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Experimental and modeling study of pure terephthalic acid (PTA) wastewater transport in the vadose zone Cuiling Wang,a Changli Liu,*a Lixin Pei,a Yajie Pang,b Yun Zhanga and Hongbing Houa PTA wastewater discharged from a factory was selected as the research object in this project and CODcr was selected as the characteristic pollution factor. Static adsorption and soil column leaching experiments of silty clay and clayey soil were carried out to study the adsorption, bio-degradation and dispersion coefficient of CODcr in PTA wastewater. Hydrus-1D was used to build the convection– diffusion model to demonstrate the migration of PTA wastewater in the vadose zone. The results indicate that silty clay and clayey soil in the vadose zone can adsorb, degrade and impede the contaminants in PTA wastewater; however, the coefficient of adsorption and degradation were very low, they were down to 0.0002 L g1, 0.0003 L g1 and 0.0097 d1, 0.0077 d1 for silty clay and clayey soil,
Received 10th October 2014 Accepted 21st November 2014
respectively. Under the virtual condition that, wastewater in the sewage pool is 5 m deep, CODcr concentration is 4000 mg L1, vadose zone is 21 m, PTA wastewater will reach the phreatic surface after 20.87 years. When wastewater in the sewage pool is 7 m with other conditions unchanged, after 17.18
DOI: 10.1039/c4em00538d rsc.li/process-impacts
years PTA wastewater will reach groundwater. The results show that there is a higher pollution risk for groundwater if we do not take any anti-seepage measures.
Environmental impact China is one of the main PTA (Pure terephthalic acid) producers in the world. Studies show that approximately 3–4 m3 of wastewater is generated for each ton of PTA manufactured; moreover, the major aromatic compounds found in PTA wastewater are pure terephthalic acid (PTA), phthalic acid (PA), terephthalic acid (TA), benzoic acid (BA), 4-carboxybenzaldehyde (4-CBA), and p-toluic acid (p-Tol). The literature suggests PTA wastewater is toxic to living organisms and the harmful substances in PTA wastewater could reach groundwater. The fate of PTA wastewater through the vadose zone will provide basis for managers to minimize its adverse impact. This study will provide basic data for pollution prevention and remediation.
Introduction Pure terephthalic acid (PTA) is the most important starting material for making synthetic products1 and is a raw material or intermediate for plastic plasticizers, pesticides and polyester lms.2,3 China is one of the main PTA producers in the world. In recent years, the world PTA production has kept growing rapidly from 26.12 million tons in 2002 to 28.80 million tons in 2011, among which China accounted for 2.6 million tons in 2002, but soared to 15.40 million tons in 2011.4 Along with the increasing manufacture of PTA, wastewater has increased greatly too. Studies show that approximately 3–4 m3 of wastewater is generated for each ton of PTA manufactured, and the CODcr (chemical oxygen demand was determined by means of a
Institute of Hydrogeology and Environmental Geology, Zhonghua street No. 268, Shi Jiazhuang, hebei, China. E-mail: [email protected]; Fax: +86-031188021225; Tel: +86 15033115252
b
Center for Hydrogeology and Environmental Geology Survey, Qiyi road No. 135, Baoding, Hebei, China. E-mail: [email protected]; Fax: +86-0312-5908611; Tel: +86 15232905328
This journal is © The Royal Society of Chemistry 2015
the standard method of dichromate reux) concentration is around 4000–10 000 mg L1. The major aromatic compounds found in PTA wastewater are pure terephthalic acid (PTA), phthalic acid (PA), terephthalic acid (TA), benzoic acid (BA), 4-carboxybenzaldehyde (4-CBA), p-toluic acid (p-Tol), and minor concentrations of methyl acetate, 4-formylbenzoic acid, and p-xylene.5–7 It is well known that PTA is toxic to living organisms and the toxic concentration or dose of PTA was as high as over 1000 mg L1.8 PA can induce reproductive and developmental toxicity, as well as disrupt endocrine function.9 Moreover, TA can result in the impairment of testicular functions, bladder stones and bladder cancer.10,11 Generally, BA reveals that the compound could affect the growth and reproduction of freshwater organisms, sperm viability, though it is not clinically considered as a reproductive or developmental toxicant.12 According to the information available, p-Tol will cause a decrease in epididymal weight and an increase of incidence in cauda epididymal oligo/azoospermia.13 The literature suggests that PTA wastewater is toxic to living organisms,14,15 and the US Environmental Protection Agency recently added this class of
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chemicals to the list of priority pollutants. There are at least ve kinds of benzoate pollutants existing in PTA wastewater. The toxicity of pure chemical PTA is quite different from that of PTA wastewater,5 and the contribution of the ve aromatics to CODcr could reach over 75% in this wastewater;5,16 thus, the value of CODcr can be treated as the pollution factor for PTA wastewater. Due to the increasing manufacture of PTA and the toxicity of its wastewater, their impact on the environment have drawn wide attention for the preservation of natural ecosystems and environmental protection. The harmful substances in PTA wastewater can reach groundwater through land application of sewage sludge, leakage from industrial facilities, uncontrolled leachate from landlls, or wastewater irrigation.17–19 Note that PTA wastewater is a threat to groundwater. In current studies, experiments have been focused on the detection methods for PTA and degradation treatment of PTA wastewater, but no one has studied the harmful substances in PTA wastewater transport in the vadose zone. This study includes experiments and model research, with the former one evaluating the sorption capacity, distribution coefficient, degradation coefficient and dispersion coefficient of PTA wastewater in the vadose zone, and the latter one predicting the PTA wastewater transport through the vadose zone with Hydrus-1D based on the actual situation.20–22 The main objective of this paper is to study the sorption and degradation behaviour of PTA wastewater with representative soil in the vadose zone, and then simulate the transport of wastewater through the vadose zone with Hydrus-1D. The fate of PTA wastewater through the vadose zone will provide the basis for managers to minimize its adverse impact and basic data for pollution prevention and remediation.
Theoretical basis Isothermal equations Adsorption isotherms are essential for the description of how adsorbent media interact with adsorbates. Isothermal experiments are used to examine the relationship between S (mg g1) and C (mg L1), which are the pollutants amount adsorbed by unit soil and the concentration of contaminated liquid during equilibrium, respectively. The linear, Langmuir and Freundlich isothermal equations are most widely used for tting data. Linear isothermal equation: The assumed conditions of the equation are as follows: the adsorbent soil is a single, porous media. The equation is as follows: S ¼ KdC + b
(1)
Note that the distribution coefficient Kd represents the distribution ratio of the contaminant in the solid phase and liquid. It is an important parameter for solute migration ability. The bigger distribution coefficient is, the easier the contaminant is to adsorb, and more difficult to migrate. Langmuir isothermal equation: The Langmuir isothermal model assumes that the adsorption of each molecule is covered by a monolayer and each
390 | Environ. Sci.: Processes Impacts, 2015, 17, 389–397
molecule attached to the surface has an equal activation of adsorption energy.23 The Langmuir model is as follows: S¼
Sm Kl C 1 þ Kl C
(2)
where Kl is the Langmuir equilibrium constant, which is related to the affinity of binding sites, and Sm represents the maximum adsorption capacity. Freundlich isothermal equation: The Freundlich isotherm assumes that the uptake of adsorbate occurs on a heterogeneous surface by multilayer adsorption and the amount of adsorbate adsorbed increases innitely with an increase in concentration.24 The equation can be written as follows: S ¼ KfCn, where Kf is an indicator of the adsorption capacity and n is the adsorption intensity.25
Model basis Many models have been developed to simulate the transport of pollutants in the vadose zone and the most famous models are based on convection–dispersion equations and consider such mechanisms as convection, dispersion, sorption and degradation.26 Convection is pollutant transport at a velocity equivalent to groundwater movement, whereas dispersion, including molecular diffusion and mechanical dispersion, is the solute spatial variation due to aquifer permeability, uid mixing and molecular diffusion.27–29 Sorption is the retention of solute in the soil phase by means of partitioning between the aqueous phase and solid. 30 Degradation is pollutant utilized by microorganisms for energy to grow or is consumed as a reactant in a chemical reaction. When contaminants mainly transport vertically in the vadose zone, it can be simplied as a vertical one-dimensional problem and the convection–diffusion equation is described as follows:31 vc v2 C vc r vs vc r vs vc ¼D 2 v þ 1þ (3) vt vx vx n vc vt n vc vt The rst, second, third and fourth items on the right side of the equation are dispersion, convection, sorption and degradation, respectively. In this equation, s is the sorption concentration of the pollutants, t is the time, r is the dry bulk density, n is the effective porosity, D is the dispersion coefficient, and v is the average pore water velocity. When the isothermal model is in compliance with the linear model and the degradation process conforms with the rstvs order degradation model, then ¼ Kd , and the equation can be vc expressed as follows:31 v2 C vC vC lRd C ¼ Rd (4) v vx2 vx vt Kd where the retardation coefficient Rd ¼ 1 þ r , which describes n the retention effect of pollutants at migration. D
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Environmental Science: Processes & Impacts
With these assumptions, the initial conditions turn out to be
Table 2
C(x,0) ¼ C0(x) and boundary conditions are as follows:
Sample 1 Sample 2
C(0,t) ¼ C1(t)
Parameters of wastewater from the PTA factory CODcr (mg L1)
NH4+ (mg L1)
NO3 (mg L1)
NO2 (mg L1)
pH
2037 23 968
9.50 130.0
540.0 69.0
: h.0 qs 2
1 m K h ¼ Ks Se l 1 1 Se m
(10)
Fig. 5
Comparison between the true values and simulated line.
Ks is permeability coefficient (cm s1), m, n are the soil water characteristic equations of the parameters, and l is oen 0.5.44 Water movement parameters in the vadose zone were predicted using neutral network of Hydrus-1D according to the soil particles which is shown in Table 1, the results shown in Table 5. The parameters used for the simulation are given in Table 6. The observed and simulated BTCs of CODcr are presented in Fig. 5. By good correlation results between the BTCs for PTA wastewater and the simulated curve, R2 are 0.921 and 0.976, respectively. It is concluded that, under the conditions of reasonable model and parameter, the simulation results can represent the pollutants migration in actual conditions.
and m ¼ 1 1/n (n > 1), Se ¼
q qr qs qr
Pollution prediction (11)
where qr and qs are the residual water content and saturation moisture content (%), respectively. Se is the effective saturation,
The thickness of the vadose zone is 28 m in actual situations, and the simplied prole of the vadose zone is listed in Fig. 6. In actual situations, the lower boundary is the phreatic surface, the negative pressure is 0, and upper and lower boundaries are the solute concentration ux and zero concentration gradient boundary, respectively. Regardless of the effect of precipitation and evaporation, the initial condition is negative pressure at different depth calculated by formula (8), assuming that the temperature of the vadose zone is 17 C, the initial concentration of CODcr is 4000 mg L1, PTA wastewater is 5 m and 7 m in the sewage pool, where there is no seepageproong. According to these conditions, prediction of the breakthrough curve and concentration contour in 50 years, the simulated results are shown in Fig. 7 and 8, respectively.
Table 5
Fig. 4 The BTCs of Cl in the soil columns.
394 | Environ. Sci.: Processes Impacts, 2015, 17, 389–397
Parameters of water in the vadose zone
Silty clay Clayey soil
qr (%)
qs (%)
a (cm1)
n
Ks (cm s1)
0.06 0.07
37.16 36.92
0.0078 0.0082
1.5135 1.5267
3.67 105 3.61 105
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Environmental Science: Processes & Impacts Parameters used in the simulation
Published on 21 November 2014. Downloaded by Freie Universitaet Berlin on 06/07/2015 06:59:14.
Silty clay Clayey soil
Kd (L g1)
r (g cm3)
qs
V (cm h1)
D (cm2 h1)
Rd
Ks (cm h1)
DL (cm)
0.0002 0.0003
1.70 1.72
0.372 0.369
0.176 0.168
0.038 0.012
1.048 1.073
0.132 0.130
0.216 0.071
Fig. 6
Simplified profile of the vadose zone.
Fig. 7
The predicted curves of CODCr for 5 m deep PTA wastewater.
From Fig. 7, aer 20.87 years, the CODCr in PTA wastewater will reach the phreatic surface and aer 10 years, 20 years, 30 years, 40 years and 50 years, the CODcr in the phreatic surface (21 m deep) will be 0.16 mg L1, 0.16 mg L1, 5.23 mg L1, 153.85 mg L1 and 4000.00 mg L1, respectively. From Fig. 8, when the PTA wastewater is 7 m deep and the other conditions unchanged, in only 17.18 years, the CODcr in PTA wastewater will reach groundwater, and aer 10 years, 20 years, 30 years, 40 years and 50 years, the CODcr in the phreatic surface (21 m deep) will be 0.16 mg L1, 0.16 mg L1, 1.38 mg L1, 348.46 mg L1 and 4000 mg L1, respectively. When the PTA wastewater is 5 m deep, the CODcr concentration is 4000 mg L1 and the vadose zone is 21 m thick, and aer 20.87 years, the CODcr in PTA wastewater will reach the phreatic surface. However, when the PTA wastewater is 7 m deep and the other conditions unchanged, in only 17.18 years, the wastewater will reach groundwater.
Fig. 8 Predicted curves of CODcr for 7 m deep PTA wastewater.
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This study provides basic data for pollution prevention and remediation, as well as convenience for design management strategies to minimize the adverse impacts.
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Conclusions Detailed experimentation was undertaken to determine the parameters as well as to simulate the ow conditions through the column, and then simulate the contaminants in PTA wastewater transport in the vadose zone. It was noted that under ideal conditions, the adsorption quantity of soils for PTA wastewater increases with the contact time, and then becomes constant aer equilibrium is reached (aer 24 h). The largest adsorption quantity of silty clay and clayey soil are 0.66 mg g1 and 1.05 mg g1 at balance, respectively. The adsorption capacity of silty clay is corresponding to the theoretical value determined according to Langmuir isotherm model, but the experiment value of clayey soil is bigger than theory value, because actual process in theory is more complicated than that in ideal situation. So the actual adsorption capacity is greater than the theoretical value. The batch equilibrium data corresponds to the linear, Freundlich and Langmuir isotherm models, and all experimental data were very satisfactory with these isotherm models, due to the R2 value, which was between 0.900 and 0.999. Both the Langmuir and Freundlich isotherms models exhibited a good t to the sorption data of CODcr in the PTA wastewater. However, the model was much better for the Freundlich isotherm, and the value of Kf was 0.0019 and 0.0012 for silty clay and clayey soil, respectively, indicating a positive adsorption of CODcr in soil. The Sm for silty clay and clayey soil are 0.6659 mg g1 and 0.8017 mg g1, respectively. The distribution coefficient Kd for silty clay and clayey soil are 0.0002 L g1 and 0.0003 L g1, respectively. All the results show that the adsorption capacity of CODcr in PTA wastewater adsorption to silty clay and clayey soil is lower than the other adsorbents; thus, the pollutants in PTA wastewater can easily to migrate into groundwater. It also showed that the rst order degradation kinetic models for silty clay and clayey soil can be summarized as: C ¼ 737.14e0.0097t and C ¼ 766.23e0.0077t, respectively. The degradation coefficient was found to be quite low and indicated that CODcr in PTA wastewater is mobile and less degradable in the soil studied. PTA wastewater transport in the vadose zone was simulated by HYDRUS-1D, and the results showed that when PTA wastewater is 5 m deep, the CODcr concentration is 4000 mg L1 and the vadose zone is 21 m thick; moreover, aer 20.87 years the PTA wastewater will reach the phreatic surface. However, when the PTA wastewater is 7 m deep and the other conditions unchanged, wastewater will reach groundwater in only 17.18 years. This study will provide basic data for pollution prevention and remediation, as well as convenience for design management strategies to minimize the adverse impacts.
396 | Environ. Sci.: Processes Impacts, 2015, 17, 389–397
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Acknowledgements This work was supported by the National Science and Technology Support Project (Project number 2012BAJ11B04) and the Geological Survey Projects (Project number 12120114011701).
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Experimental and modeling study of pure terephthalic acid (PTA) wastewater transport in the vadose zone Cuiling Wang,a Changli Liu,*a Lixin Pei,a Yajie Pang,b Yun Zhanga and Hongbing Houa PTA wastewater discharged from a factory was selected as the research object in this project and CODcr was selected as the characteristic pollution factor. Static adsorption and soil column leaching experiments of silty clay and clayey soil were carried out to study the adsorption, bio-degradation and dispersion coefficient of CODcr in PTA wastewater. Hydrus-1D was used to build the convection– diffusion model to demonstrate the migration of PTA wastewater in the vadose zone. The results indicate that silty clay and clayey soil in the vadose zone can adsorb, degrade and impede the contaminants in PTA wastewater; however, the coefficient of adsorption and degradation were very low, they were down to 0.0002 L g1, 0.0003 L g1 and 0.0097 d1, 0.0077 d1 for silty clay and clayey soil,
Received 10th October 2014 Accepted 21st November 2014
respectively. Under the virtual condition that, wastewater in the sewage pool is 5 m deep, CODcr concentration is 4000 mg L1, vadose zone is 21 m, PTA wastewater will reach the phreatic surface after 20.87 years. When wastewater in the sewage pool is 7 m with other conditions unchanged, after 17.18
DOI: 10.1039/c4em00538d rsc.li/process-impacts
years PTA wastewater will reach groundwater. The results show that there is a higher pollution risk for groundwater if we do not take any anti-seepage measures.
Environmental impact China is one of the main PTA (Pure terephthalic acid) producers in the world. Studies show that approximately 3–4 m3 of wastewater is generated for each ton of PTA manufactured; moreover, the major aromatic compounds found in PTA wastewater are pure terephthalic acid (PTA), phthalic acid (PA), terephthalic acid (TA), benzoic acid (BA), 4-carboxybenzaldehyde (4-CBA), and p-toluic acid (p-Tol). The literature suggests PTA wastewater is toxic to living organisms and the harmful substances in PTA wastewater could reach groundwater. The fate of PTA wastewater through the vadose zone will provide basis for managers to minimize its adverse impact. This study will provide basic data for pollution prevention and remediation.
Introduction Pure terephthalic acid (PTA) is the most important starting material for making synthetic products1 and is a raw material or intermediate for plastic plasticizers, pesticides and polyester lms.2,3 China is one of the main PTA producers in the world. In recent years, the world PTA production has kept growing rapidly from 26.12 million tons in 2002 to 28.80 million tons in 2011, among which China accounted for 2.6 million tons in 2002, but soared to 15.40 million tons in 2011.4 Along with the increasing manufacture of PTA, wastewater has increased greatly too. Studies show that approximately 3–4 m3 of wastewater is generated for each ton of PTA manufactured, and the CODcr (chemical oxygen demand was determined by means of a
Institute of Hydrogeology and Environmental Geology, Zhonghua street No. 268, Shi Jiazhuang, hebei, China. E-mail: [email protected]; Fax: +86-031188021225; Tel: +86 15033115252
b
Center for Hydrogeology and Environmental Geology Survey, Qiyi road No. 135, Baoding, Hebei, China. E-mail: [email protected]; Fax: +86-0312-5908611; Tel: +86 15232905328
This journal is © The Royal Society of Chemistry 2015
the standard method of dichromate reux) concentration is around 4000–10 000 mg L1. The major aromatic compounds found in PTA wastewater are pure terephthalic acid (PTA), phthalic acid (PA), terephthalic acid (TA), benzoic acid (BA), 4-carboxybenzaldehyde (4-CBA), p-toluic acid (p-Tol), and minor concentrations of methyl acetate, 4-formylbenzoic acid, and p-xylene.5–7 It is well known that PTA is toxic to living organisms and the toxic concentration or dose of PTA was as high as over 1000 mg L1.8 PA can induce reproductive and developmental toxicity, as well as disrupt endocrine function.9 Moreover, TA can result in the impairment of testicular functions, bladder stones and bladder cancer.10,11 Generally, BA reveals that the compound could affect the growth and reproduction of freshwater organisms, sperm viability, though it is not clinically considered as a reproductive or developmental toxicant.12 According to the information available, p-Tol will cause a decrease in epididymal weight and an increase of incidence in cauda epididymal oligo/azoospermia.13 The literature suggests that PTA wastewater is toxic to living organisms,14,15 and the US Environmental Protection Agency recently added this class of
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chemicals to the list of priority pollutants. There are at least ve kinds of benzoate pollutants existing in PTA wastewater. The toxicity of pure chemical PTA is quite different from that of PTA wastewater,5 and the contribution of the ve aromatics to CODcr could reach over 75% in this wastewater;5,16 thus, the value of CODcr can be treated as the pollution factor for PTA wastewater. Due to the increasing manufacture of PTA and the toxicity of its wastewater, their impact on the environment have drawn wide attention for the preservation of natural ecosystems and environmental protection. The harmful substances in PTA wastewater can reach groundwater through land application of sewage sludge, leakage from industrial facilities, uncontrolled leachate from landlls, or wastewater irrigation.17–19 Note that PTA wastewater is a threat to groundwater. In current studies, experiments have been focused on the detection methods for PTA and degradation treatment of PTA wastewater, but no one has studied the harmful substances in PTA wastewater transport in the vadose zone. This study includes experiments and model research, with the former one evaluating the sorption capacity, distribution coefficient, degradation coefficient and dispersion coefficient of PTA wastewater in the vadose zone, and the latter one predicting the PTA wastewater transport through the vadose zone with Hydrus-1D based on the actual situation.20–22 The main objective of this paper is to study the sorption and degradation behaviour of PTA wastewater with representative soil in the vadose zone, and then simulate the transport of wastewater through the vadose zone with Hydrus-1D. The fate of PTA wastewater through the vadose zone will provide the basis for managers to minimize its adverse impact and basic data for pollution prevention and remediation.
Theoretical basis Isothermal equations Adsorption isotherms are essential for the description of how adsorbent media interact with adsorbates. Isothermal experiments are used to examine the relationship between S (mg g1) and C (mg L1), which are the pollutants amount adsorbed by unit soil and the concentration of contaminated liquid during equilibrium, respectively. The linear, Langmuir and Freundlich isothermal equations are most widely used for tting data. Linear isothermal equation: The assumed conditions of the equation are as follows: the adsorbent soil is a single, porous media. The equation is as follows: S ¼ KdC + b
(1)
Note that the distribution coefficient Kd represents the distribution ratio of the contaminant in the solid phase and liquid. It is an important parameter for solute migration ability. The bigger distribution coefficient is, the easier the contaminant is to adsorb, and more difficult to migrate. Langmuir isothermal equation: The Langmuir isothermal model assumes that the adsorption of each molecule is covered by a monolayer and each
390 | Environ. Sci.: Processes Impacts, 2015, 17, 389–397
molecule attached to the surface has an equal activation of adsorption energy.23 The Langmuir model is as follows: S¼
Sm Kl C 1 þ Kl C
(2)
where Kl is the Langmuir equilibrium constant, which is related to the affinity of binding sites, and Sm represents the maximum adsorption capacity. Freundlich isothermal equation: The Freundlich isotherm assumes that the uptake of adsorbate occurs on a heterogeneous surface by multilayer adsorption and the amount of adsorbate adsorbed increases innitely with an increase in concentration.24 The equation can be written as follows: S ¼ KfCn, where Kf is an indicator of the adsorption capacity and n is the adsorption intensity.25
Model basis Many models have been developed to simulate the transport of pollutants in the vadose zone and the most famous models are based on convection–dispersion equations and consider such mechanisms as convection, dispersion, sorption and degradation.26 Convection is pollutant transport at a velocity equivalent to groundwater movement, whereas dispersion, including molecular diffusion and mechanical dispersion, is the solute spatial variation due to aquifer permeability, uid mixing and molecular diffusion.27–29 Sorption is the retention of solute in the soil phase by means of partitioning between the aqueous phase and solid. 30 Degradation is pollutant utilized by microorganisms for energy to grow or is consumed as a reactant in a chemical reaction. When contaminants mainly transport vertically in the vadose zone, it can be simplied as a vertical one-dimensional problem and the convection–diffusion equation is described as follows:31 vc v2 C vc r vs vc r vs vc ¼D 2 v þ 1þ (3) vt vx vx n vc vt n vc vt The rst, second, third and fourth items on the right side of the equation are dispersion, convection, sorption and degradation, respectively. In this equation, s is the sorption concentration of the pollutants, t is the time, r is the dry bulk density, n is the effective porosity, D is the dispersion coefficient, and v is the average pore water velocity. When the isothermal model is in compliance with the linear model and the degradation process conforms with the rstvs order degradation model, then ¼ Kd , and the equation can be vc expressed as follows:31 v2 C vC vC lRd C ¼ Rd (4) v vx2 vx vt Kd where the retardation coefficient Rd ¼ 1 þ r , which describes n the retention effect of pollutants at migration. D
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With these assumptions, the initial conditions turn out to be
Table 2
C(x,0) ¼ C0(x) and boundary conditions are as follows:
Sample 1 Sample 2
C(0,t) ¼ C1(t)
Parameters of wastewater from the PTA factory CODcr (mg L1)
NH4+ (mg L1)
NO3 (mg L1)
NO2 (mg L1)
pH
2037 23 968
9.50 130.0
540.0 69.0
: h.0 qs 2
1 m K h ¼ Ks Se l 1 1 Se m
(10)
Fig. 5
Comparison between the true values and simulated line.
Ks is permeability coefficient (cm s1), m, n are the soil water characteristic equations of the parameters, and l is oen 0.5.44 Water movement parameters in the vadose zone were predicted using neutral network of Hydrus-1D according to the soil particles which is shown in Table 1, the results shown in Table 5. The parameters used for the simulation are given in Table 6. The observed and simulated BTCs of CODcr are presented in Fig. 5. By good correlation results between the BTCs for PTA wastewater and the simulated curve, R2 are 0.921 and 0.976, respectively. It is concluded that, under the conditions of reasonable model and parameter, the simulation results can represent the pollutants migration in actual conditions.
and m ¼ 1 1/n (n > 1), Se ¼
q qr qs qr
Pollution prediction (11)
where qr and qs are the residual water content and saturation moisture content (%), respectively. Se is the effective saturation,
The thickness of the vadose zone is 28 m in actual situations, and the simplied prole of the vadose zone is listed in Fig. 6. In actual situations, the lower boundary is the phreatic surface, the negative pressure is 0, and upper and lower boundaries are the solute concentration ux and zero concentration gradient boundary, respectively. Regardless of the effect of precipitation and evaporation, the initial condition is negative pressure at different depth calculated by formula (8), assuming that the temperature of the vadose zone is 17 C, the initial concentration of CODcr is 4000 mg L1, PTA wastewater is 5 m and 7 m in the sewage pool, where there is no seepageproong. According to these conditions, prediction of the breakthrough curve and concentration contour in 50 years, the simulated results are shown in Fig. 7 and 8, respectively.
Table 5
Fig. 4 The BTCs of Cl in the soil columns.
394 | Environ. Sci.: Processes Impacts, 2015, 17, 389–397
Parameters of water in the vadose zone
Silty clay Clayey soil
qr (%)
qs (%)
a (cm1)
n
Ks (cm s1)
0.06 0.07
37.16 36.92
0.0078 0.0082
1.5135 1.5267
3.67 105 3.61 105
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Environmental Science: Processes & Impacts Parameters used in the simulation
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Silty clay Clayey soil
Kd (L g1)
r (g cm3)
qs
V (cm h1)
D (cm2 h1)
Rd
Ks (cm h1)
DL (cm)
0.0002 0.0003
1.70 1.72
0.372 0.369
0.176 0.168
0.038 0.012
1.048 1.073
0.132 0.130
0.216 0.071
Fig. 6
Simplified profile of the vadose zone.
Fig. 7
The predicted curves of CODCr for 5 m deep PTA wastewater.
From Fig. 7, aer 20.87 years, the CODCr in PTA wastewater will reach the phreatic surface and aer 10 years, 20 years, 30 years, 40 years and 50 years, the CODcr in the phreatic surface (21 m deep) will be 0.16 mg L1, 0.16 mg L1, 5.23 mg L1, 153.85 mg L1 and 4000.00 mg L1, respectively. From Fig. 8, when the PTA wastewater is 7 m deep and the other conditions unchanged, in only 17.18 years, the CODcr in PTA wastewater will reach groundwater, and aer 10 years, 20 years, 30 years, 40 years and 50 years, the CODcr in the phreatic surface (21 m deep) will be 0.16 mg L1, 0.16 mg L1, 1.38 mg L1, 348.46 mg L1 and 4000 mg L1, respectively. When the PTA wastewater is 5 m deep, the CODcr concentration is 4000 mg L1 and the vadose zone is 21 m thick, and aer 20.87 years, the CODcr in PTA wastewater will reach the phreatic surface. However, when the PTA wastewater is 7 m deep and the other conditions unchanged, in only 17.18 years, the wastewater will reach groundwater.
Fig. 8 Predicted curves of CODcr for 7 m deep PTA wastewater.
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This study provides basic data for pollution prevention and remediation, as well as convenience for design management strategies to minimize the adverse impacts.
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Conclusions Detailed experimentation was undertaken to determine the parameters as well as to simulate the ow conditions through the column, and then simulate the contaminants in PTA wastewater transport in the vadose zone. It was noted that under ideal conditions, the adsorption quantity of soils for PTA wastewater increases with the contact time, and then becomes constant aer equilibrium is reached (aer 24 h). The largest adsorption quantity of silty clay and clayey soil are 0.66 mg g1 and 1.05 mg g1 at balance, respectively. The adsorption capacity of silty clay is corresponding to the theoretical value determined according to Langmuir isotherm model, but the experiment value of clayey soil is bigger than theory value, because actual process in theory is more complicated than that in ideal situation. So the actual adsorption capacity is greater than the theoretical value. The batch equilibrium data corresponds to the linear, Freundlich and Langmuir isotherm models, and all experimental data were very satisfactory with these isotherm models, due to the R2 value, which was between 0.900 and 0.999. Both the Langmuir and Freundlich isotherms models exhibited a good t to the sorption data of CODcr in the PTA wastewater. However, the model was much better for the Freundlich isotherm, and the value of Kf was 0.0019 and 0.0012 for silty clay and clayey soil, respectively, indicating a positive adsorption of CODcr in soil. The Sm for silty clay and clayey soil are 0.6659 mg g1 and 0.8017 mg g1, respectively. The distribution coefficient Kd for silty clay and clayey soil are 0.0002 L g1 and 0.0003 L g1, respectively. All the results show that the adsorption capacity of CODcr in PTA wastewater adsorption to silty clay and clayey soil is lower than the other adsorbents; thus, the pollutants in PTA wastewater can easily to migrate into groundwater. It also showed that the rst order degradation kinetic models for silty clay and clayey soil can be summarized as: C ¼ 737.14e0.0097t and C ¼ 766.23e0.0077t, respectively. The degradation coefficient was found to be quite low and indicated that CODcr in PTA wastewater is mobile and less degradable in the soil studied. PTA wastewater transport in the vadose zone was simulated by HYDRUS-1D, and the results showed that when PTA wastewater is 5 m deep, the CODcr concentration is 4000 mg L1 and the vadose zone is 21 m thick; moreover, aer 20.87 years the PTA wastewater will reach the phreatic surface. However, when the PTA wastewater is 7 m deep and the other conditions unchanged, wastewater will reach groundwater in only 17.18 years. This study will provide basic data for pollution prevention and remediation, as well as convenience for design management strategies to minimize the adverse impacts.
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Acknowledgements This work was supported by the National Science and Technology Support Project (Project number 2012BAJ11B04) and the Geological Survey Projects (Project number 12120114011701).
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