In-Line Seawater Phosphate Detection with Ion-Exchange Membrane Reagent Delivery

Abstract

There is an urgent need for reliable seawater phosphate measuring tools to better assess eutrophication. Today, most accepted sensing approaches are based on the established colorimetric molybdenum blue assay. It requires one to modify the sample to strongly acidic conditions and to add various reagents, principally molybdate and reducing agent (e.g. ascorbic acid), to form a blue colored phosphate complex that is subsequently detected spectrophotometrically. The associated need for large sample and mobile phase reservoirs and mixing coils are, unfortunately, not ideally adapted for the development of operationally simple in situ sensing instruments. It is here demonstrated for the first time that the key reagents needed to achieve phosphate detection by the molybdate method may be delivered by passive counter transport across ion-exchange membranes. A cation-exchange Donnan exclusion membrane placed in contact with a sample flow (450 µm thick) is shown to provide the strongly acidic conditions (pH~1) necessary for phosphate determination. Proton transport is driven, via cation-exchange, by the high sodium content of the seawater sample. Molybdate was similarly released through an anion-exchange membrane by chloride counter transport. Consequently, an in-line flow system containing the two membrane modules in series was used for delivering both hydrogen and molybdate ions into the sample to form the desired phosphomolybdate complex for subsequent spectrophotometric detection. A linear calibration graph in the range of 0.1-10 µM phosphate (3-300 ppb inorganic P) was achieved, which is sufficiently attractive for environmental work. A range of seawater samples was tested and the results from this membrane delivery device showed no significant differences compared to the classical molybdate assay chosen as the reference method.

Electrochemical monitoring of marine nutrients: From principle to application

“…An effective and simple strategy to overcome this to optimize the ISE membrane [104,105]. Among the numerous available membranes [106,107], the ionophore-based membrane has been successfully utilized for dihydrogen phosphate detection [108]. Therefore, it remains difficult to monitor phosphate in seawater using a potentiometric sensor.…”

Polymeric Membrane Marine Sensors with a Regenerable Antibiofouling Coating Based on Surface Modification of a Dual-Functionalized Magnetic Composite

“…As a well-established analytical technique, polymeric membrane ion-selective electrodes (ISEs) have been widely applied in environmental analysis due to their excellent selectivity, rapid response, ease of use, and high reliability. − In recent years, many marine analyzers based on polymeric membrane ISEs have been developed for monitoring of pH, nutrients, , alkali/alkaline earth metals, heavy metals, and carbonate . However, maintaining the analytical accuracy of a marine sensor for long-term monitoring is still a challenge due to the problem of marine biofouling. − The formation of marine biofouling on a substrate surface is generally considered to follow four time-dependent stages including the adsorption of organic molecules within seconds, the attachment of bacterial cells within hours, the formation of a dense biofilm over days, and the adhesion and growth of large marine invertebrates. , It has been found out that marine biofouling can deteriorate the polymeric membrane sensor’s performance in terms of sensitivity and lifetime. , …”

A Colorimetric Dip Strip Assay for Detection of Low Concentrations of Phosphate in Seawater

“…However, the Molybdenum blue-based spectrophotometry method introduced by Murphy and Riley [20] and improved by successive researchers [21][22][23] has been broadly accepted as a standard method for the determination of phosphate in water [1]. In this method, ammonium molybdate in a strong acidic condition reacts with orthophosphate (Equation ( 1)) to form a Keggin ion [PMo12O40] 3− and then is reduced by ascorbic acid (Equation (2)) to generate the molybdenum blue complex [24]. Potassium antimonyl tartrate, as a source of antimony, has been frequently used for increasing the rate of reaction and eliminating the need for a heating process to form the stable molybdenum blue product [25][26][27].…”

“…In the monitoring of water quality, phosphates are commonly referred to as orthophosphates [ 1 ]. Excessive concentrations of phosphate due to the predominant usage of phosphate-based pesticides and fertilizers [ 2 ], along with intentional or accidental inappropriate human activities [ 3 , 4 ], are known to be harmful to environmental aquatic systems. The US Environmental Protection Agency (US EPA) has set the desired limit of 0.150 ppm or 150 parts-per-billion (ppb) for the total phosphate concentration in streams flowing into a lake or reservoir [ 5 ].…”