both before and after the PAC adsorption tests were fractionated into two fractions (i.e., the hydrophobic (HPO) and hydrophilic (HPI) fractions) by using DAX-8 resin. The procedure followed a resin fractionation procedure proposed by Leenheer, Marhaba et al., Kanokkantapong et al., and Rakruam and Wattanachira [ 17 - 20 ]. It was acceptable that the difference between the masses of un-fractionated and fractionated organic matters be in the range of 10-15% (% diff.) using DAX-8 resin [ 21 ].
3. Principles of the Adsorption Mechanisms
3.1. Adsorption Kinetics
Kinetic model of a pseudo second order model was used to describe the data of DOM removal kinetics. This model has been the most widely used to describe time evolution of adsorption under non-equilibrium conditions. The kinetic model can be expressed by Eq. (1)-(3) [ 22 ].
where k 2 (g/mg.min) are the rate constants of pseudo second-order. q t and q e are the amount of DOC adsorbed (mg/g) at t time and equilibrium, respectively.
3.2. Adsorption Isotherms
Adsorption isotherms were evaluated by comparing the obtained data with the Langmuir and Freundlich models. The Langmuir isotherm describes monolayer sorption onto a surface, with sorption occurring only onto sites. There are no interactions between the molecules. The model’s equation is as follows [ 23 ]:
where q 0 is the amount of DOC adsorbed per unit weight of PAC in forming a complete monolayer on the surface (mg/g), q e is the total amount of DOC adsorbed per unit weight of PAC at equilibrium (mg/g), C e is the concentration of the DOC in the solution at equilibrium (mg/L), and K L is the constant of sorption (L/mg).
The Freundlich isotherm can be applied with non-ideal sorption on heterogeneous surfaces and multilayer sorption. The model is expressed by the following equation [ 23 ]:
where q e is the total amount of DOC adsorbed per unit weight of PAC at equilibrium (mg/g), C e is the concentration of the DOC in the solution at equilibrium (mg/L), K F is the Freundlich constant which can express the capacity of the adsorption process (L/g), and n is the Freundlich constant, which explains the intensity of adsorption (dimensionless).
4. Results and Discussion
4.1. Leachate Contaminated Groundwater
The characteristics of the leachate contaminated groundwater are reported in Table 1 . DOC, DON, UV-254, and specific UV absorbance (SUVA) values were 3.4-7.5 mg/L, 0.17-0.60 mg/L, 0.128-0.161 cm -1 , and 1.71-4.20 L/mg•m, respectively.
|Alkalinity (mg/L as CaCO 3 )||179-202|
|NH 3 -N (mg/L)||0.04-0.49|
|UV-254 (cm -1 )||0.128-0.161|
Characteristics of the leachate contaminated groundwater.
A high value of conductivity in the groundwater might be due to the large amount of ions in the leachate. The DOC concentration was relatively higher than the common DOC concentration (<1.0 mg/L) found in groundwater [ 24 ]. This could be from leachate contamination. In addition, the results show a low value of UV-254. This implied that the leachate contaminated groundwater contained a small number of aromatic and double-bond organic compounds. The SUVA value was calculated by dividing the UV-254 value with the DOC concentration and multiplying it by 100. The SUVA resulting value was utilized to indicate which type of treatment process was appropriate for the DOMs in the water [ 25 ]. When the SUVA value is higher than 4, a high amount of hydrophobic organic matters are present, and the coagulation process would be the suggested approach. In this research, the SUVA value was in a rage of 1.71-4.20 L/mg•m. Hence, the adsorption process was preferred over coagulation.
4.2. Properties of PAC
The surface morphology of the PAC is illustrated in Fig. 1 . It shows the SEM images of the PAC at different magnifications: 50 (a), 500 (b), and 2500 (c). The PAC provided an unsmooth porous structure.
SEM images of PAC.
The specific surface area, average pore diameter, and total pore volume of the adsorbent were determined using the BET technique. The specific surface area, total pore volume, and average pore size of the PAC were observed at 647 m2/g, 0.46 cc/g, and 28.5 Å, respectively. The specific surface area of PAC used in this study was in the range between 500 and 3000 m2/g, which was consistent with others [ 26 - 27 ], while PAC pore size of 28.5 Å was considered as mesopores. The high specific surface area of the PAC allowed it to have a high organic matter adsorption capacity. This was reported in a study by Leyva and co-workers, who found that adsorbents with higher specific surface areas and total pore volumes could reach higher adsorption efficiencies [ 28 ].
4.3. Adsorption Kinetics and Isotherms
Figure 2 illustrates the kinetic adsorption of DOC by the PAC. The PAC adsorbed DOC rapidly in the first 30 minutes. For instance, the DOC adsorbed on PAC was increased with increase contact time to around 2.0 mg/g and then the adsorption gradually slowed until it reached equilibrium after 3 hours.
Kinetic adsorption of DOC adsorbed on PAC.
Kinetic adsorption processes could be divided into three main steps: (i) diffusion across the solute film surrounding the surface of adsorbent; (ii) intra-particle diffusion and (iii) physical adsorption or chemical adsorption at active site of adsorbent [ 29 ]. Our result reveals that the pseudo second order model suitably described the DOC adsorption by PAC using leachate contaminated groundwater with coefficients of determination and rate constant of 0.998 and 0.73 g/mg•min, respectively. It suggests that rate-limiting step of the DOC adsorption probably relates to the chemisorption, involving valence forces through then sharing or exchange of electrons.
A plot of the pseudo second-order kinetic model.
The adsorption isotherms of the Langmuir and Freundlich models were evaluated. It was found that the adsorption data of DOC on the PAC best fit with the Freundlich isotherm model with regard to R2 (R2 = 0.97) as shown in Fig. 4 . The K F and n constants were 2.51 mg/g and 0.336, respectively.
In this study, the DOC adsorption by PAC followed the Freundlich isotherm, which was assumed to have a heterogeneous surface consisting of active sites with different adsorption potentials. Furthermore, the experimental data indicated that DOC adsorption was not restricted to the formation of the monolayer. The result demonstrated that PAC had adsorption capacity to adsorb DOC with K F value of 2.51 mg/L. It was similar to K F value of antrazine adsorption by PAC (K F of 2.59 mg/L) [ 30 ]. DOC removal by PAC was possibly associated to the interaction between carboxylic function group of DOM and positive charged reactive site of PAC.
A plot of the Freundlich model.
4.4. Treatment of Leachate Contaminated Groundwater by PAC
4.4. Treatment of Leachate Contaminated Groundwater by PAC
4.4. Treatment of Leachate Contaminated Groundwater by PAC
4.4. Treatment of Leachate Contaminated Groundwater by PAC
In this investigation, PAC adsorption was able to moderately improve the leachate contaminated groundwater. DOC, DON, and THMFP could be reduced by 55%, 57%, and 73%, respectively. The HPO fraction showed a higher THMFP compared to the hydrophilic fraction (HPI), and HPO fraction was identified as high reactivity toward THMFP. Although the HPO fraction could be successfully reduced by 87% after PAC adsorption, its specific THMFP remained high. The formation of THMs did not only depend on the concentration of a precursor but also the DOM fractions that were fractionated. Although the HPO fraction was treated efficiently, the specific THMFP of the HPO fraction remained high, requiring the use of an additional water treatment prior to the chlorine disinfection process.
The research fund from Research Administration Center, Office of the University, Chiang Mai University (Research ID: R000011326), Thailand, is greatly appreciated.
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