Surface modification of natural zeolite by chitosan and its use for nitrate removal in cold regions
Introduction
A significant proportion of pollution present in the Arctic and Antarctic is from oil or its derivatives (Filler et al., 2008). The main contaminant sources include; industrial or military sites (active or abandoned), scientific research stations and remote communities. It is generally believed that fuel contamination in cold regions is more damaging than temperate climates as slow natural attenuation rates mean that high concentrations may last for many years. As a result, in sensitive areas, often more active remediation options are sought. Such options range from high cost “dig and haul” approaches, to more passive such as air-sparging, composting and land-farming. These passive systems aim accelerate metabolism of fuel contaminants by hydrocarbon degrading bacteria through the addition of oxygen and/or fertilizers.
The appropriate management of nutrient concentrations in soil and soil water, particularly nitrogen, has been found critical to the success of these types of remediation systems. Ammonium is generally the preferred nitrogen source (Filler et al., 2008). Under aerobic conditions, ammonium may be oxidised to nitrite and nitrate. The produced nitrate may either be metabolized by plants or, under anaerobic conditions, be further transformed into nitrogen gas via de-nitrification processes. However, if conditions conducive to these endpoints do not prevail, as nitrate is repelled by negatively charged colloids and is highly soluble, it may enter groundwater systems. This may become a concern in the design of certain passive remediation systems as generally water bodies containing nitrate concentrations above 50 ppm are considered contaminated (Australian Government, 2004). Therefore the development of materials to adsorb nitrate, either to enable its use in other microbial processes or to prevent high concentrations entering into the surrounding environment would be beneficial.
Strong- or weak-base anion exchange materials are generally used to capture nitrate anions. Strong-base anion exchangers have high exchange capacities and are generally synthesized as resins such as Amberlite IRA 900 (Baes et al., 1997, Orlando et al., 2002b). However, synthetic resins are often not suitable for many in-situ applications due to their potential hazard as another contaminant source. Weak-base anion exchangers may also be synthesized from many types of natural products such as sugarcane, rice hull (Orlando et al., 2002a), persimmon tealeaf, coconut husk (Orlando et al., 2002b), wheat residue (Wang et al., 2007) and amine modified coconut coir (Baes et al., 1997). Weak-base anion exchange material is able to remove anions from strong acids like Cl−, SO42−, NO3− and do so generally better in acidic conditions with pH < 5 (Noble and Terry, 2004).
It has also been demonstrated that chitosan beads are a promising material for nitrate sorption with capacities measured in the vicinity of 1.68 mmol g− 1 (Chatterjee et al., 2009). Chitosan is a semi-crystalline polymer and it is easily obtained by the alkaline hydrolysis of the amino groups of chitin in a strong base. A disadvantage of these beads is their reported weak mechanical strength (Guibal, 2004). Particle break up alters hydraulic conductivity and hence the contact time between the material and the nitrate ions, resulting in a negative impact to nitrate uptake. In attempts to increase the strength of chitosan based materials, workers have cross-linked the polymer and also have used chitosan to coat other more stable materials, such as membranes, so that chemical reactivity is maintained with increased mechanical strength (Guibal, 2004, Chatterjee et al., 2009).
In this study, chitosan is used to coat a zeolite material. Zeolites are well known in the environmental fields as a good cation exchanger. However, without modification, natural zeolites have very small or no affinity to remove anionic contaminants (Widiastuti et al., 2008). By coating the zeolite with chitosan, it may enable the capture of nitrate anions. To evaluate the performance of the material produced, surface characterization and laboratory scale batch equilibrium tests were conducted at room and cold temperatures.
Section snippets
Sorbent material
Chitosan flakes were obtained from Kyowa Techno Corporate Limited, Japan with a deacetylation degree of 85% and approximate molecular weight of 100,000. Natural zeolite with mesh size 8/15 was obtained from Castle Mountain with mineral composition clinoptilolite (85%), mordenite (15%), minor components of quartz, feldspar and montmorillonite.
Reagents
AR grade hydrochloric acid (37 wt.%), hydrogen peroxide (30 vol.%), sulfuric acid (98 wt.%), ethanol (99.7 wt.%) and sodium carbonate were supplied by Merck
Physical Characterisation
The surface morphologies of acid-washed zeolite and the Ch-Z as obtained by SEM are presented in Fig. 1. These images show the apparent covering of the zeolite surface with chitosan. However, panels C and D indicate that while chitosan did cover the zeolite surface it was not uniform and patchy in sections.
The FTIR spectra of Ch-Z (A) and acid-washed zeolite (B) are shown in Fig. 2. The two materials were found to exhibit similar spectra. This result is not unexpected as the zeolite is a much
Conclusions
The Ch-Z has a nitrate exchange capacity comparable to other weak-base anion exchangers for capturing nitrate, even though chitosan did not provide a full coating of the zeolite material. Protonation with hydrochloric acid enabled a higher maximum nitrate exchange capacity compared to sulfuric acid. Even though the nitrate exchange capacity was reasonable, the material was shown to be more selective towards sulfate and chloride. However, as this material is being developed for use in polar
Acknowledgements
This study was supported by the Particulate Fluid Processing Centre, a Special Research Centre of the Australian Research Council and the Australian Antarctic Division.
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