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An integrated approach for simultaneous immobilization of lead in both contaminated soil and groundwater: Laboratory test and numerical modeling
Accepted Manuscript Title: An integrated approach for simultaneous immobilization of lead in both contaminated soil and groundwater: Laboratory test and numerical modeling Authors: Yihan Dai, Yuan Liang, Xiaoyun Xu, Ling Zhao, Xinde Cao PII: DOI: Reference:
S0304-3894(17)30615-5 http://dx.doi.org/doi:10.1016/j.jhazmat.2017.08.023 HAZMAT 18788
To appear in:
Journal of Hazardous Materials
Received date: Revised date: Accepted date:
13-4-2017 19-7-2017 9-8-2017
Please cite this article as: Yihan Dai, Yuan Liang, Xiaoyun Xu, Ling Zhao, Xinde Cao, An integrated approach for simultaneous immobilization of lead in both contaminated soil and groundwater: Laboratory test and numerical modeling, Journal of Hazardous Materialshttp://dx.doi.org/10.1016/j.jhazmat.2017.08.023 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
An integrated approach for simultaneous immobilization of lead in both contaminated soil and groundwater: Laboratory test and numerical modeling
Yihan Dai1, Yuan Liang2, Xiaoyun Xu1, Ling Zhao1, Xinde Cao1, * 1
School of Environmental Science and Engineering, Shanghai Jiao Tong University,
Shanghai 200240, China 2
School of Environmental Science and Engineering, Suzhou University of Science
and Technology, Suzhou 215009, China
*
Corresponding author. Tel: +86 21 3420 2841. E-mail: [email protected].
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GRAPHICAL ABSTRACT
2
HIGHLIGHTS . Integrated approach for remediation of Pb-contaminated soil and groundwater. . High removal of Pb from groundwater in pre-amended contaminated surface soil. . High retention capacity and immobilization ability of Pb in phosphate-amended soil. . Deriving the profile distribution and treatment capacity of Pb via HYDRUS simulation.
Abstract In this study, we demonstrated the feasibility of an integrated remediation approach for simultaneous immobilization of Pb in both soil and groundwater. The laboratory test was conducted via column experiment by pumping Pb-contaminated groundwater into the pre-amended contaminated surface soils to identify its retention and immobilization ability of Pb. HYDRUS modeling was undertaken to simulate Pb distribution and permissible treatment capacity in the remediation. The experiment results showed that phosphate- and biochar-amended soils were highly effective in removing Pb from contaminated groundwater, with the removal reaching up to 94.2% and 84.5%, respectively. However, phosphate amendment was more effective in immobilizing Pb with TCLP extracted Pb reduced by 18.3%–51.5%, compared to the control, while the reduction for biochar amendment was less than 13.5%. The modeling indicated that phosphate-amended soil could immobilize 509 g Pb m-2 soil under the environmentally-relevant conditions, given both groundwater and soil quality criteria being met. Our study demonstrated that the integrated system with phosphate amendment is fairly feasible for simultaneous remediation of both Pb-contaminated soil and groundwater. 3
Keywords: Contaminated soil and groundwater; HYDRUS modeling; Immobilization; Integrated remediation method; Pb
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1. Introduction Soil and corresponding groundwater contamination is often closely related at a specific site. The heavy metals in soil may dissolve into aqueous phase of soil and seep through the alluviums and fractured zones, leading to groundwater contamination [1]. Meanwhile, due to the fluctuation of groundwater level, the heavy metals in contaminated groundwater could accumulate in soil, causing soil pollution [2]. With the heavy metal contamination of soil and groundwater becoming a matter of utmost concern to public health, a variety of methods have been developed for remediation of contaminated soil or groundwater. The common technologies for remediation of heavy metals contaminated soils include chemical immobilization [3], phytoremediation [4], and soil washing [5], et al. Among these remediation methods, chemical immobilization is a promising soil remediation technique with the advantages of simplicity, rapidity, low costs, and high applicability [6]. Chemical immobilization relies on addition of the soil amendments to help retain metals in the stable solid phase by sorption, precipitation, complexation, ion exchange or redox process, thereby decreasing mobility and bioavailability of metals [7]. Phosphorus-bearing materials [8] and biochar [9] are two widely applied amendments that have already been proven effective in immobilizing heavy metals in contaminated soils. Mignardi et al. [10] reported the high effectiveness of phosphate treatment for Cd, Cu, Pb, and Zn immobilization in mine waste soils. Ahmad et al. [11] indicated that application of biochar was effective in decreasing the phytoavailability of Pb and Sb in the studied army firing range soil. For groundwater cleanup, one of the most commonly used technologies is pump-and-treat system, which removes the contaminated groundwater by pumping and use of aboveground treatment facilities [12]. Pump-and-treat method has been 5
proven a simple and effective way for the remediation of contaminated groundwater [13]. Diels and Vanbroekhoven [14] showed that pump-and-treat method could efficiently remove Cd, Cr, and Zn from contaminated groundwater. Although a variety of methods have been developed for individual remediation of heavy metal contaminated soil or groundwater, these separate methods are often expensive and time-consuming at a specific site and sometimes less effective at achieving the proposed level of cleanup. Therefore, the high attention should be paid to develop the technology for simultaneously remediating both contaminated soil and groundwater. There have been a few reports about the simultaneous remediation of soil and groundwater contaminated by organic pollutants [15, 16]. Tsitonaki et al. [15] showed that persulfate could be used for in situ chemical oxidation of organic compounds both in the soil and groundwater. Paria [16] reported that organic contaminants in soil and groundwater could be removed through micellar solubilization and adsorption of surfactant. However, the technology used for simultaneous remediation of heavy metal contaminated soil and groundwater lacks of study. Our previous work proposed a method of coupling the chemical immobilization with the pump-and-treat method for simultaneously remediating heavy metal contaminated soil and groundwater [2]. In this approach, contaminated groundwater is pumped through a series of pipes and then introduced into top contaminated soil that was pretreated with amendments. The contaminants from water and soil are immobilized in the top soil and the treated water percolates gravitationally through the soil profile. In this paper, we attempted to provide practical guidance for engineering implementation of this integrated remediation approach. Numerical modeling was conducted for the potential to bridge the gap between laboratory test and engineering applications [17]. Among the software simulating solute movement, HYDRUS-1D 6
was widely used to model the transport of contaminants in soils and aquifers [18, 19]. Trenouth et al. [20] used HYDRUS-1D to simulate the treatment of heavy metals from storm water runoff by soil amendments and design a best thickness of amendments. The overall objective of this study was to demonstrate the feasibility of the proposed method under environmentally-relevant conditions via laboratory test and HYDRUS-1D modeling. Specifically, this study were: (i) to determine the retention ability of pre-amended soil for Pb from contaminated groundwater, (ii) to determine the immobilization and underlying mechanisms of Pb in soil and from groundwater, and (iii) to simulate the Pb distribution in soil profile and its remediating capacity of moderately Pb-contaminated soil and groundwater.
2. Materials and methods 2.1 Characterization of the soil and amendments materials The contaminated soil was collected from a site adjacent to a lead refinery metallurgical plant in Henan, China. The preliminary characterization showed that the soil was mainly contaminated by Pb, its concentration was 873 mg kg-1, excessively above the limit of 500 mg kg-1 for the China Environmental Quality Standard for Soil [21]. The amendments for Pb immobilization included P-bearing material (PT) and dairy manure-derived biochar (DM). The P-bearing material amendment was a mixture of phosphate mine tailing and triple superphosphate fertilizer with a 1:1 molar ratio of P [22]. The biochar was produced from dairy manure at 350 oC under O2-limited condition for 4 h [23]. The pH values of soil and amendment were measured by the pH/Ion 510 Bench Meter (Eutech instruments Pte Ltd/Okaon Instruments). Soil texture was analyzed 7
following the method provided by American Society for Testing and Materials [24]. Soil and amendments were digested using HNO3/H2O2 hot block digestion procedure [25]. Phosphorus in the digest was determined using the colorimetry method [26]. Concentration of heavy metals in the digest was determined using an atomic absorption spectrometry (AAS) (Jena AAS novAA350). Elemental (C, H, N) analyses on biochar was conducted using the CHNS/O Analyzer (Perkin Elmer, 2400 II). Selected physical and chemical properties of contaminated soil and amendments used in the experiment are presented in Table 1. 2.2 Soil pre-amendment Approximate 36 kg of the Pb-contaminated soil was prepared and divided equally into three parts. Two of them were mixed with 2 % (w/w) of PT and 5 % (w/w) of DM, respectively, and the third one was not treated with any amendments and designated as control. The application dosages of the amendments have been proved to be the most effective in immobilizing heavy metals in soils [27, 28]. All the control and treatments were incubated for 100 d with moisture content of 60–70 % of the maximum water holding capacity. 2.3 Laboratory-scale demonstration test 2.3.1. Soil packing in experimental device The experimental device (S1 and Figure S1 in supporting information) was made in triplicate for treatments and control. The bottom of each small cuboid was packed about 3 cm in height by 1.2 kg rough quartz sand (2-3 mm particle size) for filtrating the leaching water. Approximate 3 kg of each amended and unamended soil (about 12 cm in height) is packed above the rough quartz sand. 2.3.2. Leaching experiment To meet the study objectives, the highly contaminated groundwater was made 8
from deionized water by spiking Pb(NO3)2 at concentration of 250 mg L-1 Pb. The simulative groundwater was continuously delivered into soil by peristaltic pump at flow rate of 200~240 mL h-1. After the experiment commenced, the effluent water was collected regularly and its volume, pH value, and Pb concentration were measured and recorded for evaluating the treatment efficiency of contaminated groundwater. The breakthrough curves were plotted by the ratio of Pb concentration in the effluent to Pb concentration in influent against the loading volume. The treated amounts of Pb (Qt, g kg-1) by amended or unamended soils were calculated as follows: 𝑄𝑡 (𝑔 𝑘𝑔−1 ) =
𝑉 ∫0 𝑡 (𝐶0 −𝐶𝑡 )𝑑𝑉
(1)
1000∙𝑚
where Qt is Pb retention amount from groundwater (g kg-1), C0 is the Pb concentration in groundwater (mg L−1), Ct is the Pb concentration in effluent (mg L−1), Vt is the volume of collected effluent, which was supposed to be equal to the volume of influent (L), m is the weight of soil used in the column. To determine the immobilization effect of Pb in soil with different amendments, the soil was collected at 0-4 cm, 4-8 cm, and 8-12 cm layer from the columns at the end of the leaching test and subjected to toxicity characteristics leaching procedure (TCLP) test [25]. The TCLP method has been widely used to assess both the mobility of contaminants under simulated landfill condition and the performance of remediation techniques applied to contaminated soils [29]. Sequential extractions were executed for speciation analysis of the retained soil Pb following the BCR sequential extraction scheme as proposed by the Standards, Measurements and Testing Programme (formerly BCR) of the European Commission [30]. Meanwhile, X-ray diffraction (XRD) was used for soil characterization. XRD analysis can provide crystallographic composition. The XRD patterns were obtained on a Rigaku D/Max9
2550 PC (Rigaku Corporation, Japan) by scanning at a range from 20° to 55° at 2°/min. The total concentration of Pb in the soil was determined using AAS after the soil was digested by HNO3/H2O2 hot block method [25]. 2.4. Simulating the remediation of moderately Pb-contaminated soil and groundwater For the purpose of determining the Pb retention capacity and immobilization mechanism, a much higher Pb concentration (250 mg L-1) of contaminated groundwater was used in the laboratory test above. It is more realistic and urged to investigate remediation of moderately Pb-contaminated soil and groundwater. Thus, the HYDRUS-1D modeling was conducted under environmentally-relevant conditions in which the Pb concentrations of soil and input contaminated groundwater were set as 800 mg kg-1 and 10 mg L-1, respectively. The HYDRUS-1D is a software package for simulating water, heat and solute movement in one-dimensional variably-saturated media and has been verified against a large number of test cases [31]. In this simulation, a soil layer with the depth of 50 cm was set up. The top 20 cm was packed with a contaminated soil containing 800 mg kg-1 Pb; the bottom 30 cm was set as uncontaminated soil and without amendment; PT and DM were amended in the top 30 cm as the laboratory test (for future treatment of contaminated groundwater, the amended layer was 10 cm thicker than contaminated layer). Since the soil was supposed to be uniform in size and texture the single permeability model was used, and water transport was modeled using the van Genuchten–Mualem approach assuming no hysteresis effects. The water flux was set equal to 36 cm d-1. Solute transport simulation used the Crank–Nicolson time weighting scheme, and the chemical non-equilibrium two-site sorption model (TSM) was selected. The water flow parameters were obtained based on soil bulk density and percentages of sand, silt 10
and clay by Neural Network Predictions in HYDRUS-1D software. The longitudinal dispersivity was set as 1 cm according to the transport distance and the flow rate [32] and the immobile water content was set as 0 for it was negligible for transport under the used hydraulic conditions [33]. The adsorption isotherm coefficients in HYDRUS-1D modeling were obtained from batch sorption experiments (S2 in Supporting Information). At last, the dimensionless fraction of adsorption sites and first order rate coefficient were valued from the inverse solution of HYDRUS-1D using the laboratory test data (S3 in supporting information).
3. Results and discussion 3.1. Removal of Pb from groundwater during the integrated remediation test The breakthrough curves of Pb in the contaminated groundwater by amended or unamended soils are shown in Figure 1. After leaching of 200 L groundwater containing 250 mg L-1 Pb, Pb still remained unsaturated (C/C0<1) in soils due to its large adsorption capacity. PT and DM treatments prolonged the breakthrough points of Pb (C/C0= 0.05) which appeared at 150 L and 81.5 L in these two amended soils and increased by 129% and 24.6%, respectively, compared with 65.4 L for the control (Figure 1 and Table 2). The treated amounts of Pb retained in the PT- and DM-treated soil disposed from highly Pb-contaminated groundwater were 15.9 and 14.1 g kg-1, an increasing of 20.5% and 6.82%, respectively, compared to the control. The increase of Pb treatment amount in two amended soil could be attributed to the high affinity for Pb by phosphate materials and dairy manure-derived biochar [34, 35]. Correspondingly, Pb in the groundwater was removed by PT and DM treatments by up to 94.2% and 84.5%, respectively (Table 2). These results indicated that the removal capacity of Pb from contaminated groundwater by soil could be greatly 11
improved with PT and DM amendments. 3.2. Immobilization of Pb in soil during the integrated remediation test The concentrations of Pb in different depths of soils after 200 L loading of 250 mg L-1 Pb-containing groundwater are shown in Figure 2a. The retained Pb was mainly accumulated in the top 4 cm of soil columns, accounting for approximate 60% of the total Pb in groundwater. For PT- and DM-treated soils, the concentration of retained Pb in the top 4 cm soil increased from 23737 mg kg-1 in the control to 33087 and 28632 mg kg-1, increasing by 39.4% and 20.6%, respectively. The two amended soil showed high sorption capacities for Pb, thus retaining more groundwater Pb in the upper layer of soils. The soil pH increased from 7.2 to 7.9 by DM treatment (Table 1), which could enhance the affinity of soil surface for Pb by increasing negative surface charge and hydrolysis of Pb [36]. The increased affinity of PT-amended soil for Pb was mainly induced by Pb-P precipitation, which would be further discussed later. The immobilization effectiveness of Pb in the soils after contaminated groundwater loading was evaluated by TCLP extraction method and the results are shown in Figure 2b.The Pb leachability in TCLP extraction decreased by 18.3%, 32.5%, and 51.5% in 0–4 cm, 4–8 cm, and 8–12 cm of PT-treated soil, compared to the control. Note that the lowest decrease of TCLP-Pb in the 0–4 cm soil was due to its much higher concentration of Pb, which may be beyond the immobilization capability of amended soils [37]. The PT amendment could cause Pb in soils to shift from forms with high availability to more strongly bound Pb fractions, such as residual or insoluble fraction, then reducing its leachability. The BCR extraction analysis (Figure S2) showed prominent increase of residual fraction Pb in PT treatment from 7.05% to 20.4%, which could be attributed to the Pb–P precipitation [8]. The increasing of residual fraction was mainly from the decreasing of acid soluble 12
fraction including weakly absorbed Pb retained on the soil surface and carbonate form of Pb [38]. XRD analysis of PT-treated soil confirmed the formation of insoluble Pb-phosphate precipitation, in which the main peaks of Pb5(PO4)3OH or Pb5(PO4)3Cl appeared at 2θ= 29.8 and 30.1 (Figure 3). In fact, pyromorphite-like minerals (Pb5(PO4)3X, X=Cl, OH, F) have been proven to be the most stable environmental Pb compounds under a wide range of pH and Eh natural conditions [8]. In addition, the formation of Pb3(PO4)2 and PbHPO4 were also found in XRD patterns at 2θ= 29.0 and 2θ= 31.2, 33.0 and 35.5, respectively (Figure 3), and it was also responsible for the reduction of Pb leachability according to Cao et al. [39]. Like PT-treated soil, the leachability of Pb in the DM-treated soil was also reduced by 5.05%–13.5% in the three layer soils, compare to the control (Figure 2b). XRD patterns of DM-treated soils (Figure 3) showed the main peak of PbSO4 at 2θ= 27.6, the mineral is a strongly bound Pb fractions [40]. There was a possible formation of Pb5(PO4)3OH or Pb5(PO4)3Cl appearing at 2θ= 29.8 and 30.1 (Figure 3). The DM biochar contained as much as 0.64% P (Table 1), which favors the precipitation of Pb phosphate minerals [27]. In addition, the remarkable increase of reducible fraction Pb from 25.7% to 33.5% in BCR analysis (Figure S2) indicated the formation of Fe/Mn oxides bound Pb resulted from the high concentration of Fe (6,160 mg kg-1) and Mn (440 mg kg-1) in DM biochar (Table 1) [36]. Besides, surface complexation with active carboxyl and hydroxyl functional groups of the biochar was also the possible mechanisms for Pb immobilization by biochar [41], which might be responsible for the increase of oxidizable fraction Pb from 2.85% in control to 7.39% in DM-amended soil (Figure S2) [38]. It should be noted that although DM treatment retained much Pb in the soil, the stability of retained Pb was lower than that in PT treatment in terms of higher TCLP leachability (34.0–73.2% vs 18.9–64.7%) (Figure 13
2b). The less stability could be attributed to Pb in the DM-treated soil existing as the forms with relatively lower stability, such as PbSO4, Pb bound to Fe/Mn oxides, and Pb complexed with O-containing groups, these Pb forms could be readily extracted in the acidic TCLP fluid [25]. Overall, the results from lab demonstration test above indicated that PT and DM amendment could not only efficiently remove Pb from contaminated groundwater, but also have the ability to immobilize those Pb in the soil, especially for PT amendment, showing a prospect of simultaneously remediating Pb-contaminated soil and groundwater. 3.3. Prospective modeling of the integrated remediation system The inverse solution of HYDRUS-1D showed a good performance in fitting the cumulative system losses for Pb in the column leachate (R2=0.930-0.999, Figure S4) with the chemical non-equilibrium two-site sorption model (TSM). The selected TSM model has been validated in many studies [18, 19] for effectively modeling the sorption and transport of heavy metals in soils. Chotpantarat et al. [18] found that the breakthrough curves were well described by the TSM model in both binary and multi-metal systems. According to Santos and his co-workers [19], simulated transfers were found more realistic in metal-contaminated agricultural soils when using the TSM model. With solute transport parameters obtained in the inverse solution (S3 in supporting information), the remediation of a moderately Pb-contaminated soil (800 mg kg-1) and groundwater (10 mg L-1) site by the integrated approach was simulated by HYDRUS-1D. A suite of simulations on the dissolved Pb concentration in 50 cm soils in depth was undertaken to elucidate the potential treatment capacity of Pb-contaminated groundwater (Figure 4a). According to China Quality Standard for 14
Ground Water [42] in which the standard for Pb is 0.01 mg L-1, PT- and DM-amended soils could continuously treat Pb-contaminated groundwater for 2328 h and 1632 h before the Pb effluent concentration in 50 cm depth exceed the standard, much higher than that of unamended soil (1415 h). By these times, the accumulated retention of Pb in top 30 cm PT- and DM-amended soil was greatly increased by 43.5% and 12.5%, respectively, compared with the control (Figure 4b). Especially in the PT-amended soil, the retention capacity of Pb under the infiltration of 10 mg L-1 Pb-contaminated groundwater reached up to 3745 mg kg-1, higher than that of DM-amended soil (3141 mg kg-1) and about 1.5 times that in unamended soil (2557 mg kg-1). As a result, the potential treatment capacities of 10 mg L-1 Pb-contaminated groundwater in PT- and DM-amended soil were approximately 35 t m-2 and 24 t m-2 (in term of contaminated groundwater flux), an increasing of 64.5% and 15.3%, respectively, compared with the control. These simulations demonstrated the effectiveness of PT and DM amendments to shrink the groundwater pollution of Pb. Besides, the stability of Pb retained in soil after treating contaminated groundwater should also be evaluated in calculating the potential remediating capacities of this integrated system. TCLP extraction tests were used to evaluate the potential stability of soil Pb in above simulations (S4 in Supporting Information). The TCLP-Pb and total-Pb showed a good liner relation (R2=0.994 and 0.999 for PT and DM, respectively) (Figure 5). Therefore, the thresholds for total-Pb in PT- and DM-amended soils could be derived as about 4014 mg kg-1 and 873 mg kg-1, respectively, based on the TCLP regulatory limit of 5 mg L-1 [43]. For unamended soil containing 800 mg kg-1, its TCLP-Pb was 6.09 mg L-1 which already exceeded the 5 mg L-1 limit. This clearly showed that unamended soil didn’t have sufficient immobilization ability in spite of the high retention capacity (Figure 4b). In PT-amended soil, the total-Pb obtained from the 15
simulations was 3745 mg kg-1, much lower than the derived threshold (4014 mg kg-1). It indicated that not only the treated water percolated downward could be clean, the Pb retained in PT-amended soil was also stable enough. Therefore, the PT amendment was favorable to be used in this integrated system. By calculation, the eventual remediating capacity of Pb (both Pb retained from groundwater and already exist in soil) in this integrated system with PT amendment was about 509 g Pb m-2 soil. However, in DM-amended soil, the soil total Pb reached up to 3141 mg kg-1 (Figure 4b), which was far beyond the threshold of 873 mg kg-1 derived from TCLP tests, the TCLP-Pb was 25.9 mg L-1, much higher than the 5 mg L-1 limit. This indicated that the Pb retained in the DM-treated soil was not stable enough. As a fact, the initial DM-amended soil had TCLP-Pb (about 4.33 mg L-1) close to the 5 mg L-1 limit and could not afford further groundwater loading. The poor stability of Pb in DM-amended soil made it not suitable for further treating Pb-contaminated groundwater after remediating contaminated soil.
4. Conclusions Both phosphate- and biochar-treated soils were highly effective in removing Pb from contaminated groundwater. However, the Pb retained in biochar-amended soil could be present as PbSO4, Pb bound to Fe/Mn oxides in biochar, and Pb complexed with O-containing groups of biochar. These Pb forms are less stable, showing the limited immobilization ability. Therefore, biochar-amended soil was not suitable for loading much extra Pb from contaminated groundwater after soil Pb remediation. On the contrary, phosphate-amended soil possessed both high Pb retention capacity and immobilization ability due to the formation of insoluble Pb-phosphate precipitates, thus making it be more applicable to simultaneously remediate Pb-contaminated soil 16
and groundwater in this integrated remediation approach. The modeling suggested that the integrated approach with phosphate amendment was sufficient to meet the treatment requirements to Pb-contaminated soil and groundwater site in terms of the potential treatment capacity of this integrated system. Both laboratory test and numerical modeling demonstrated that the integrated remediation approach with phosphate amendment was fairly feasible to remediate Pb-contaminated soil and groundwater simultaneously.
Acknowledgments This work was supported in part by China Ministry of Environmental Protection (No. 201509035), National Natural Science Foundation of China (No. 21537002, 21377081, 21607099), and Key Laboratory of Bio-organic fertilizer Creation of Ministry of Agriculture (No. BOFC2015KA04).
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Reference [1] H. El Khalil, O. El Hamiani, G. Bitton, N. Ouazzani, A. Boularbah, Heavy metal contamination from mining sites in South Morocco: Monitoring metal content and toxicity of soil runoff and groundwater, Environmental Monitoring and Assessment, 136 (2008) 147-160. [2] Y. Liang, X. Cao, L. Zhao, E. Arellano, Biochar- and phosphate-induced immobilization of heavy metals in contaminated soil and water: implication on simultaneous remediation of contaminated soil and groundwater, Environmental Science and Pollution Research, 21 (2014) 4665-4674. [3] P. Castaldi, L. Santona, P. Melis, Heavy metal immobilization by chemical amendments in a polluted soil and influence on white lupin growth, Chemosphere, 60 (2005) 365-371. [4] R.L. Chaney, C.L. Broadhurst, T. Centofanti, Phytoremediation of soil metals, Current Opinion in Biotechnology, 8 (1997) 279-284. [5] G. Dermont, M. Bergeron, G. Mercier, M. Richer-Lafleche, Soil washing for metal removal: A review of physical/chemical technologies and field applications, Journal of Hazardous Materials, 152 (2008) 1-31. [6] R.A. Wuana, F.E. Okieimen, Heavy Metals in Contaminated Soils: A Review of Sources, Chemistry, Risks and Best Available Strategies for Remediation, Isrn Ecology, 2011 (2011). [7] J. Kumpiene, A. Lagerkvist, C. Maurice, Stabilization of As, Cr, Cu, Pb and Zn in soil using amendments - A review, Waste Management, 28 (2008) 215-225. [8] P. Miretzky, A. Fernandez-Cirelli, Phosphates for Pb immobilization in soils: a review, Environmental Chemistry Letters, 6 (2008) 121-133. [9] M. Ahmad, A.U. Rajapaksha, J.E. Lim, M. Zhang, N. Bolan, D. Mohan, M. Vithanage, S.S. Lee, Y.S. Ok, Biochar as a sorbent for contaminant management in soil and water: A review, Chemosphere, 99 (2014) 19-33. [10] S. Mignardi, A. Corami, V. Ferrini, Evaluation of the effectiveness of phosphate treatment for the remediation of mine waste soils contaminated with Cd, Cu, Pb, and Zn, Chemosphere, 86 (2012) 354-360. [11] M. Ahmad, S.S. Lee, J.E. Lim, S.-E. Lee, J.S. Cho, D.H. Moon, Y. Hashimoto, Y.S. Ok, Speciation and phytoavailability of lead and antimony in a small arms range soil amended with 18
mussel shell, cow bone and biochar: EXAFS spectroscopy and chemical extractions, Chemosphere, 95 (2014) 433-441. [12] M.R. Higgins, T.M. Olson, Life-Cycle Case Study Comparison of Permeable Reactive Barrier versus Pump-and-Treat Remediation, Environmental Science & Technology, 43 (2009) 9432-9438. [13] M.O. Rivett, S.W. Chapman, R.M. Allen-King, S. Feenstra, J.A. Cherry, Pump-and-treat remediation of chlorinated solvent contamination at a controlled field-experiment site, Environmental Science & Technology, 40 (2006) 6770-6781. [14] L. Diels, K. Vanbroekhoven, Remediation of Metal and Metalloid Contaminated Groundwater, Springer Netherlands, 2008. [15] A. Tsitonaki, B. Petri, M. Crimi, H. Mosbaek, R.L. Siegrist, P.L. Bjerg, In Situ Chemical Oxidation of Contaminated Soil and Groundwater Using Persulfate: A Review, Critical Reviews in Environmental Science and Technology, 40 (2010) 55-91. [16] S. Paria, Surfactant-enhanced remediation of organic contaminated soil and water, Advances in Colloid and Interface Science, 138 (2008) 24-58. [17] N.T. Basta, S.L. Mcgowen, Evaluation of chemical immobilization treatments for reducing heavy metal transport in a smelter-contaminated soil, Environmental Pollution, 127 (2004) 73-82. [18] S. Chotpantarat, S.K. Ong, C. Sutthirat, K. Osathaphan, Competitive modeling of sorption and transport of Pb2+, Ni2+, Mn2+ AND Zn2+ under binary and multi-metal systems in lateritic soil columns, Geoderma, 189-190 (2012) 278-287. [19] D.R.D. Santos, P. Cambier, F.J.K. Mallmann, J. Labanowski, I. Lamy, D. Tessier, F.V. Oort, Prospective modeling with Hydrus-2D of 50 years Zn and Pb movements in low and moderately metal-contaminated agricultural soils, Journal of Contaminant Hydrology, 145 (2013) 54-66. [20] W.R. Trenouth, B. Gharabaghi, Soil amendments for heavy metals removal from stormwater runoff discharging to environmentally sensitive areas, Journal of Hydrology, 529 (2015) 1478-1487. [21] SAC, Environmental Quality Standard for Soil (GB15618-1995), Standardization Administration of the People's Republic of China (SAC), Beijing, China, 1995. [22] Y. Fang, X. Cao, L. Zhao, Effects of phosphorus amendments and plant growth on the 19
mobility of Pb, Cu, and Zn in a multi-metal-contaminated soil, Environmental Science and Pollution Research, 19 (2012) 1659-1667. [23] X. Cao, W. Harris, Properties of dairy-manure-derived biochar pertinent to its potential use in remediation, Bioresource Technology, 101 (2010) 5222-5228. [24] ASTM, Annual book of ASTM standards, section 4: Construction Vol. 04.08. Soil and rock; dimension stone; geosynthetics, ASTM, (1993). [25] EPA, Test Method for Evaluating Solid Waste: SW- 846. Third edition, Environmental Protection Agency, Office of Solid Waste and Emergency Response, EPA, (2010). [26] A.L. Page, Methods of soil analysis. Part 2. Chemical and microbiological properties, Wi American Society of Agronomy Inc & Soil Science Society of America Inc, (1982). [27] X. Cao, L. Ma, Y. Liang, B. Gao, W. Harris, Simultaneous Immobilization of Lead and Atrazine in Contaminated Soils Using Dairy-Manure Biochar, Environmental Science & Technology, 45 (2011) 4884-4889. [28] X. Cao, Y. Liang, L. Zhao, H. Le, Mobility of Pb, Cu, and Zn in the phosphorus-amended contaminated soils under simulated landfill and rainfall conditions, Environmental Science and Pollution Research, 20 (2013) 5913-5921. [29] D.C. Tsang, W.E. Olds, P.A. Weber, A.C. Yip, Soil stabilisation using AMD sludge, compost and lignite: TCLP leachability and continuous acid leaching, Chemosphere, 93 (2013) 2839-2847. [30] G. Rauret, J.F. López-Sánchez, A. Sahuquillo, R. Rubio, C. Davidson, A. Ure, P. Quevauviller, Improvement of the BCR three step sequential extraction procedure prior to the certification of new sediment and soil reference materials, Journal of Environmental Monitoring Jem, 1 (1999) 57. [31] J. Simunek, M.T. van Genuchten, M. Sejna, Development and applications of the HYDRUS and STANMOD software packages and related codes, Vadose Zone Journal, 7 (2008) 587-600. [32] J. Vanderborght, H. Vereecken, Review of dispersivities for transport modeling in soils, Vadose Zone Journal, 6 (2007) 29-52. [33] S. Jellali, E. Diamantopoulos, H. Kallali, S. Bennaceur, M. Anane, N. Jedidi, Dynamic sorption of ammonium by sandy soil in fixed bed columns: Evaluation of equilibrium and non-equilibrium transport processes, Journal of Environmental Management, 91 (2010) 897-905. 20
[34] X.D. Cao, L.Q. Ma, D.R. Rhue, C.S. Appel, Mechanisms of lead, copper, and zinc retention by phosphate rock, Environmental Pollution, 131 (2004) 435-444. [35] X. Xu, X. Cao, L. Zhao, Comparison of rice husk- and dairy manure-derived biochars for simultaneously removing heavy metals from aqueous solutions: Role of mineral components in biochars, Chemosphere, 92 (2013) 955-961. [36] T.-Y. Jiang, J. Jiang, R.-K. Xu, Z. Li, Adsorption of Pb(II) on variable charge soils amended with rice-straw derived biochar, Chemosphere, 89 (2012) 249-256. [37] R. Melamed, X. Cao, M. Chen, L.Q. Ma, Field assessment of lead immobilization in a contaminated soil after phosphate application, Science of the Total Environment, 305 (2003) 117. [38] L. Rodríguez, E. Ruiz, J. Alonso-Azcárate, J. Rincón, Heavy metal distribution and chemical speciation in tailings and soils around a Pb–Zn mine in Spain, Journal of Environmental Management, 90 (2009) 1106. [39] X. Cao, D. Dermatas, X. Xu, G. Shen, Immobilization of lead in shooting range soils by means of cement, quicklime, and phosphate amendments, Environmental Science and Pollution Research, 15 (2008) 120-127. [40] M.V. Ruby, A. Davis, A. Nicholson, In situ formation of lead phosphates in soils as a method to immobilize lead, Environmental Science & Technology, 28 (1994) 646. [41] H. Lu, W. Zhang, Y. Yang, X. Huang, S. Wang, R. Qiu, Relative distribution of Pb2+ sorption mechanisms by sludge-derived biochar, Water Research, 46 (2012) 854-862. [42] AQSIQ, Quality Standards for Ground Water (GB/T 14848-1993), General Administration of Quality Supervision, Inspection and Quarantine of the People's Republic of China (AQSIQ), Beijing, China, (1993). [43] Federal Register. Rules and regulations. 55 (61), (1990), 11863-11877.
21
Table 1 Selected physico-chemical properties of soils, groundwater, and amendments Contaminated Contaminated Amendments soil
groundwater
PT a
DM
pH
7.2
6.0
7.5
9.1
Sand+silt (2-2,000 μm)
69.9 %
—b
—
—
Clay (<2 μm)
18.9 %
—
—
—
P
0.045 mg kg-1 —
17.0 %
0.64 %
Pb
873 mg kg-1
250 mg L-1
2.80 mg kg-1
BDL
Cd
6.50 mg kg-1
—
BDLc
BDL
Zn
287 mg kg-1
—
228 mg kg-1
523 mg kg-1
Fe
—
—
—
Mn
—
—
—
440 mg kg-1
C
—
—
—
44.7 %
H
—
—
—
2.20 %
O
—
—
—
2.00 %
S
—
—
—
0.58 %
a
6,160
mg
kg-1
PT and DM mean phosphate amendment and dairy manure derived biochar
amendment, respectively. b
Not determined
c
Below detection limit
22
Table 2 The treatment effectiveness of pre-amended soil for the Pb-contaminated groundwater in the integrated remediation test PT-soila
DM-soil
CK-soil
The volume of treated groundwater (L)
202
200
200
The volume of breakthrough points (L)
150
81.5
65.4
The amounts retained in soil from
15.9
14.1
13.2
94.2
84.5
79.5
groundwater (g kg-1) Removal rate of Pb in groundwater (%) a
CK-soil, PT-soil, and DM-soil represent the control, PT-amended soil, and
DM-amended soil, respectively.
23
FIGURE CAPTIONS
Figure 1. Breakthrough curves of Pb with contaminated groundwater loading in the unamended and amended soils. Original soil Pb: 873 mg kg-1, original groundwater Pb: 250 mg L-1 Figure 2. Concentration of Pb in different depths of soils (a) and leachability of Pb in TCLP extract of soils (b) after loaded with 200 L groundwater containing 250 mg L-1 Pb Figure 3. XRD patterns of the soils with different treatments, 1: Pb5(PO4)3OH or Pb5(PO4)3Cl; 2: PbHPO4; 3: Pb3(PO4)2; 4: PbSO4 Figure 4. HYDRUS-1D simulated dissolved Pb concentration in the leachate through 50 cm depth of amended and unamended soils during the Pb-contaminated groundwater infiltration (a) and the depth-discrete Pb accumulation profiles in PT-, DM-amended and unamended soils after 2328-h, 1632-h and 1415-h Pb-contaminated groundwater remediation, respectively (b). The dissolved Pb concentration exceeded China Quality Standard for Ground Water [42] at 2328 h, 1632 h, and 1415h for PT-, DM-amended and unamended soils, respectively Figure 5. Pb concentrations in TCLP leachate versus different total Pb concentrations in soil. The TCLP-Pb in PT- and DM-amended soil might exceed the TCLP regulatory limit for Pb (5 mg L-1) if their total Pb concentration reached up to 4014 mg kg-1 and 873 mg kg-1, respectively
24
0.5
PT DM CK C/C0= 0.05
Pb (C/C0)
0.4
0.3
0.2
0.1
0.0 0
50
100
150
200
Volume (L) Figure 1. Breakthrough curves of Pb with contaminated groundwater loading in the unamended and amended soils. Original soil Pb: 873 mg kg-1, original groundwater Pb: 250 mg L-1
25
a
35000
PT DM CK
Soil Pb (mg/kg)
30000
25000
10000
5000
0
0~4cm
4~8cm
8~12cm
90
b
80
PT DM CK
TCLP-Pb (%)
70 60 50 40 30 20 10 0
0~4cm
4~8cm
8~12cm
Figure 2. Concentration of Pb in different depths of soils (a) and leachability of Pb in TCLP extract of soils (b) after loaded with 200 L groundwater containing 250 mg L-1 Pb
26
4
3
3 2
PT
2 1 1
1
2
2 1
DM
CK
20
22
24
26
28
30
32
34
36
Two-Theta (deg) Figure 3. XRD patterns of the soils with different treatments, 1: Pb5(PO4)3OH or Pb5(PO4)3Cl; 2: PbHPO4; 3: Pb3(PO4)2; 4: PbSO4
27
PT DM CK Quality standard
-1
Groundwater Pb (mg L )
0.010
0.008
0.006
0.004
0.002
a 0.000 0
500
1000
1500
2000
2500
Time (hours) -1
Soil Pb (mg kg ) 0
500
1000
1500
2000
2500
3000
3500
4000
0
Depth (cm)
-10
-20
PT DM CK
-30
-40
b -50
Figure 4. HYDRUS-1D simulated dissolved Pb concentration in the leachate through 50 cm depth of amended and unamended soils during the Pb-contaminated groundwater infiltration (a) and the depth-discrete Pb accumulation profiles in PT-, DM-amended and unamended soils after 2328-h, 1632-h and 1415-h Pb-contaminated groundwater remediation, respectively (b). The dissolved Pb concentration exceeded China Quality Standard for Ground Water [42] at 2328 h, 1632 h, and 1415h for PT-, DM-amended and unamended soils, respectively
28
20 18
PT fitted line for PT DM fitted line for DM TCLP limit
-1
TCLP-Pb (mg L )
16 14 12 10 8 6 4 2 0 0
1000
2000
3000
4000
5000
6000
7000
8000
-1
Total-Pb (mg kg ) Figure 5. Pb concentrations in TCLP leachate versus different total Pb concentrations in soil. The TCLP-Pb in PT- and DM-amended soil might exceed the TCLP regulatory limit for Pb (5 mg L-1) if their total Pb concentration reached up to 4014 mg kg-1 and 873 mg kg-1, respectively.
29
S0304-3894(17)30615-5 http://dx.doi.org/doi:10.1016/j.jhazmat.2017.08.023 HAZMAT 18788
To appear in:
Journal of Hazardous Materials
Received date: Revised date: Accepted date:
13-4-2017 19-7-2017 9-8-2017
Please cite this article as: Yihan Dai, Yuan Liang, Xiaoyun Xu, Ling Zhao, Xinde Cao, An integrated approach for simultaneous immobilization of lead in both contaminated soil and groundwater: Laboratory test and numerical modeling, Journal of Hazardous Materialshttp://dx.doi.org/10.1016/j.jhazmat.2017.08.023 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
An integrated approach for simultaneous immobilization of lead in both contaminated soil and groundwater: Laboratory test and numerical modeling
Yihan Dai1, Yuan Liang2, Xiaoyun Xu1, Ling Zhao1, Xinde Cao1, * 1
School of Environmental Science and Engineering, Shanghai Jiao Tong University,
Shanghai 200240, China 2
School of Environmental Science and Engineering, Suzhou University of Science
and Technology, Suzhou 215009, China
*
Corresponding author. Tel: +86 21 3420 2841. E-mail: [email protected].
1
GRAPHICAL ABSTRACT
2
HIGHLIGHTS . Integrated approach for remediation of Pb-contaminated soil and groundwater. . High removal of Pb from groundwater in pre-amended contaminated surface soil. . High retention capacity and immobilization ability of Pb in phosphate-amended soil. . Deriving the profile distribution and treatment capacity of Pb via HYDRUS simulation.
Abstract In this study, we demonstrated the feasibility of an integrated remediation approach for simultaneous immobilization of Pb in both soil and groundwater. The laboratory test was conducted via column experiment by pumping Pb-contaminated groundwater into the pre-amended contaminated surface soils to identify its retention and immobilization ability of Pb. HYDRUS modeling was undertaken to simulate Pb distribution and permissible treatment capacity in the remediation. The experiment results showed that phosphate- and biochar-amended soils were highly effective in removing Pb from contaminated groundwater, with the removal reaching up to 94.2% and 84.5%, respectively. However, phosphate amendment was more effective in immobilizing Pb with TCLP extracted Pb reduced by 18.3%–51.5%, compared to the control, while the reduction for biochar amendment was less than 13.5%. The modeling indicated that phosphate-amended soil could immobilize 509 g Pb m-2 soil under the environmentally-relevant conditions, given both groundwater and soil quality criteria being met. Our study demonstrated that the integrated system with phosphate amendment is fairly feasible for simultaneous remediation of both Pb-contaminated soil and groundwater. 3
Keywords: Contaminated soil and groundwater; HYDRUS modeling; Immobilization; Integrated remediation method; Pb
4
1. Introduction Soil and corresponding groundwater contamination is often closely related at a specific site. The heavy metals in soil may dissolve into aqueous phase of soil and seep through the alluviums and fractured zones, leading to groundwater contamination [1]. Meanwhile, due to the fluctuation of groundwater level, the heavy metals in contaminated groundwater could accumulate in soil, causing soil pollution [2]. With the heavy metal contamination of soil and groundwater becoming a matter of utmost concern to public health, a variety of methods have been developed for remediation of contaminated soil or groundwater. The common technologies for remediation of heavy metals contaminated soils include chemical immobilization [3], phytoremediation [4], and soil washing [5], et al. Among these remediation methods, chemical immobilization is a promising soil remediation technique with the advantages of simplicity, rapidity, low costs, and high applicability [6]. Chemical immobilization relies on addition of the soil amendments to help retain metals in the stable solid phase by sorption, precipitation, complexation, ion exchange or redox process, thereby decreasing mobility and bioavailability of metals [7]. Phosphorus-bearing materials [8] and biochar [9] are two widely applied amendments that have already been proven effective in immobilizing heavy metals in contaminated soils. Mignardi et al. [10] reported the high effectiveness of phosphate treatment for Cd, Cu, Pb, and Zn immobilization in mine waste soils. Ahmad et al. [11] indicated that application of biochar was effective in decreasing the phytoavailability of Pb and Sb in the studied army firing range soil. For groundwater cleanup, one of the most commonly used technologies is pump-and-treat system, which removes the contaminated groundwater by pumping and use of aboveground treatment facilities [12]. Pump-and-treat method has been 5
proven a simple and effective way for the remediation of contaminated groundwater [13]. Diels and Vanbroekhoven [14] showed that pump-and-treat method could efficiently remove Cd, Cr, and Zn from contaminated groundwater. Although a variety of methods have been developed for individual remediation of heavy metal contaminated soil or groundwater, these separate methods are often expensive and time-consuming at a specific site and sometimes less effective at achieving the proposed level of cleanup. Therefore, the high attention should be paid to develop the technology for simultaneously remediating both contaminated soil and groundwater. There have been a few reports about the simultaneous remediation of soil and groundwater contaminated by organic pollutants [15, 16]. Tsitonaki et al. [15] showed that persulfate could be used for in situ chemical oxidation of organic compounds both in the soil and groundwater. Paria [16] reported that organic contaminants in soil and groundwater could be removed through micellar solubilization and adsorption of surfactant. However, the technology used for simultaneous remediation of heavy metal contaminated soil and groundwater lacks of study. Our previous work proposed a method of coupling the chemical immobilization with the pump-and-treat method for simultaneously remediating heavy metal contaminated soil and groundwater [2]. In this approach, contaminated groundwater is pumped through a series of pipes and then introduced into top contaminated soil that was pretreated with amendments. The contaminants from water and soil are immobilized in the top soil and the treated water percolates gravitationally through the soil profile. In this paper, we attempted to provide practical guidance for engineering implementation of this integrated remediation approach. Numerical modeling was conducted for the potential to bridge the gap between laboratory test and engineering applications [17]. Among the software simulating solute movement, HYDRUS-1D 6
was widely used to model the transport of contaminants in soils and aquifers [18, 19]. Trenouth et al. [20] used HYDRUS-1D to simulate the treatment of heavy metals from storm water runoff by soil amendments and design a best thickness of amendments. The overall objective of this study was to demonstrate the feasibility of the proposed method under environmentally-relevant conditions via laboratory test and HYDRUS-1D modeling. Specifically, this study were: (i) to determine the retention ability of pre-amended soil for Pb from contaminated groundwater, (ii) to determine the immobilization and underlying mechanisms of Pb in soil and from groundwater, and (iii) to simulate the Pb distribution in soil profile and its remediating capacity of moderately Pb-contaminated soil and groundwater.
2. Materials and methods 2.1 Characterization of the soil and amendments materials The contaminated soil was collected from a site adjacent to a lead refinery metallurgical plant in Henan, China. The preliminary characterization showed that the soil was mainly contaminated by Pb, its concentration was 873 mg kg-1, excessively above the limit of 500 mg kg-1 for the China Environmental Quality Standard for Soil [21]. The amendments for Pb immobilization included P-bearing material (PT) and dairy manure-derived biochar (DM). The P-bearing material amendment was a mixture of phosphate mine tailing and triple superphosphate fertilizer with a 1:1 molar ratio of P [22]. The biochar was produced from dairy manure at 350 oC under O2-limited condition for 4 h [23]. The pH values of soil and amendment were measured by the pH/Ion 510 Bench Meter (Eutech instruments Pte Ltd/Okaon Instruments). Soil texture was analyzed 7
following the method provided by American Society for Testing and Materials [24]. Soil and amendments were digested using HNO3/H2O2 hot block digestion procedure [25]. Phosphorus in the digest was determined using the colorimetry method [26]. Concentration of heavy metals in the digest was determined using an atomic absorption spectrometry (AAS) (Jena AAS novAA350). Elemental (C, H, N) analyses on biochar was conducted using the CHNS/O Analyzer (Perkin Elmer, 2400 II). Selected physical and chemical properties of contaminated soil and amendments used in the experiment are presented in Table 1. 2.2 Soil pre-amendment Approximate 36 kg of the Pb-contaminated soil was prepared and divided equally into three parts. Two of them were mixed with 2 % (w/w) of PT and 5 % (w/w) of DM, respectively, and the third one was not treated with any amendments and designated as control. The application dosages of the amendments have been proved to be the most effective in immobilizing heavy metals in soils [27, 28]. All the control and treatments were incubated for 100 d with moisture content of 60–70 % of the maximum water holding capacity. 2.3 Laboratory-scale demonstration test 2.3.1. Soil packing in experimental device The experimental device (S1 and Figure S1 in supporting information) was made in triplicate for treatments and control. The bottom of each small cuboid was packed about 3 cm in height by 1.2 kg rough quartz sand (2-3 mm particle size) for filtrating the leaching water. Approximate 3 kg of each amended and unamended soil (about 12 cm in height) is packed above the rough quartz sand. 2.3.2. Leaching experiment To meet the study objectives, the highly contaminated groundwater was made 8
from deionized water by spiking Pb(NO3)2 at concentration of 250 mg L-1 Pb. The simulative groundwater was continuously delivered into soil by peristaltic pump at flow rate of 200~240 mL h-1. After the experiment commenced, the effluent water was collected regularly and its volume, pH value, and Pb concentration were measured and recorded for evaluating the treatment efficiency of contaminated groundwater. The breakthrough curves were plotted by the ratio of Pb concentration in the effluent to Pb concentration in influent against the loading volume. The treated amounts of Pb (Qt, g kg-1) by amended or unamended soils were calculated as follows: 𝑄𝑡 (𝑔 𝑘𝑔−1 ) =
𝑉 ∫0 𝑡 (𝐶0 −𝐶𝑡 )𝑑𝑉
(1)
1000∙𝑚
where Qt is Pb retention amount from groundwater (g kg-1), C0 is the Pb concentration in groundwater (mg L−1), Ct is the Pb concentration in effluent (mg L−1), Vt is the volume of collected effluent, which was supposed to be equal to the volume of influent (L), m is the weight of soil used in the column. To determine the immobilization effect of Pb in soil with different amendments, the soil was collected at 0-4 cm, 4-8 cm, and 8-12 cm layer from the columns at the end of the leaching test and subjected to toxicity characteristics leaching procedure (TCLP) test [25]. The TCLP method has been widely used to assess both the mobility of contaminants under simulated landfill condition and the performance of remediation techniques applied to contaminated soils [29]. Sequential extractions were executed for speciation analysis of the retained soil Pb following the BCR sequential extraction scheme as proposed by the Standards, Measurements and Testing Programme (formerly BCR) of the European Commission [30]. Meanwhile, X-ray diffraction (XRD) was used for soil characterization. XRD analysis can provide crystallographic composition. The XRD patterns were obtained on a Rigaku D/Max9
2550 PC (Rigaku Corporation, Japan) by scanning at a range from 20° to 55° at 2°/min. The total concentration of Pb in the soil was determined using AAS after the soil was digested by HNO3/H2O2 hot block method [25]. 2.4. Simulating the remediation of moderately Pb-contaminated soil and groundwater For the purpose of determining the Pb retention capacity and immobilization mechanism, a much higher Pb concentration (250 mg L-1) of contaminated groundwater was used in the laboratory test above. It is more realistic and urged to investigate remediation of moderately Pb-contaminated soil and groundwater. Thus, the HYDRUS-1D modeling was conducted under environmentally-relevant conditions in which the Pb concentrations of soil and input contaminated groundwater were set as 800 mg kg-1 and 10 mg L-1, respectively. The HYDRUS-1D is a software package for simulating water, heat and solute movement in one-dimensional variably-saturated media and has been verified against a large number of test cases [31]. In this simulation, a soil layer with the depth of 50 cm was set up. The top 20 cm was packed with a contaminated soil containing 800 mg kg-1 Pb; the bottom 30 cm was set as uncontaminated soil and without amendment; PT and DM were amended in the top 30 cm as the laboratory test (for future treatment of contaminated groundwater, the amended layer was 10 cm thicker than contaminated layer). Since the soil was supposed to be uniform in size and texture the single permeability model was used, and water transport was modeled using the van Genuchten–Mualem approach assuming no hysteresis effects. The water flux was set equal to 36 cm d-1. Solute transport simulation used the Crank–Nicolson time weighting scheme, and the chemical non-equilibrium two-site sorption model (TSM) was selected. The water flow parameters were obtained based on soil bulk density and percentages of sand, silt 10
and clay by Neural Network Predictions in HYDRUS-1D software. The longitudinal dispersivity was set as 1 cm according to the transport distance and the flow rate [32] and the immobile water content was set as 0 for it was negligible for transport under the used hydraulic conditions [33]. The adsorption isotherm coefficients in HYDRUS-1D modeling were obtained from batch sorption experiments (S2 in Supporting Information). At last, the dimensionless fraction of adsorption sites and first order rate coefficient were valued from the inverse solution of HYDRUS-1D using the laboratory test data (S3 in supporting information).
3. Results and discussion 3.1. Removal of Pb from groundwater during the integrated remediation test The breakthrough curves of Pb in the contaminated groundwater by amended or unamended soils are shown in Figure 1. After leaching of 200 L groundwater containing 250 mg L-1 Pb, Pb still remained unsaturated (C/C0<1) in soils due to its large adsorption capacity. PT and DM treatments prolonged the breakthrough points of Pb (C/C0= 0.05) which appeared at 150 L and 81.5 L in these two amended soils and increased by 129% and 24.6%, respectively, compared with 65.4 L for the control (Figure 1 and Table 2). The treated amounts of Pb retained in the PT- and DM-treated soil disposed from highly Pb-contaminated groundwater were 15.9 and 14.1 g kg-1, an increasing of 20.5% and 6.82%, respectively, compared to the control. The increase of Pb treatment amount in two amended soil could be attributed to the high affinity for Pb by phosphate materials and dairy manure-derived biochar [34, 35]. Correspondingly, Pb in the groundwater was removed by PT and DM treatments by up to 94.2% and 84.5%, respectively (Table 2). These results indicated that the removal capacity of Pb from contaminated groundwater by soil could be greatly 11
improved with PT and DM amendments. 3.2. Immobilization of Pb in soil during the integrated remediation test The concentrations of Pb in different depths of soils after 200 L loading of 250 mg L-1 Pb-containing groundwater are shown in Figure 2a. The retained Pb was mainly accumulated in the top 4 cm of soil columns, accounting for approximate 60% of the total Pb in groundwater. For PT- and DM-treated soils, the concentration of retained Pb in the top 4 cm soil increased from 23737 mg kg-1 in the control to 33087 and 28632 mg kg-1, increasing by 39.4% and 20.6%, respectively. The two amended soil showed high sorption capacities for Pb, thus retaining more groundwater Pb in the upper layer of soils. The soil pH increased from 7.2 to 7.9 by DM treatment (Table 1), which could enhance the affinity of soil surface for Pb by increasing negative surface charge and hydrolysis of Pb [36]. The increased affinity of PT-amended soil for Pb was mainly induced by Pb-P precipitation, which would be further discussed later. The immobilization effectiveness of Pb in the soils after contaminated groundwater loading was evaluated by TCLP extraction method and the results are shown in Figure 2b.The Pb leachability in TCLP extraction decreased by 18.3%, 32.5%, and 51.5% in 0–4 cm, 4–8 cm, and 8–12 cm of PT-treated soil, compared to the control. Note that the lowest decrease of TCLP-Pb in the 0–4 cm soil was due to its much higher concentration of Pb, which may be beyond the immobilization capability of amended soils [37]. The PT amendment could cause Pb in soils to shift from forms with high availability to more strongly bound Pb fractions, such as residual or insoluble fraction, then reducing its leachability. The BCR extraction analysis (Figure S2) showed prominent increase of residual fraction Pb in PT treatment from 7.05% to 20.4%, which could be attributed to the Pb–P precipitation [8]. The increasing of residual fraction was mainly from the decreasing of acid soluble 12
fraction including weakly absorbed Pb retained on the soil surface and carbonate form of Pb [38]. XRD analysis of PT-treated soil confirmed the formation of insoluble Pb-phosphate precipitation, in which the main peaks of Pb5(PO4)3OH or Pb5(PO4)3Cl appeared at 2θ= 29.8 and 30.1 (Figure 3). In fact, pyromorphite-like minerals (Pb5(PO4)3X, X=Cl, OH, F) have been proven to be the most stable environmental Pb compounds under a wide range of pH and Eh natural conditions [8]. In addition, the formation of Pb3(PO4)2 and PbHPO4 were also found in XRD patterns at 2θ= 29.0 and 2θ= 31.2, 33.0 and 35.5, respectively (Figure 3), and it was also responsible for the reduction of Pb leachability according to Cao et al. [39]. Like PT-treated soil, the leachability of Pb in the DM-treated soil was also reduced by 5.05%–13.5% in the three layer soils, compare to the control (Figure 2b). XRD patterns of DM-treated soils (Figure 3) showed the main peak of PbSO4 at 2θ= 27.6, the mineral is a strongly bound Pb fractions [40]. There was a possible formation of Pb5(PO4)3OH or Pb5(PO4)3Cl appearing at 2θ= 29.8 and 30.1 (Figure 3). The DM biochar contained as much as 0.64% P (Table 1), which favors the precipitation of Pb phosphate minerals [27]. In addition, the remarkable increase of reducible fraction Pb from 25.7% to 33.5% in BCR analysis (Figure S2) indicated the formation of Fe/Mn oxides bound Pb resulted from the high concentration of Fe (6,160 mg kg-1) and Mn (440 mg kg-1) in DM biochar (Table 1) [36]. Besides, surface complexation with active carboxyl and hydroxyl functional groups of the biochar was also the possible mechanisms for Pb immobilization by biochar [41], which might be responsible for the increase of oxidizable fraction Pb from 2.85% in control to 7.39% in DM-amended soil (Figure S2) [38]. It should be noted that although DM treatment retained much Pb in the soil, the stability of retained Pb was lower than that in PT treatment in terms of higher TCLP leachability (34.0–73.2% vs 18.9–64.7%) (Figure 13
2b). The less stability could be attributed to Pb in the DM-treated soil existing as the forms with relatively lower stability, such as PbSO4, Pb bound to Fe/Mn oxides, and Pb complexed with O-containing groups, these Pb forms could be readily extracted in the acidic TCLP fluid [25]. Overall, the results from lab demonstration test above indicated that PT and DM amendment could not only efficiently remove Pb from contaminated groundwater, but also have the ability to immobilize those Pb in the soil, especially for PT amendment, showing a prospect of simultaneously remediating Pb-contaminated soil and groundwater. 3.3. Prospective modeling of the integrated remediation system The inverse solution of HYDRUS-1D showed a good performance in fitting the cumulative system losses for Pb in the column leachate (R2=0.930-0.999, Figure S4) with the chemical non-equilibrium two-site sorption model (TSM). The selected TSM model has been validated in many studies [18, 19] for effectively modeling the sorption and transport of heavy metals in soils. Chotpantarat et al. [18] found that the breakthrough curves were well described by the TSM model in both binary and multi-metal systems. According to Santos and his co-workers [19], simulated transfers were found more realistic in metal-contaminated agricultural soils when using the TSM model. With solute transport parameters obtained in the inverse solution (S3 in supporting information), the remediation of a moderately Pb-contaminated soil (800 mg kg-1) and groundwater (10 mg L-1) site by the integrated approach was simulated by HYDRUS-1D. A suite of simulations on the dissolved Pb concentration in 50 cm soils in depth was undertaken to elucidate the potential treatment capacity of Pb-contaminated groundwater (Figure 4a). According to China Quality Standard for 14
Ground Water [42] in which the standard for Pb is 0.01 mg L-1, PT- and DM-amended soils could continuously treat Pb-contaminated groundwater for 2328 h and 1632 h before the Pb effluent concentration in 50 cm depth exceed the standard, much higher than that of unamended soil (1415 h). By these times, the accumulated retention of Pb in top 30 cm PT- and DM-amended soil was greatly increased by 43.5% and 12.5%, respectively, compared with the control (Figure 4b). Especially in the PT-amended soil, the retention capacity of Pb under the infiltration of 10 mg L-1 Pb-contaminated groundwater reached up to 3745 mg kg-1, higher than that of DM-amended soil (3141 mg kg-1) and about 1.5 times that in unamended soil (2557 mg kg-1). As a result, the potential treatment capacities of 10 mg L-1 Pb-contaminated groundwater in PT- and DM-amended soil were approximately 35 t m-2 and 24 t m-2 (in term of contaminated groundwater flux), an increasing of 64.5% and 15.3%, respectively, compared with the control. These simulations demonstrated the effectiveness of PT and DM amendments to shrink the groundwater pollution of Pb. Besides, the stability of Pb retained in soil after treating contaminated groundwater should also be evaluated in calculating the potential remediating capacities of this integrated system. TCLP extraction tests were used to evaluate the potential stability of soil Pb in above simulations (S4 in Supporting Information). The TCLP-Pb and total-Pb showed a good liner relation (R2=0.994 and 0.999 for PT and DM, respectively) (Figure 5). Therefore, the thresholds for total-Pb in PT- and DM-amended soils could be derived as about 4014 mg kg-1 and 873 mg kg-1, respectively, based on the TCLP regulatory limit of 5 mg L-1 [43]. For unamended soil containing 800 mg kg-1, its TCLP-Pb was 6.09 mg L-1 which already exceeded the 5 mg L-1 limit. This clearly showed that unamended soil didn’t have sufficient immobilization ability in spite of the high retention capacity (Figure 4b). In PT-amended soil, the total-Pb obtained from the 15
simulations was 3745 mg kg-1, much lower than the derived threshold (4014 mg kg-1). It indicated that not only the treated water percolated downward could be clean, the Pb retained in PT-amended soil was also stable enough. Therefore, the PT amendment was favorable to be used in this integrated system. By calculation, the eventual remediating capacity of Pb (both Pb retained from groundwater and already exist in soil) in this integrated system with PT amendment was about 509 g Pb m-2 soil. However, in DM-amended soil, the soil total Pb reached up to 3141 mg kg-1 (Figure 4b), which was far beyond the threshold of 873 mg kg-1 derived from TCLP tests, the TCLP-Pb was 25.9 mg L-1, much higher than the 5 mg L-1 limit. This indicated that the Pb retained in the DM-treated soil was not stable enough. As a fact, the initial DM-amended soil had TCLP-Pb (about 4.33 mg L-1) close to the 5 mg L-1 limit and could not afford further groundwater loading. The poor stability of Pb in DM-amended soil made it not suitable for further treating Pb-contaminated groundwater after remediating contaminated soil.
4. Conclusions Both phosphate- and biochar-treated soils were highly effective in removing Pb from contaminated groundwater. However, the Pb retained in biochar-amended soil could be present as PbSO4, Pb bound to Fe/Mn oxides in biochar, and Pb complexed with O-containing groups of biochar. These Pb forms are less stable, showing the limited immobilization ability. Therefore, biochar-amended soil was not suitable for loading much extra Pb from contaminated groundwater after soil Pb remediation. On the contrary, phosphate-amended soil possessed both high Pb retention capacity and immobilization ability due to the formation of insoluble Pb-phosphate precipitates, thus making it be more applicable to simultaneously remediate Pb-contaminated soil 16
and groundwater in this integrated remediation approach. The modeling suggested that the integrated approach with phosphate amendment was sufficient to meet the treatment requirements to Pb-contaminated soil and groundwater site in terms of the potential treatment capacity of this integrated system. Both laboratory test and numerical modeling demonstrated that the integrated remediation approach with phosphate amendment was fairly feasible to remediate Pb-contaminated soil and groundwater simultaneously.
Acknowledgments This work was supported in part by China Ministry of Environmental Protection (No. 201509035), National Natural Science Foundation of China (No. 21537002, 21377081, 21607099), and Key Laboratory of Bio-organic fertilizer Creation of Ministry of Agriculture (No. BOFC2015KA04).
17
Reference [1] H. El Khalil, O. El Hamiani, G. Bitton, N. Ouazzani, A. Boularbah, Heavy metal contamination from mining sites in South Morocco: Monitoring metal content and toxicity of soil runoff and groundwater, Environmental Monitoring and Assessment, 136 (2008) 147-160. [2] Y. Liang, X. Cao, L. Zhao, E. Arellano, Biochar- and phosphate-induced immobilization of heavy metals in contaminated soil and water: implication on simultaneous remediation of contaminated soil and groundwater, Environmental Science and Pollution Research, 21 (2014) 4665-4674. [3] P. Castaldi, L. Santona, P. Melis, Heavy metal immobilization by chemical amendments in a polluted soil and influence on white lupin growth, Chemosphere, 60 (2005) 365-371. [4] R.L. Chaney, C.L. Broadhurst, T. Centofanti, Phytoremediation of soil metals, Current Opinion in Biotechnology, 8 (1997) 279-284. [5] G. Dermont, M. Bergeron, G. Mercier, M. Richer-Lafleche, Soil washing for metal removal: A review of physical/chemical technologies and field applications, Journal of Hazardous Materials, 152 (2008) 1-31. [6] R.A. Wuana, F.E. Okieimen, Heavy Metals in Contaminated Soils: A Review of Sources, Chemistry, Risks and Best Available Strategies for Remediation, Isrn Ecology, 2011 (2011). [7] J. Kumpiene, A. Lagerkvist, C. Maurice, Stabilization of As, Cr, Cu, Pb and Zn in soil using amendments - A review, Waste Management, 28 (2008) 215-225. [8] P. Miretzky, A. Fernandez-Cirelli, Phosphates for Pb immobilization in soils: a review, Environmental Chemistry Letters, 6 (2008) 121-133. [9] M. Ahmad, A.U. Rajapaksha, J.E. Lim, M. Zhang, N. Bolan, D. Mohan, M. Vithanage, S.S. Lee, Y.S. Ok, Biochar as a sorbent for contaminant management in soil and water: A review, Chemosphere, 99 (2014) 19-33. [10] S. Mignardi, A. Corami, V. Ferrini, Evaluation of the effectiveness of phosphate treatment for the remediation of mine waste soils contaminated with Cd, Cu, Pb, and Zn, Chemosphere, 86 (2012) 354-360. [11] M. Ahmad, S.S. Lee, J.E. Lim, S.-E. Lee, J.S. Cho, D.H. Moon, Y. Hashimoto, Y.S. Ok, Speciation and phytoavailability of lead and antimony in a small arms range soil amended with 18
mussel shell, cow bone and biochar: EXAFS spectroscopy and chemical extractions, Chemosphere, 95 (2014) 433-441. [12] M.R. Higgins, T.M. Olson, Life-Cycle Case Study Comparison of Permeable Reactive Barrier versus Pump-and-Treat Remediation, Environmental Science & Technology, 43 (2009) 9432-9438. [13] M.O. Rivett, S.W. Chapman, R.M. Allen-King, S. Feenstra, J.A. Cherry, Pump-and-treat remediation of chlorinated solvent contamination at a controlled field-experiment site, Environmental Science & Technology, 40 (2006) 6770-6781. [14] L. Diels, K. Vanbroekhoven, Remediation of Metal and Metalloid Contaminated Groundwater, Springer Netherlands, 2008. [15] A. Tsitonaki, B. Petri, M. Crimi, H. Mosbaek, R.L. Siegrist, P.L. Bjerg, In Situ Chemical Oxidation of Contaminated Soil and Groundwater Using Persulfate: A Review, Critical Reviews in Environmental Science and Technology, 40 (2010) 55-91. [16] S. Paria, Surfactant-enhanced remediation of organic contaminated soil and water, Advances in Colloid and Interface Science, 138 (2008) 24-58. [17] N.T. Basta, S.L. Mcgowen, Evaluation of chemical immobilization treatments for reducing heavy metal transport in a smelter-contaminated soil, Environmental Pollution, 127 (2004) 73-82. [18] S. Chotpantarat, S.K. Ong, C. Sutthirat, K. Osathaphan, Competitive modeling of sorption and transport of Pb2+, Ni2+, Mn2+ AND Zn2+ under binary and multi-metal systems in lateritic soil columns, Geoderma, 189-190 (2012) 278-287. [19] D.R.D. Santos, P. Cambier, F.J.K. Mallmann, J. Labanowski, I. Lamy, D. Tessier, F.V. Oort, Prospective modeling with Hydrus-2D of 50 years Zn and Pb movements in low and moderately metal-contaminated agricultural soils, Journal of Contaminant Hydrology, 145 (2013) 54-66. [20] W.R. Trenouth, B. Gharabaghi, Soil amendments for heavy metals removal from stormwater runoff discharging to environmentally sensitive areas, Journal of Hydrology, 529 (2015) 1478-1487. [21] SAC, Environmental Quality Standard for Soil (GB15618-1995), Standardization Administration of the People's Republic of China (SAC), Beijing, China, 1995. [22] Y. Fang, X. Cao, L. Zhao, Effects of phosphorus amendments and plant growth on the 19
mobility of Pb, Cu, and Zn in a multi-metal-contaminated soil, Environmental Science and Pollution Research, 19 (2012) 1659-1667. [23] X. Cao, W. Harris, Properties of dairy-manure-derived biochar pertinent to its potential use in remediation, Bioresource Technology, 101 (2010) 5222-5228. [24] ASTM, Annual book of ASTM standards, section 4: Construction Vol. 04.08. Soil and rock; dimension stone; geosynthetics, ASTM, (1993). [25] EPA, Test Method for Evaluating Solid Waste: SW- 846. Third edition, Environmental Protection Agency, Office of Solid Waste and Emergency Response, EPA, (2010). [26] A.L. Page, Methods of soil analysis. Part 2. Chemical and microbiological properties, Wi American Society of Agronomy Inc & Soil Science Society of America Inc, (1982). [27] X. Cao, L. Ma, Y. Liang, B. Gao, W. Harris, Simultaneous Immobilization of Lead and Atrazine in Contaminated Soils Using Dairy-Manure Biochar, Environmental Science & Technology, 45 (2011) 4884-4889. [28] X. Cao, Y. Liang, L. Zhao, H. Le, Mobility of Pb, Cu, and Zn in the phosphorus-amended contaminated soils under simulated landfill and rainfall conditions, Environmental Science and Pollution Research, 20 (2013) 5913-5921. [29] D.C. Tsang, W.E. Olds, P.A. Weber, A.C. Yip, Soil stabilisation using AMD sludge, compost and lignite: TCLP leachability and continuous acid leaching, Chemosphere, 93 (2013) 2839-2847. [30] G. Rauret, J.F. López-Sánchez, A. Sahuquillo, R. Rubio, C. Davidson, A. Ure, P. Quevauviller, Improvement of the BCR three step sequential extraction procedure prior to the certification of new sediment and soil reference materials, Journal of Environmental Monitoring Jem, 1 (1999) 57. [31] J. Simunek, M.T. van Genuchten, M. Sejna, Development and applications of the HYDRUS and STANMOD software packages and related codes, Vadose Zone Journal, 7 (2008) 587-600. [32] J. Vanderborght, H. Vereecken, Review of dispersivities for transport modeling in soils, Vadose Zone Journal, 6 (2007) 29-52. [33] S. Jellali, E. Diamantopoulos, H. Kallali, S. Bennaceur, M. Anane, N. Jedidi, Dynamic sorption of ammonium by sandy soil in fixed bed columns: Evaluation of equilibrium and non-equilibrium transport processes, Journal of Environmental Management, 91 (2010) 897-905. 20
[34] X.D. Cao, L.Q. Ma, D.R. Rhue, C.S. Appel, Mechanisms of lead, copper, and zinc retention by phosphate rock, Environmental Pollution, 131 (2004) 435-444. [35] X. Xu, X. Cao, L. Zhao, Comparison of rice husk- and dairy manure-derived biochars for simultaneously removing heavy metals from aqueous solutions: Role of mineral components in biochars, Chemosphere, 92 (2013) 955-961. [36] T.-Y. Jiang, J. Jiang, R.-K. Xu, Z. Li, Adsorption of Pb(II) on variable charge soils amended with rice-straw derived biochar, Chemosphere, 89 (2012) 249-256. [37] R. Melamed, X. Cao, M. Chen, L.Q. Ma, Field assessment of lead immobilization in a contaminated soil after phosphate application, Science of the Total Environment, 305 (2003) 117. [38] L. Rodríguez, E. Ruiz, J. Alonso-Azcárate, J. Rincón, Heavy metal distribution and chemical speciation in tailings and soils around a Pb–Zn mine in Spain, Journal of Environmental Management, 90 (2009) 1106. [39] X. Cao, D. Dermatas, X. Xu, G. Shen, Immobilization of lead in shooting range soils by means of cement, quicklime, and phosphate amendments, Environmental Science and Pollution Research, 15 (2008) 120-127. [40] M.V. Ruby, A. Davis, A. Nicholson, In situ formation of lead phosphates in soils as a method to immobilize lead, Environmental Science & Technology, 28 (1994) 646. [41] H. Lu, W. Zhang, Y. Yang, X. Huang, S. Wang, R. Qiu, Relative distribution of Pb2+ sorption mechanisms by sludge-derived biochar, Water Research, 46 (2012) 854-862. [42] AQSIQ, Quality Standards for Ground Water (GB/T 14848-1993), General Administration of Quality Supervision, Inspection and Quarantine of the People's Republic of China (AQSIQ), Beijing, China, (1993). [43] Federal Register. Rules and regulations. 55 (61), (1990), 11863-11877.
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Table 1 Selected physico-chemical properties of soils, groundwater, and amendments Contaminated Contaminated Amendments soil
groundwater
PT a
DM
pH
7.2
6.0
7.5
9.1
Sand+silt (2-2,000 μm)
69.9 %
—b
—
—
Clay (<2 μm)
18.9 %
—
—
—
P
0.045 mg kg-1 —
17.0 %
0.64 %
Pb
873 mg kg-1
250 mg L-1
2.80 mg kg-1
BDL
Cd
6.50 mg kg-1
—
BDLc
BDL
Zn
287 mg kg-1
—
228 mg kg-1
523 mg kg-1
Fe
—
—
—
Mn
—
—
—
440 mg kg-1
C
—
—
—
44.7 %
H
—
—
—
2.20 %
O
—
—
—
2.00 %
S
—
—
—
0.58 %
a
6,160
mg
kg-1
PT and DM mean phosphate amendment and dairy manure derived biochar
amendment, respectively. b
Not determined
c
Below detection limit
22
Table 2 The treatment effectiveness of pre-amended soil for the Pb-contaminated groundwater in the integrated remediation test PT-soila
DM-soil
CK-soil
The volume of treated groundwater (L)
202
200
200
The volume of breakthrough points (L)
150
81.5
65.4
The amounts retained in soil from
15.9
14.1
13.2
94.2
84.5
79.5
groundwater (g kg-1) Removal rate of Pb in groundwater (%) a
CK-soil, PT-soil, and DM-soil represent the control, PT-amended soil, and
DM-amended soil, respectively.
23
FIGURE CAPTIONS
Figure 1. Breakthrough curves of Pb with contaminated groundwater loading in the unamended and amended soils. Original soil Pb: 873 mg kg-1, original groundwater Pb: 250 mg L-1 Figure 2. Concentration of Pb in different depths of soils (a) and leachability of Pb in TCLP extract of soils (b) after loaded with 200 L groundwater containing 250 mg L-1 Pb Figure 3. XRD patterns of the soils with different treatments, 1: Pb5(PO4)3OH or Pb5(PO4)3Cl; 2: PbHPO4; 3: Pb3(PO4)2; 4: PbSO4 Figure 4. HYDRUS-1D simulated dissolved Pb concentration in the leachate through 50 cm depth of amended and unamended soils during the Pb-contaminated groundwater infiltration (a) and the depth-discrete Pb accumulation profiles in PT-, DM-amended and unamended soils after 2328-h, 1632-h and 1415-h Pb-contaminated groundwater remediation, respectively (b). The dissolved Pb concentration exceeded China Quality Standard for Ground Water [42] at 2328 h, 1632 h, and 1415h for PT-, DM-amended and unamended soils, respectively Figure 5. Pb concentrations in TCLP leachate versus different total Pb concentrations in soil. The TCLP-Pb in PT- and DM-amended soil might exceed the TCLP regulatory limit for Pb (5 mg L-1) if their total Pb concentration reached up to 4014 mg kg-1 and 873 mg kg-1, respectively
24
0.5
PT DM CK C/C0= 0.05
Pb (C/C0)
0.4
0.3
0.2
0.1
0.0 0
50
100
150
200
Volume (L) Figure 1. Breakthrough curves of Pb with contaminated groundwater loading in the unamended and amended soils. Original soil Pb: 873 mg kg-1, original groundwater Pb: 250 mg L-1
25
a
35000
PT DM CK
Soil Pb (mg/kg)
30000
25000
10000
5000
0
0~4cm
4~8cm
8~12cm
90
b
80
PT DM CK
TCLP-Pb (%)
70 60 50 40 30 20 10 0
0~4cm
4~8cm
8~12cm
Figure 2. Concentration of Pb in different depths of soils (a) and leachability of Pb in TCLP extract of soils (b) after loaded with 200 L groundwater containing 250 mg L-1 Pb
26
4
3
3 2
PT
2 1 1
1
2
2 1
DM
CK
20
22
24
26
28
30
32
34
36
Two-Theta (deg) Figure 3. XRD patterns of the soils with different treatments, 1: Pb5(PO4)3OH or Pb5(PO4)3Cl; 2: PbHPO4; 3: Pb3(PO4)2; 4: PbSO4
27
PT DM CK Quality standard
-1
Groundwater Pb (mg L )
0.010
0.008
0.006
0.004
0.002
a 0.000 0
500
1000
1500
2000
2500
Time (hours) -1
Soil Pb (mg kg ) 0
500
1000
1500
2000
2500
3000
3500
4000
0
Depth (cm)
-10
-20
PT DM CK
-30
-40
b -50
Figure 4. HYDRUS-1D simulated dissolved Pb concentration in the leachate through 50 cm depth of amended and unamended soils during the Pb-contaminated groundwater infiltration (a) and the depth-discrete Pb accumulation profiles in PT-, DM-amended and unamended soils after 2328-h, 1632-h and 1415-h Pb-contaminated groundwater remediation, respectively (b). The dissolved Pb concentration exceeded China Quality Standard for Ground Water [42] at 2328 h, 1632 h, and 1415h for PT-, DM-amended and unamended soils, respectively
28
20 18
PT fitted line for PT DM fitted line for DM TCLP limit
-1
TCLP-Pb (mg L )
16 14 12 10 8 6 4 2 0 0
1000
2000
3000
4000
5000
6000
7000
8000
-1
Total-Pb (mg kg ) Figure 5. Pb concentrations in TCLP leachate versus different total Pb concentrations in soil. The TCLP-Pb in PT- and DM-amended soil might exceed the TCLP regulatory limit for Pb (5 mg L-1) if their total Pb concentration reached up to 4014 mg kg-1 and 873 mg kg-1, respectively.
29