The aim of this study was to investigate the efficiency of four different mineral liners (clay, bentonite, kaoline, and zeolite) which could be utilized to prevent the transport of phenolic compounds to groundwater through alternative liner systems. Four laboratory-scale HDPE reactors with 80 cm height and 40 cm inner diameter were operated for a period of 180 days. Results indicated that the transport of mono- or dichlorophenols is significantly prevented by the liner systems used, while the transport of highly chlorinated phenolic compounds cannot be prevented by the landfill liner system effectively. Highly chlorinated phenolic compounds in groundwater can be found in higher concentrations than the leachate, as a result of the degradation and transformation of these compounds. Thus, the analysis of highly chlorinated phenolic compounds such as 2,4,6-TCP, 2,3,6-TCP, 3,4,5-TCP, and PCP is of great significance for the studies to be conducted on the contamination of groundwater around landfills.
In recent years, the characteristics of wastes in MSW landfills have varied according to the changing habits of consumers. The characteristics of leachate generated from sanitary landfill sites vary according to the waste compounds disposed and the physical, chemical, and biological processes occurring in landfills. Household hazardous wastes such as batteries, paints, oils, electrical products, and pharmaceuticals have a negative impact on environmental and human health [
In studies conducted to determine the pollutants in both leachate and the leachate contaminated groundwater, numerous hazardous substances were identified [
Diffusion constitutes the main transport mechanism for the pollutant transport through landfill liners to groundwater [
In recent years, there has been an increase in the number of studies conducted on the effects of leachate on the contamination of water resources [
The aim of this study was to investigate the performance of different liner materials for the prevention of transport of phenolic compounds to groundwater. Four different liners (clay, bentonite, zeolite, and kaoline) with the same thickness (20 cm) were used in the simulated reactors. Phenol, 2,4-dichlorophenol (2,4-DCP), 2,6-dichlorophenol (2,6-DCP), 2,4,6-trichlorophenol (2,4,6-TCP), 2,4,5-trichlorophenol (2,4,5-TCP), 2,3,6-trichlorophenol (2,3,6-TCP), 3,4,5-trichlorophenol (3,4,5-TCP), and pentachlorophenol (PCP) concentrations in leachate and groundwater samples were identified and the performance of different liner materials was determined in order to evaluate the transport of phenolic compounds.
Four lab-scale reactors with a height of 80 cm and an inner diameter of 40 cm were used in order to determine the transport of phenolic compounds in landfill leachate through liner systems (Figure
Reactors used in the study.
The upper part of the liner was filled with 20 liters of leachate (app. 17 cm height) obtained from Odayeri Sanitary Landfill located on the European Side of Istanbul, Turkey. According to the US and the European standards, the maximum leachate load on landfill liner should be 20–50 cm [
The liner system of reactors is comprised of 2 mm thick HDPE geomembrane and 20 cm thick natural minerals (R1: clay, R2: bentonite, R3: kaoline, and R4: zeolite). The liner systems are compacted to prevent the possibilities of leakage at commissioning stage. Mineral layers are filled by the optimum water content obtained from soil analysis. The lower part of the reactors was filled with 25 liters of distilled water representing groundwater.
Leachate characterization was realized before the upper parts of the reactors were filled. pH, electrical conductivity, COD, BOD, TOC, CI−, TKN, NH3-N, and
To determine phenol and chlorinated phenolic compounds in leachate and groundwater samples, solid phase microextraction (SPME) method was used as conducted by Ribeiro et al. [
pH, hydraulic conductivity, soil classification, compaction test [
The mean values of leachate characterization analyses realized on 3 different leachate samples are given in Table
Leachate characteristics.
Parameter | Value |
---|---|
pH | 7.8 |
Conductivity (mS/cm) | 33 |
COD (mg/L) | 22000 |
BOD (mg/L) | 13000 |
TOC (mg/L) | 6200 |
TKN (mg/L) | 2700 |
TP (mg/L) | 14 |
NH3 (mg/L) | 2500 |
Cl− (mg/L) | 4400 |
S |
500 |
Phenol ( |
26 |
2,4-Dichlorophenol ( |
46 |
2,6-Dichlorophenol ( |
2.4 |
2,4,6-Trichlorophenol ( |
9.5 |
2,4,5-Trichlorophenol ( |
8.7 |
2,3,6-Trichlorophenol ( |
2.7 |
3,4,5-Trichlorophenol ( |
ND |
Pentachlorophenol ( |
0.8 |
In the result of repeated experiments, as the values for conventional parameters were near to each other, determined values for phenolic compounds were not near due to volatile properties of phenolic compounds. The studies conducted to determine the phenolic compounds in the leachate also demonstrated that different derivatives of monochlorophenol and pentachlorophenol were present. In the study conducted by Öman and Junestedt [
The results of the characterization experiments of clay, bentonite, kaoline, and zeolite used as landfill liner materials in this study are given in Table
Properties of the mineral materials used in the study.
Material | pH | Clay content (%) | Silt content (%) | Soil classification | Hydraulic conductivity (m/s) | Optimum water content (%) | CEC (meq/100 g) |
---|---|---|---|---|---|---|---|
Clay (R1) | 7.4 | 85 | 15 | CH-inorganic clays of high plasticity | 6.3 × 10−8 | 27 | 19.3 |
Bentonite (R2) | 8.8 | 89 | 11 | CH-inorganic clays of high plasticity | 2.7 × 10−10 | 41 | 48.6 |
Kaoline (R3) | 7.7 | 57 | 43 | ML-inorganic silts with slight plasticity | 3.1 × 10−7 | 23.5 | 10.1 |
Zeolite (R4) | 8.1 | 19 | 81 | MH-inorganic silts with high plasticity | 8.8 × 10−8 | 40 | 20.2 |
The gradual changes of phenolic compounds in leachate and groundwater samples taken from R1, R2, R3, and R4 reactors are given in Figures
Variations of the phenolic compounds in groundwater and leachate samples of R1 reactor (
Variations of the phenolic compounds in groundwater and leachate samples of R2 reactor (
Variations of the phenolic compounds in groundwater and leachate samples of R3 reactor (
Variations of the phenolic compounds in groundwater and leachate samples of R4 reactor (
The average phenol concentrations in leachate samples of R1, R2, R3, and R4 reactors were 6.91, 6.91, 6.67, and 6.77
The average 2,4-DCP concentrations in leachate samples during 180 days of operation were determined as 11.7, 11.6, 15.5, and 19.4
A similar trend can be seen for 2,6-DCP in leachate samples, but the decrease was not observed as fast as 2,4-DCP. The observed leachate concentrations were 3.25
Average concentrations of 2,4,6-TCP were determined as 2.92, 3.45, 2.78, and 2.73
The average PCP concentrations for R1, R2, R3, and R4 reactors were determined as 2.26, 0.94, 0.83, and 0.64
The dominant mechanism for the transport of phenol and phenolic compounds from leachate to groundwater is molecular diffusion. Geomembranes are ineffective in organic contaminant transport [
Adsorption can also be considered as a fundamental affecting the migration of phenolic compounds to groundwater by landfill liners. Chaouati et al. [
Four lab-scale HDPE reactors with different liner materials (clay, bentonite, kaoline, and zeolite) were used in order to determine the transport of phenolic compounds in landfill leachate through liner systems. Leachate samples used in the study correspond to a middle-aged landfill leachate. The reduction of all phenolic compounds to phenol as an end product caused the high concentrations of phenol in raw leachate. According to the soil analysis, bentonite seems to have the lowest hydraulic conductivity and a higher adsorption capacity.
Experimental results indicated that the mineral materials used in landfill liners will not have an effect on mono- and dichlorophenol transport to groundwater. The observed decrease of these compounds in leachate samples was derived from biodegradation and sorption mechanisms. The reason for the high concentrations of highly chlorinated phenolic compounds in groundwater samples can be explained by the transformation of phenolic compounds under anaerobic conditions. Highly chlorinated phenolic compounds demonstrated slower degradation compared to other phenolic compounds resulting in a substantial migration of these compounds to groundwater.
As a general result, mono- and dichlorophenols migration can be prevented by mineral layers, but these layers are ineffective for polychlorinated phenolic compounds. Additionally, the four mineral layers used in this study failed to prevent the transport of contaminants. However, the migration of phenolic compounds to groundwater can be decreased with the use of zeolite material in landfill liners. It was also determined that the transport of highly chlorinated phenolic compounds from mineral layer is possible through different processes. Due to degradation and transformation activities, these compounds can be found in groundwater with higher concentrations than the leachate. Therefore, the analysis of highly chlorinated compounds such as 2,4,6-TCP, 2,3,6-TCP, 3,4,5-TCP, and PCP is of great significance for the studies to be conducted on the contamination of groundwater around landfills by leachate.
As a result of this work, the best of four different materials used may be zeolite because of high silt content and adsorption capacity. Adsorption studies by using zeolite and/or zeolite + clay mixture can be conducted to determine the best material minimizing the migration of leachate contaminants to groundwater for the future studies.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This research has been supported by the Scientific Research Projects Coordinatorship of Yildiz Technical University (BAP) (Project no. 2011-05-02-KAP01), Istanbul, Turkey. The authors wish to acknowledge ISTAC (Istanbul Metropolitan Municipality Environment Protection and Waste Material Recycling Inc.) for their support during the study.