Title

Removal of nitrate from contaminated groundwater by floating alginate beads

Date of Award

2019

Degree Type

Thesis

Degree Name

Doctor of Philosophy

School

School of Engineering

First Advisor

Dr Guangzhi Sun

Second Advisor

Dr Gordon Lucas

Field of Research Code

090409

Abstract

Elevated levels of dissolved nitrate in waterways, particularly in groundwater, is a common and serious environmental problem that remains largely unsolved despite intensive research efforts. Major risks posed by dissolved nitrate in water are two-fold: (a) it is hazardous to human health, and (b) it contributes to eutrophication. Many environmental regulators in developed communities have recommended that the level of nitrate in drinking water should not exceed 10 mg/L. the major source of nitrate contamination is discharging agricultural wastes into surface water. This contamination can later find its way into groundwater resources by penetration through soil (which can be amplified by high irrigation in contaminated sites) due to high solubility and mobility of nitrate ions. For decades, researchers worldwide have studied different routes to tackle the problem of nitrate contaminated groundwater. Among numerous studies, many researchers have proposed application of chemicals to reduce nitre to other nitrogenous compounds (which are either easier to treat or less toxic). Among these studies, nanoparticles have demonstrated significant potentials in removing nitrate, as reactive nanoparticles can bind nitrate ions onto their surfaces, acting as both adsorbents and reactants to convert nitrate. Many studies have evaluated effectiveness of zero valent iron nanoparticles (NZVI) and magnetite nanoparticles (MNP) in converting nitrate to ammonium. However, the fate of nitrate after its transformation to ammonium has remained an unanswered question. Many of the previous studies have also neglected the possibility of the agglomeration and post-contamination of water resources by nanoparticles.

In this study, sodium alginate powder was used to immobilize the NZVI and MNP to avoid postcontamination issues and agglomeration of nanoparticle. In the experimental studies, NZVIs were successfully embedded in multiple batches of lab synthesized alginate beads, which have the III characteristics of initially sinking in water but later floating on the surface. A methodology was introduced to produce NP-impregnated alginate beads, and later impacts of alginate presence was assessed on the efficiency of the nanoparticles in removing nitrate. To protect the reactivity of NZVI, magnetite nanoparticles were simultaneously embedded in the beads to provide electron mediating sites for increasing the efficiency of nitrate removal by NZVI and prolong NZVI age.

This PhD thesis reports a comprehensive experimental approach on the use of NZVI/MNP to reduce nitrate-nitrogen (NO3 --N) to ammoniacal-nitrogen (NH4 +-N) and then immobilise NH4 +-N; thereby allowing nitrogen to be separated and potentially removed from polluted groundwater. Two different common adsorbents including powdered activated carbon (PAC) and zeolite were evaluated based on their capacity in adsorbing ammonium. In addition, a novel method was used to make the alginate beads floating on the surface of the treated water. This method combined with stabilization of nanoparticles (inside the beads) is potential to assure easier recovery of the nanoparticles after treatment and avoid any post-contamination issue.

Overall, this PhD project covers two aspects: experimental studies and process simulation. The Experimental results have: (a) determined a process of synthesizing the NZVI-impregnated beads of desirable characteristics for acting as floating reactors in groundwater flume, (b) quantified the efficiencies of the synthesised beads to chemically reduce NO3 --N and then immobilise nitrogen inside the beads for potential removal from the water, and (c) demonstrated the beneficial effects of adding MNPs into NZVI impregnated beads. The process simulation aspect of this research has produced simple theoretical illustration of the nitrate transformation process by a MATLAB code to describe the nitrate removal processes in these nanoparticle-impregnated beads and verify the experimental results.

Finally, this research has laid the foundation for using nanoparticle-impregnated floating beads to provide in situ treatment of nitrate-contaminated water, which can potentially be a viable technical alternative to conventional technologies, such as permeable reactive barrier (PRB) systems, for groundwater remediation.

Access Note

Access to this thesis is embargoed until 12 February 2021. At the expiration of the embargo period, access to the thesis will be restricted to current ECU staff and students. Email queries to library@ecu.edu.au

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