Hydrocarbon adsorption performance and regeneration stability of diphenyldichlorosilane coated zeolite and its application in permeable reactive barriers: Column studies

Highlights

• A column study was conducted to determine the hydrocarbon sorption ability of DPDSCI coated zeolite.

• The influence of temperature and contaminant flow rate on the adsorption performance was assessed.

• The long-term stability of the DPDSCI coated zeolite was found to be excellent.

• Computer modelling, was applied to explore the hydrocarbon adsorption mechanism and could predict results obtained.

Abstract

The adsorption behaviour and long-term stability of diphenoldichlorosilane (DPDSCI) coated zeolite was investigated in a column study by studying the toluene adsorption performance and material regeneration at various temperatures. Computer modelling (CXTFIT and MATLAB) was adapted to explore the adsorption mechanism, including the calculation of the axial dispersion coefficient and maximum adsorption capacity.

The results indicate that DPDSCI coated zeolite presented a higher adsorption capacity towards toluene than that found in previous batch experiments. Under high flow rates, temperature had little influence on the adsorption behaviour. Regeneration tests prove that this material may be cleaned and reused at least three times without significant reduction in adsorption effectiveness. Axial dispersion studies show that transitional flow occurs during the adsorption process and is dominated by diffusion processes. Good agreement between experimental data and model prediction are achieved with an R² value between 0.97 and 0.99 obtained for all tests. Overall, the column tests demonstrate DPDSCI coated zeolite may be suitable for the remediation of sites contaminated with hydrocarbons, even in cold regions and variable water flows.

Introduction

Worldwide, accidental spills; unsatisfactory disposal of industrial chemicals; poor agricultural practices and mining activities has resulted in water, especially groundwater being seriously contaminated by pollutants, including heavy metals, hydrocarbons and pesticides amongst others [1]. As humans increase their presence across the globe, this not only occurs in temperate climates, but also polar regions [2]. Therefore, it is necessary to develop suitable technologies for contaminated site remediation in both temperate and cold climates. Previous researchers [3] have found that the Permeable Reactive Barrier (PRB) technologies are suitable for application in situ [4]. The key factors that influence PRB performance include; the characteristics of the reactive media such as porosity and particle size as well as contaminant type, nutrient availability and microbial activity [5,6].

Previous studies have applied Granular Activated Carbon (GAC) or Surfactant Modified Zeolite (SMZ) as principal reactive materials for laboratory or field scale column experiments towards the removal of hydrocarbon contaminants, such as BTEX, phenanthrene or catechol under temperate temperatures [[7], [8], [9], [10], [11], [12]]. As these are commercial products, they are relatively easy to access and are cost competitive [7]. Evaluation of the work conducted reveals that for most applications, GAC exhibits superior adsorption behaviour as compared to SMZ [7,13]. This is likely due to the porous granular structure and surface functional groups of GAC. In contrast, although SMZ does not exhibit as strong as adsorption potential for hydrocarbons, the base material, zeolite has a high cation exchange capacity and so may be used to remove metal pollutants [14]. This indicates that SMZ may be able to adsorb both hydrocarbon and heavy metals.

As most column tests present in literature are conducted at room temperatures, their use in low temperatures need interpretation [4,15]. The results from a large scale installation in the Antarctic indicate that GAC is too fragile to withstand repeated freeze-thaw cycles and breaks down, thereby changing the flow path through the PRB itself [16]. Also, it was found that the surfactant coating of SMZ may leach after several regenerations, as the surfactant is only held by electrostatic bonds on the zeolite surface, thereby influencing the surface capacity of SMZ for hydrocarbons [7,17]. The limitations of both GAC and SMZ indicate that a novel composite material that possesses the high hydrocarbon adsorption ability of GAC, ion capture ability and rigid structure of SMZ, and a stable coating is still required.

Based on these requirements, chlorosilane is introduced as a coating for the zeolite [18]. It has been reported that chlorosliane may be grafted to the surface of zeolite via a series of chemical reactions [18]. This surface coating improves the hydrocarbon adsorption capacity without damage to the ion capture ability [19]. Regeneration tests showed that this coating is highly stable due to the presence of covalent bonds [19]. Among the common chlorosilanes available, diphenoldichlorosilane (DPDSCI) was found to exhibit a better affinity towards hydrocarbon contaminants, especially the BTEX and aromatic compounds, as they have similar structures [18]. Tests investigating temperature dependence, showed that DPDSCI coated zeolite performed well even under cold temperatures i.e. 4 °C [20]. Although chloroslianes are highly reactive materials and may hydrolyse and liberate hydrochloride acid when exposed to moisture [21], the hazards are minimized during the modification process by using acetone and water to wash the excessing DPDSCI and oven drying. As the DPDSCI is covalently bonded on the surface of zeolite, it is not be easily washed off, and this does not pollute environment. Therefore, as it is efficient to adsorb hydrocarbons and safe to the environment, we extend upon our previous work and examine the performance of the material with column studies.

To test the long-term stability of the material, previous studies have used air sparging or wet air oxidation [7,8] to remove contaminants on the surface of sorbents. However, according to the batch experiments of DPDSCI coated zeolite conducted previously [20], it was found that water may have similar results for toluene. In addition, as it is intended that the material will be regenerated via microbial activity, a water regeneration method is applied in this study.

To appropriately simulate the various field conditions likely to be experienced, it is desirable to carry out adsorption and regeneration column tests at both temperate and cold temperatures. To investigate the flow characteristics through the column and explore the adsorption mechanisms, axial dispersion and reactive transport modelling column tests are conducted, and modelled using standard methods [13]. This will enable the evaluation of the likely performance of the material in the field.