Membrane-based Sensing Systems for Real-time Monitoring Chlorine and Ammonia

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Ever-increasing population growth and rapid industrialisation have placed enormous pressure on the environment. To address this situation, environmental authorities around the world have introduced strict monitoring programs as a tactic to better manage pollutants and reduce potential health risks. To meet the requirements of water and air quality management, environmental agencies and water utilities constantly demand real-time continuous, and preferably, in situ monitoring techniques that can support decision-making and mitigate potential risks at the earliest stage. This thesis aims to tackle these critical issues by developing a suite of membrane-based sensing systems capable of in situ, real-time, continuous monitoring chlorine and ammonia in dissolved and gaseous forms. A membrane-based colourimetric flow-injection system (MCFIS) was firstly developed to online monitor free chlorine (FC) in drinking water. The majority of water utilities currently employ in-lab tests of grabbed samples to determine the FC residual levels. Under certain circumstances, such approaches are inadequate for timely and accurate reflection of dynamically changing FC residual levels, leading to inappropriate dosages. This work presents a uniquely configured MCFIS capable of accurately and reliably online monitoring drinking water FC residual levels in nearly real-time (1−5 min per measurement) in FC concertation ranges from 0.04 mg L-1 to 6.07 mg L-1. The embedded gas-permeable membrane enables the establishment of a multiple data point-based analytical principle and makes MCFIS an interference-free, calibration-free FC monitoring system. MCFIS is highly promising for integration into modern online platforms for optimising chlorine dosing control. Highly toxic chlorine gas imposes serious health risks in the workplace. The ability to on-site monitor instantaneous and time-weighted average chlorine gas concentrations in a simple, sensitive, accurate and reliable manner is critical for workplace health and safety. A gaseous chlorine detection principle based on a N, N-diethyl-p-phenylenediamine sulphate salt/Cl2 colourimetric reaction-controlled membrane process was proposed and experimentally validated. A gas-permeable membrane-based portable colourimetric gaseous chlorine sensing probe (MCSP) was designed and fabricated based on the detection principle. The developed MCSP is capable of continuous real-time monitoring of instantaneous and time-weighted average chlorine gas concentrations within an analytical range of 0.009 to 2.058 mg L-1 without the need for ongoing calibration. The MCSP can be a useful analytical tool for managing the toxic chlorine gas imposed health risks in workplaces. A membrane-based conductometric ammonia sensing system (MCAS) was then developed and applied for real-time in situ monitoring of ammonia in coastal environments. Currently, the majority of existing ammonia sensors cannot be used for coastal water analysis due to high salinity induced interference. The MCAS innovatively integrated a superhydrophobic gaspermeable membrane and used glycerol as the ammonia recognition reagent modifier to achieve rapid ammonia quantification with high salinity tolerance. Investigations of key performance indicators, such as sensitivity, deployment period and biofilm effects, demonstrated that the proposed system is capable of real-time, continuous trace monitoring of ammonia in coastal and marine waters with a detection limit of 1 μg L-1 and without the need for ongoing calibration. MCAS can be a useful analytical tool to obtain long-term continuous spatial and temporal ammonia flux profiles, which provides information to better understand nitrification and denitrification processes in coastal environments. The MCAS was further developed to real-time monitor soil ammonia volatilization. The current soil ammonia volatilization monitoring methods are passive sampling methods and require tedious sampling processes and complex analysis procedures, and most importantly, are unable to provide dynamic information on the ammonia volatilization process. In this work, a new analytical principle capable of directly quantifying real-time ammonia volatilization flux was proposed and experimentally validated. The MCAS embedded with the proposed analytical principle successfully permitted real-time monitoring of ammonia volatilization from different types of soil with a sensitivity down to ppt level. The information obtained with this system is valuable for improving agricultural fertilisation practices and soil nitrogen management. The membrane-based sensing systems proposed here can be readily configured to measure other gaseous compounds by changing the recognition reagent and the transducer (optical or electrochemical). These types of systems could open a new horizon for membrane-based sensing system development. The reported analytical principle could also be adapted to improve the accuracy and reliability of other sensors, especially for field-based sensing applications.