Chem. Res. Chin. Univ. 2013, 29(1), 37—41 doi: 10.1007/s40242-013-2303-8

——————————— *Corresponding author. E-mail: zhangyl8@jlu.edu.cn Received August 14, 2012; accepted October 24, 2012. Supported by the National Key Scientific and Technological Project of China(No.2009ZX07419-03) and the Specific

Research on Public Service of Environmental Protection in China(No.201009009). © Jilin University, The Editorial Department of Chemical Research in Chinese Universities and Springer-Verlag GmbH

In situ Remediation of Petroleum Contaminated Groundwater by Permeable Reactive Barrier with Hydrothermal

Palygorskite as Medium

ZHANG Sheng-yu, ZHANG Yu-ling*, SU Xiao-si and ZHANG Ying Institute of Water Resources and Environmental,

Key Lab of Groundwater Resources and Environment, Ministry of Education, Jilin University, Changchun 130026, P. R. China

Abstract The permeable reactive barrier(PRB) has proven to be a cost-effective technique to remediate the petro-leum contaminated groundwater at a northeast field site in China. In this study, the geology, hydrogeology and con-tamination characterization of the field site were investigated and the natural hydrothermal palygorskite was chosen as a reactive medium. Furthermore, the adsorption of the total petroleum hydrocarbons(TPH) in the groundwater onto hydrothermal palygorskite and the adsorption kinetics were investigated. The results indicate that the removal rates of TPH, benzene, naphthalene and phenantharene could all reach up to 90% by hydrothermal palygorskite with a diameter of 0.25―2.00 mm that had been thermally pretreated at 140 °C. The adsorption of TPH onto hydrothermal palygorskite after pretreatment followed a pseudo-second-order kinetic model and a Langmuir adsorption isotherm, suggesting that the theoretic adsorption capacity of hydrothermal palygorskite for adsorbate could be 4.2 g/g. Scan-ning electron microscopy(SEM), infrared spectroscopy(IR), X-ray diffraction(XRD) and X-ray fluorescence spec-troscopy(XRF) were carried out to analyze the adsorption mechanism. The results reveal that hydrothermal palygors-kite is a fibrous silicate mineral enriched in Mg and Al with large surface area and porosity. The dense cluster acicular and fibrous crystal of hydrothermal palygorskite, and its effect polar group ―OH played an important role in the physical and chemical adsorption processes of it for contaminants. This study has demonstrated hydrothermal paly-gorskite is a reliable reactive medium for in situ remediation of petroleum contaminated groundwater at field sites. Keywords Petroleum contaminated groundwater; Hydrothermal palygorskite; Total petroleum hydrocarbon(TPH); Adsorption

1 Introduction

Accidental releases of petroleum products cause serious groundwater pollution in world wide. It is urgent to research and develop secure, economically feasible and effective petro-leum removal technologies. The Permeable Reactive Bar-rier(PRB) has proven to be capable of rapidly reducing the concentration of some chemicals of concern(COCs) by up to several orders of magnitude[1,2]. Various metals, organic- contaminants and nitrate-contaminants have been removed by PRB in recent years[3―5]. However, the reactive medium is the key for PRB to remediate contaminated groundwater success-fully[6]. Recently, many researches focused on pursuing safe and cost-effective reactive medium for PRB[7,8].

Hydrothermal palygorskite is a natural one-dimensional nanoscale clay mineral that belongs to the hormite group[9,10]. It has a large specific surface area, strong adsorption ability and good colloidal features due to its unique structure of layered chain crystal and fine rod-like, fibrous crystal morphology with

structural ions and superficial ions[11―14]. Hydrothermal paly-gorskite has been nicknamed the “special clays”, leading to more effective application of the clay in various fields[15,16]. Many chemical pretreatment methods may enhance the adsorp-tion effect of hydrothermal palygorskite for contaminants, but they are hard to compete with physical pretreatment methods due to overall costs. Till now, there have been some experi-mental researches on the use of hydrothermal palygorskite as a PRB reaction medium for groundwater treatment. However, there have been no researches on petroleum contamination groundwater remediation. The objects in this study are: (1) to investigate the geology, hydrogeology and contamination cha-racteristics of petroleum-contaminated groundwater at a field site in Northeast China; (2) to evaluate the sorption of the total petroleum hydrocarbons(TPH) onto hydrothermal palygorskite; (3) to estimate the effectiveness of PRB in situ remediation with hydrothermal palygorskite as medium for the petro-leum-contaminated groundwater at field sites.

38   Chem. Res. Chin. Univ. Vol.29

2 Methods

2.1 Pretreatment of Hydrothermal Palygorskite

Commercially pure hydrothermal palygorskite(pure hy-drothermal palygorskite ≥89.02%, Lingshou Country, Hebei Province, China) was washed with pure distilled water in an ultrasonic cleaner. Then, the hydrothermal palygorskite was heated up at 100 °C to 150 °C at an interval of 10 °C in a fur-nace, kept at the desired temperature for 2 h and allowed to cool in the furnace. Each of 7 heat treated hydrothermal paly-gorskite samples(2 g each) was added to 100 mL of petroleum contaminated groundwater and shaken at 10 °C, 108 r/min for 1 h in shaking incubator for subsequent TPH testing. In this study, 20 mL of carbon tetrachloride with 100 mL of water sample containing 2 g of hydrothermal palygorskite was used to extract the oil and analyzed by infrared spectrophotometry oil measuring instrument(JDS-108U+, Jiguang Technology Co. Ltd., Jilin Province, China). Following the heat treatment expe-riments, the hydrothermal palygorskite particles with different diameters(<0.074 mm, 0.074―0.1 mm, 0.1―0.25 mm, 0.25―0.5 mm, 0.5―2 mm, >2 mm) were used to adsorb the petroleum contamination. Samples(2 g each) for each particle size were respectively added to 100 mL of petroleum contami-nated groundwater and shaken at 10 °C, 108 r/min for 1 h in a shaking incubator for subsequent TPH testing.

2.2 Adsorption of Featured Contaminants onto Hydrothermal Palygorskite

The solutions of benezene, naphthalene, phenanthrene or octadecane were prepared individually and the concentrations were 145.5 mg/L, 95.25, 444.65 or 796.63 μg/L, respectively. Pretreated hydrothermal palygorskite(2 g) was added to 100 mL of every solution mentioned above and covered with alu-minium foil to avoid the evaporation of the solution. The solu-tions were shaken at 10 °C, 108 r/min for 1 h in a shaking in-cubator.

2.3 Kinetics of TPH Adsorption

The experiments were carried out at a constant initial TPH concentration of 54.07 mg/L at 10 °C, 108 r/min. Pretreatment hydrothermal palygorskite(2 g) was added in the petroleum contaminated groundwater(100 mL), and covered with alumi-nium foil to avoid the evaporation of the solution. The pre-determined time was set intervals. For the first 30 min, sampling was done every other 10 min. For the next 120 min, sampling was collected every other 30 min. For the last 120 min, sampling was collected every other 60 min.

2.4 Adsorption Thermodynamics of TPH Process and Modeling

The adsorption experiment was carried out by adding 2 g of hydrothermal palygorskite to 100 mL of petroleum conta-minated groundwater with the initial TPH concentration in a range of 0.48 to 13.03 mg/L at pH=7.4, which was put into a

250 mL conical flask. The conical flask with the mixture was then covered with aluminium foil to avoid the evaporation of the solution before it was put into the incubator shaker which operated at 108 r/min and constant temperature 10 ºC until it reached equilibrium, and then, the subsequent TPH testing was operated.

2.5 Adsorption Mechanism of Petroleum Conta-mination onto Hydrothermal Palygorskite

The microstructure, mineral composition and functional group variation of hydrothermal palygorskite before and after TPH adsorption were characterized with a scanning electron microscope(JSM-6700F, JEOL Ltd., Japan), a Fourier trans-form near infrared spectrometer(VERTEX70, Brooke optical instruments Corporation, Germany), an X-ray diffractome-ter(XD-3, Shimadzu Corporation, Japan) and an X-ray fluores-cence spectrometer(XRF-1800X, Shimadzu Corporation, Ja-pan)[17―19].

3 Results and Discussion

3.1 Effects of Pretreatment Methods

The residual TPH concentrations of petroleum contami-nated groundwater subjected to the adsorption of the hydro-thermal palygorskite preheated at different temperatures were: 1.85, 1.53, 1.48, 0.72, 0.07 and 9.77 mg/L, respectively. The results show that the heat pretreated hydrothermal palygorskite performed better(in terms of removal of TPH) than the non-pretreated hydrothermal palygorskite in a range of 100―150 °C(Fig.1). It indicates that heat treatment of natural hydrothermal palygorskite could purge the inner crystal chan-nel, increase its surface area, and finally lead to improved ad-sorption performance. The removal of TPH by the thermally pretreated hydrothermal palygorskite increased greatly in a range of 100―140 °C and reached a maximum above 98%. The adsorption capacity of nature palygorskite is constrained by its crystal structure because the crystal structure allow the molecules smaller than its pore diameter into the pores and the bigger molecules than the pore diameter only can be adsorbed on the surface[20]. However, heat treatment can enhance the palygorskite adsorption capacity for TPH. The heat treatment can remove parts of crystallization water in hydrothermal palygorskite and consequently higher surface activity and

Fig.1 Effect of heat treatment temperature on TPH removal by palygorskite

No.1 ZHANG Sheng-yu et al. 39

adsorptivity occur[21], indicating that the heat treatment could enhance the palygorskite adsorption capacity for TPH. The adsorption efficiency of hydrothermal palygorskite decreased when the heat treatment temperature was higher than 150 °C, which is probably due to the fact that this higher temperature led to the collapse of the hydrothermal palygorskite structure, aggregation of fiber and decrement of the specific surface area[22].

The hydrothermal palygorskite heated at 140 °C was sub-sequently used to screen for its particle size range. The mass fraction of hydrothermal palygorskite with different size ranges were 30%, 57.1%, 17.3%, 16.1%, 5.7% and 3.5%, respectively. The removal efficiencies of TPH by the hydrothermal paly-gorskite in different particle size ranges are shown in Fig.2. It can be concluded from Fig.2 that the removal efficiencies gradually increased with the decrease of the particle size within the experimental ranges, which is due to the fact that the sma- ller particle sizes have larger surface areas and higher adsorp-tion capacities. During the removal of TPH, the active sites on the surface of hydrothermal palygorskite can be fully utilized to improve its overall adsorption capacity. The effects of the par-ticle sizes on TPH removal varied slightly and the removal efficiency was stable at near 98% when the particle size was less than 0.25 mm. However, the smaller the particle size of hydrothermal palygorskite is, the lower the permeability of it is in the actual process of groundwater contamination remediation, which may increase the hydraulic retention time but decrease the medium permeability and increase clogging potential. Ac-cording to the particle size classification, the primary particle size distribution ranged from 2―0.5 mm, which possessed the mass percentage of approximately 60%. Therefore, the particle size range of 2―0.5 mm was selected.

Fig.2 Effect of particle size on TPH removal by

palygorskite Particle size/mm: a. >2; b. 2―0.5; c. 0.5―0.25; d. 0.25―0.1; e. 0.1―0.074; f. <0.074.

3.2 Adsorption Effect of Hydrothermal Palygors-kite for Featured Pollutants

Through the adsorption experiment for 1 h, the concentra-tions of featured petroleum hydrocarbons(benzene, naphthalene, phenanthrene and octadecane) were 145.5 mg/L, 95.25, 444.65 and 796.63 μg/L, respectively. The adsorption effects of hy-drothermal palygorskite for the featured petroleum hydrocar-bons are shown in Fig.3.

It can be seen that the total removal of benzene, naphtha-lene, phenanthrene or octadecane in groundwater by

hydrothermal palygorskite was 94.7%, 99.8%, 99.7% or 94.0% respectively. The adsorption capacity of hydrothermal paly-gorskite for these hydrocarbons, from high to low, was in the order of phenanthrene>naphthalene>benzene>octadecane. This indicates that hydrothermal palygorskite has selective adsorp-tion for hydrocarbons among which the adsorption capacity of hydrothermal palygorskite for aromatics is greater than that for the saturated hydrocarbons. These have provided a reliable basis for the utilization of hydrothermal palygorskite at conta-minated field sites.

Fig.3 Removal effect of featured petroleum hydrocarbons by palygorskite

3.3 Kinetics of TPH Adsorption

After adsorption for 240 min, the TPH concentration from an initial concentration of 54.07 mg/L decreased to 35.8, 14.9, 11.1, 11.2, 11.2, 11.1, 11.01, 11.2 mg/L respectively, corres-ponding to the different adsorption time. In Fig.4, the adsorp-tion efficiency of hydrothermal palygorskite for TPH increased rapidly during the first 10―30 min, and the adsorption reached saturation equilibrium after adsorption for 30 min, the optimal time adsorption of hydrothermal palygorskite for TPH after pretreatment.

Fig.4 Adsorption variation of petroleum hydrocarbons by palygorskite with time

The common equations(Elovich equation, two-constant rate equation, parabolic diffusion equation, pseudo-first order kinetic model and pseudo-second order kinetic model) were used to fit the data of the Kinetics of TPH adsorption experi-ments, with the result shown in Table 1.

Total adsorption of TPH by hydrothermal palygorskite could be well fitted by a pseudo-second-order kinetic model as shown in Table 1 with the noted fitting equation and correlation coefficient. This indicates that the adsorption kinetics of oil pollution on hydrothermal palygorskite follows a pseudo- second-order model with correlation coefficient R2=0.9973 (Table 1).

40   Chem. Res. Chin. Univ. Vol.29

Table 1 Equation constants and correlation coefficients*

Equation Parameter

a b R2 Elovich equation, qt=a+blnt 0.8189 0.2795 0.5191 Two-constant rate equation, lnqt=a+blnt –0.1331 0.1894 0.4916 Parabolic diffusion equation, qt=a+bt1/2 1.4538 0.0581 0.3457 Pseudo-first order kinetic model, lnqt=a+bt 0.4854 0.0017 0.2094 Pseudo-second order kinetic model, t/qt=a+bt 2.1345 0.4537 0.9973

* qt is the adsorption capacity of pretreated hydrothermal palygorskite for TPH at time t(mg/g); t (min) is the adsorption time of the experiment; a is the

correlation coefficient of TPH initial concentration; b is the adsorption rate constant of activation energy of adsorption; R2 is correlation coefficient.

3.4 Adsorption Thermodynamics of TPH onto Hydrothermal Palygorskite

As shown in Fig.5, the driving force of the adsorption of hydrothermal polygorskite for TPH increased with increasing initial concentration of TPH, which in turn promoted the adsorption of TPH onto hydrothermal palygorskite and led to increasing adsorption capacity.

Fig.5 Effect of initial concentration of TPH on adsorp-tion of hydrothermal palygorskite for TPH

The results in Fig.6 show that the adsorption of TPH onto hydrothermal palygorskite increased with increasing initial TPH concentration in the manner of the Langmuir isotherm in the experimental concentration range[Qe

–1=0.4974×ce–1+0.2345,

R2=0.9764, where ce is the equilibrium TPH concentration in the solution(mg/L); Qe is the equilibrium concentration on the hydrothermal palygorskite(mg/g)], suggesting that the theoretic adsorption capacity of hydrothermal palygorskite for TPH could be 4.2 g/g.

Fig.6 Adsorption isotherm model fitting of palygorskite

3.5 Adsorption Mechanism of Petroleum Conta-minants onto Hydrothermal Palygorskite

3.5.1 Microstructure Comparison of Hydrothermal Palygorskite Before and After Adsorbing TPH from Groundwater

The original hydrothermal palygorskite shows dense

cluster acicular and fibrous crystal with a length of over 2 µm [Fig.7(A)], which is typical of hydrothermal palygorskite with many micro channels and high porosity. The crystal morpholo-gy of the hydrothermal palygorskite after adsorbing TPH was not obvious and the original loose pores were filled with petro-leum pollutants[Fig.7(B)], indicating that hydrothermal paly-gorskite could effectively adsorb the petroleum hydrocarbon components.

Fig.7 SEM images of palygorskite before(A) and

after(B) adsorbing TPH 3.5.2 Hydrothermal Palygorskite Mineral Composi-tion

The results in Fig.8 indicate that the hydrothermal paly-gorskite contained a variety of mineral species such as quartz, alkali feldspar, plagioclase, calcite, Arl mixed layer, illite, kao-linite, siderite and hematite. The inorganic chemical composi-tions of hydrothermal palygorskite before and after heat treat-ment are listed in Table 2.

Fig.8 XRD patterns of hydrothermal palygorskite before(a) and after(b) heat treatment

No.1 ZHANG Sheng-yu et al. 41

Table 2 Mineral components(mass fraction) of hydrothermal palygorskite Component Before heat treatment After heat treatment Component Before heat treatment After heat treatment

SiO2 35.47×10–2 35.92×10–2 Zn 63.9×10–6 58.7×10–6 Al2O3 8.44×10–2 8.14×10–2 Co 30.1×10–6 30.6×10–6 TFe2O3 8.51×10–2 9.02×10–2 Ni 63.6×10–6 72.3×10–6 CaO 21.03×10–2 20.42×10–2 Rb 39.0×10–6 36.5×10–6 MgO 22.89×10–2 22.50×10–2 Sr 157×10–6 184×10–6 K2O 1.28×10–2 1.40×10–2 Cr 102×10–6 96.1×10–6 Na2O 1.17×10–2 1.26×10–2 Zr 144×10–6 135×10–6 Ti 10156×10–6 10489×10–6 Ba 424×10–6 404×10–6 Mn 1532×10–6 1297×10–6 V 336×10–6 357×10–6 P 3607×10–6 4400×10–6 Y 5.04×10–6 4.09×10–6 Cu 62.1×10–6 67.2×10–6 Th 8.28×10–6 8.32×10–6 Pb 57.9×10–6 54.5×10–6 According to the Mossbauer spectrum, the ideal structural

formula of hydrothermal palygorskite is {Mg5[Si4O10]2(OH)2· (OH2)4}·4H2O. The results in Table 2 show that the atomic ratios of the hydrothermal palygorskite used in this study were as following: n(Mg)/n(Si)=0.83, n(Al)/n(Si)=0.27 and n(Fe)/n(Si)=0.36. The hydrothermal palygorskite investigated in this study was different from the reported ones in literatures where they were rich in iron, magnesium and calcium. The chemical components of preheated hydrothermal palygorskite are different from those of heated hydrothermal palygorskite such as iron, manganese, phosphorus, lead and other heavy metals and microelements. These changes contributed to the adsorption of petroleum hydrocarbon substances onto it. 3.5.3 Infrared Spectra

It can be seen from Fig.9 that there are feature peaks on the IR spectra of hydrothermal palygorskite, and the feature peaks of hydrothermal palygorskite before and after adsorbing TPH changed. The peak at 3611 cm–1 was intracell telescopic vibrating peak of Mg―OH and Al―OH and it disappeared when hydrothermal palygorskite adsorbed TPH. Meanwhile, the peaks at 2350 and 1650 cm–1 were greatly weakened, of which the peak at 2350 cm–1 was the telescopic vibrating peak of Si―OH, the peak at 1650 cm–1 was the bending vibration peak of Mg―OH and Al―OH. This indicates that the polar group ―OH of hydrothermal palygorskite participated in the chemical adsorption processes, which is the effect functional group of hydrothermal palygorskite for adsorbing TPH.

Fig.9 Infrared spectra of hydrothermal palygorskite before(a) and after(b) adsorbing TPH

4 Conclusions The research demonstrates a suitable reactive medium

(hydrothermal palygorskite) for in situ remediation of petro-leum contaminated groundwater by PRB at field sites. This medium has a greater sorption capacity after pre-heating. The physical structure and effect functional group of hydrothermal palygorskite play an important role in the remediation of petro-leum contaminated groundwater.

References

[1] Mieles J., Zhan H. B., J. Contam. Hydrol., 2012, 134, 54 [2] Environmental Protection Agency US, Permeable Reactive Barrier

Technologies for Contaminant Remediation, 1998 [3] Muchitsch N., Nooten T. V., Bastiaens L., Kjeldsen P., J. Contam.

Hydrol., 2011, 126, 258 [4] Gibert O., Rötting T., Cortina J. L., Pablo J. D., Ayora C., Carrera J.,

Bolzicco J., J. Hazard. Mater., 2011, 191, 287 [5] Suzuki T., Oyama Y., Moribe M., Niinae M., Water Res., 2012, 46,

772 [6] Peng L., Zhang C., Li H., Qiu J., Chen Z., Lin T., Environ. Sci.

Manage., 2011, 36, 47 [7] Zhang Y., Zhang Y. L., Zhang S. Y., Song F., Huang J. Y., Zhang Y.,

Bai X. D., Adv. Mater. Res., 2012, 535―537, 2457 [8] Zhang Y., Zhang Y. L., Zhang S. Y., Wan Y. Y., Li H. X., J. Jilin Univ.,

Earth Sci. Edition, 2010, 40, 399 [9] Drits V. A., Sokolova G. V., Crystallograph, 1971, 16, 183

[10] Xu J. X., Wang W. B., Mu B., Wang A. Q., Colloids Surf. A, 2012, 405, 59

[11] Xu J. X., Zhang J. P., Wang Q., Wang A. Q., Appl. Clay Sci., 2011, 54, 118

[12] Galan E., Clay Miner., 1996, 31, 443 [13] Murray H. H., Appl. Clay Sci., 2000, 17, 207 [14] Wang M., Qian J. W., Zheng B. Q., Yang W. Y., Jiang W. F., Fang Z.,

Ye Y., Chem. J. Chinese Universities, 2004, 25(8), 1567 [15] Al-Futaisi A., Jamrah A., Al-Hanai R., Desalination, 2007, 214, 327 [16] Shariatmadari H., Mermut A. R., Benke M B., Clays Clay Miner.,

1999, 47, 44 [17] Zhai S. F., Li D., J. Chin. Electr. Microsc. Soc., 1988, 3, 277 [18] Song G. B., Liu F. S., Cao Y. G., Peng T. J., Dong F. Q., Wan P., Acta

Petrol. Sin., 1999, 15, 469 [19] Necip G., Clays Clay Miner., 1992, 40, 457 [20] Chen T. H., Geol. Anhui, 1999, 9, 199 [21] Kong D. J., Chen T. H., Liu H. B., Chen D., Xie J. J., Qing C. S., J.

Chin. Ceram. Soc., 2011, 39, 867 [22] Kuang W. X., Facey G. A., Detllie C. D., Clays Clay Miner., 2004, 52,

635