Simultaneous removal of Pb and MTBE by mixed zeolites in fixed-bed column tests

https://doi.org/10.1016/j.jes.2021.10.009Get rights and content

Highlights

  • Co-removal feasibility of MTBE and Pb with PRB was assessed in column tests.

  • Column performance of mixed zeolite reactive media was evaluated.

  • Clinoptilolite granule and powder were used to examine the effect of particle size.

  • Removal capacity of clinoptilolite powder almost tripled granule (130.6 vs 45.3 mg/g).

  • The minimum thickness and longevity became higher with clinoptilolite powder.

Abstract

The co-contamination of metals and organic pollutants, such as Pb and methyl tert-butyl ether (MTBE), in groundwater, has become a common and major phenomenon in many contaminated sites. This study evaluated the feasibility of their simultaneous removal with permeable reactive barrier (PRB) packed with mixed zeolites (clinoptilolite and ZSM-5) using fixed-bed column tests and breakthrough curve modeling. The effect of grain size on the permeability of PRB and removal efficacy was also assessed by granular and power clinoptilolite. The replacement of granular clinoptilolite by powder clinoptilolite largely reduced the breakthrough time but increased the saturation time nearly fourfold. The column adsorption capacity of clinoptilolite powders almost tripled that of clinoptilolite granules (130.6 mg/g versus 45.3 mg/g) due to higher specific surface areas. The minimum thickness and corresponding longevity of PRB were calculated as 7.12 cm and 321.5 min when 5% of granular clinoptilolite was mixed with 5% ZSM-5 and 90% sand as mixed PRB reactive media compared with 10.86 cm and 1230.2 min for the application of powder clinoptilolite. This study is expected to provide theoretical support and guidance for the practical application of mixed adsorbents in PRBs.

Introduction

Co-contamination of organic and heavy metal pollution in groundwater is common, and remediation of such sites is often complex and difficult due to co-contaminant effects and the need for more than one remedial technology or approach (Arjoon et al., 2013; Liu et al., 2018). One example is the co-contamination of methyl-tert-butyl ether (MTBE) and lead (Pb). MTBE was widely used as a petrol additive to replace leaded petrol (Zhang et al., 2019a) and is the second most widespread volatile organic compound in shallow groundwater mainly due to gasoline spill (Levchuk et al., 2014). Despite bans on MTBE in some developed countries, it remains a widespread petrol additive worldwide and the global MTBE market is projected to reach 24.5 million tonnes by 2024 (Global Industry Analysts, 2019). Pb is a commonly existing heavy metal in the environment including groundwater (Hashim et al., 2011; Ravindra and Mor, 2019). Therefore, the groundwater co-contamination of MTBE and Pb still exists commonly in a few regions. The choice of groundwater remediation techniques depends on many factors, such as treatment materials, site characterization, pollutants and co-existing substances. Permeable reactive barrier (PRB) is a promising in-situ groundwater remediation technique for the simultaneous removal of multiple contaminants with properly chosen reactive media. Reactive medium is the key component of PRBs and include zero-valent metals, carbonaceous materials, clays, limestone, etc (Obiri-Nyarko et al., 2014). Adsorbents, such as zeolites, are among the most suitable and effective reactive materials due to their high surface areas, reactivity and no precipitants formed on their surface, which can help reduce clogging of PRBs (Liu et al., 2021; Zhou et al., 2014), especially compared with ZVI which is the most commonly used reactive medium in PRBs. In addition, zeolites can occur as larger particles (such as in mm) and are free of shrink-swell behaviour which results in superior hydraulic characteristics compared with clay minerals (Apreutesei et al., 2008). The desorption of metal ions from zeolites was also found to be lower than that from bentonite (Hamidpour et al., 2010). When compared with lime, the use of zeolites in PRBs negligibly changes the pH and will not cause solonetzization or secondary pollution (Kumpiene et al., 2008). Zeolites also contain much fewer hazardous elements than some other materials such as fly ash and compost (van Herwijnen et al., 2007).

Clinoptilolite is a widespread and economic natural hydrophilic zeolite with a high specific surface area and removal efficiency for cations. It was reported to be the most commonly used zeolite as a single material or part of mixed reactive media to adsorb pollutants such as heavy metals including Pb (Al-Tabbaa and Liska, 2012; Pawluk and Fronczyk, 2015) and phosphate (Srinivasan et al., 2008). ZSM-5 is a hydrophobic zeolite and has affinity with organic pollutants. Its high adsorption capacity for MTBE removal has been proven in both batch adsorption tests (Zhang et al., 2018b) and column tests (Zhang, Jin, Lynch and Al-Tabbaa, 2018a, Zhang, 2019b). It has also been applied as one of the reactive materials in pilot-scale PRBs for pollutant removal. For example, Vignola et al. (2008, 2011b) used ZSM-5 and mordenite for in-situ sequenced PRBs located close to a coastal refinery to remediate groundwater contaminated by MTBE and hydrocarbons. The results showed that MTBE was reduced to under 10 µg/L for about 100 days. Faisal and Hmood (2015) used ZSM-5 in lab-scale column tests to remove Cd2+ from a contaminated shallow aquifer, and the PRBs started to saturate after ∼120 hr under the conditions tested. Single materials were frequently applied in the early stage of PRB technology, and combinations of materials are frequently applied nowadays (Zhou et al., 2014). This is because the application of mixed reactive media can improve permeability, reduce costs, make more mechanisms available for single- or multi-contaminant removal, and enhance and accelerate removal rates, and thus substantially improve the long-term performance of PRBs. However, the application of mixed reactive media in the simultaneous adsorption of heavy metal and MTBE has not been well studied. Multiple reactive materials have been applied in sequence, in layers or as a mixture on the lab-scale, pilot-scale or field-scale level in previous studies (Zhang, 2019). For example, zeolites have been mixed with different materials including adsorbents, reductants and precipitants, such as ZVI (Pawluk et al., 2019), fly ash (Fan et al., 2018), Fe-Mn nodule (Fan et al., 2018), granular activated carbon (Freidman et al., 2017) and limestone (Fronczyk, 2017), as mixed reactive media. The use of mixed adsorbents can reduce clogging and prolong the longevity of PRBs. For example, various adsorbents, such as clinoptilolite, organoclay and inorgano-organo-bentonite, were applied as mixed reactive media in the pilot-scale PRBs to remove heavy metals and petrol hydrocarbons from 2007 to 2015 in the Castleford site, UK (Al-Tabbaa and Liska, 2012). It should be noted that removal efficiency and permeability are both crucial parameters for PRBs. However, most studies focus on removal efficiency and the attention paid to permeability is insufficient. Apart from the formation of surface precipitates or changes of site conditions during the running of PRBs, the particle size of reactive materials is also an important influencing factor for PRB permeability. It is well known that adsorbents with smaller particle sizes generally have a higher surface area, leading to a higher adsorption capacity. However, small particles may also lead to the fouling or aggregation of the reactive materials and the clogging of PRBs. In comparison, the addition of materials with large particles can increase the hydraulic conductivity but reduce the adsorption effectiveness. The grain sizes of each material have a direct effect on the hydraulic conductivity of the reactive media and this is particularly true for mixed reactive media. However, the relevant studies are limited.

This study aims to 1) examine the column performance of a mixture of clinoptilolite and ZSM-5 for the simultaneous removal of Pb and MTBE; 2) evaluate the effects of grain size of clinoptilolite on column performance; and 3) estimate the thickness and longevity of PRB materials. The findings are expected to provide theoretical support and guidance for the practical application of mixed adsorbents as reactive media in PRBs.

Section snippets

Materials

Hydrogen-form ZSM-5 and clinoptilolite were used in this study, and their detailed physicochemical properties can be found in our previous studies (Zhang, 2019; Zhang et al., 2021). Natural silica sand was used to mix with zeolites in the fixed-bed column as the reactive media. Its main component is quartz and the specific gravity is 2.65 (Masad et al., 1996).

Hydraulic conductivity measurement

The hydraulic conductivity of reactive media in fixed-bed column tests was determined by a constant flow rate test using a peristaltic

Hydraulic conductivity of the reactive media

Hydraulic conductivity of reactive media is an important parameter for the PRB design because PRBs should be designed more permeable than the surrounding soil to allow water to easily flow through it (Speight, 2020). Small particles may lead to the aggregation of reactive materials and the clogging of PRBs but large particles reduce the surface area of adsorbents, reducing removal effectiveness. Therefore, sand with a large particle size was mixed with zeolites to increase the hydraulic

Conclusions

The column performance of a mixture of ZSM-5, clinoptilolite and sand in terms of the co-adsorption of MTBE and Pb was evaluated using fixed-bed column tests combined with breakthrough curve modeling. The conclusions are as follows:

  • (1)

    The Dose-Response model can describe the breakthrough curves of the MTBE adsorption onto a mixed reactive medium containing clinoptilolite granules and ZSM-5, while the Logit, Thomas, Yoon-Nelson models can well describe the MTBE adsorption onto mixed reactive media

Acknowledgments

This study is supported by a China Scholarship Council (CSC) Ph.D. studentship and the National Key R&D Program of China (No. 2020YFC1808201).

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