Measurement of polyacrylamide polymers in water and wastewater using an in-line UV–vis spectrophotometer

Abstract

Measurement of polymers in water and wastewater is a major challenge, and there is currently no quick and simple method that can achieve this. This study presents a method for quantification of polyacrylamide concentration using an in-line UV–vis spectrophotometer that can generate real-time data. Absorbance spectra of seven polymers were established in the wavelength range of 191.5–750 nm and the highest absorbance was recorded at 191.5 nm for all polymers. UV absorbance of the polymers gradually decreased between 191.5–240 nm and a strong linear relationship (R² > 0.97) between concentration and absorbance held at 191.5, 200 and 210 nm. Detection limits were established in distilled water and in four other samples collected from water and wastewater treatment plants. Polymer chemistry and presence of organic and inorganic impurities impacted the detection limits. In distilled water samples, the lowest detection limit measured was 0.05 mg/L. In samples collected from water and wastewater treatment plants, detection limits varied between 0.07 and 1.35 mg/L depending on the polymer type and water quality. Polymers are known to be toxic to aquatic ecosystems and they are suspected carcinogens, and the presented method could be useful in monitoring the polymer concentrations and minimizing its excessive use.

Introduction

Large quantities of water-soluble polymers are used for water and wastewater treatment in treatment plants around the world [1]. Polymers are required in wastewater treatment to improve the efficiency of sludge thickening and dewatering, and in drinking water treatment as flocculants to remove suspended solids [[2], [3], [4], [5]]. Synthetic polymers can increase the size and strength of flocs formed by alum or iron-based coagulants [6]. Moreover, they can replace inorganic coagulants with significant improvement on filter run times and sludge quantities. Synthetic polymers also substantially reduce the volume of solids generated and result in important savings in sludge handling and disposal costs [7,8]. Using too little or too much polymer negatively impacts the treatment efficiency; therefore, it is important to use polymers at their optimum dose [9]. This ideally requires the ability to measure and adjust the polymer dose in-line and real-time. Polymers are also known to be toxic to aquatic ecosystems and they are suspected carcinogens; therefore preventing their excessive use is important.

Analytical and chemical methods that are currently available for measuring polymer concentrations can neither achieve in-line nor real-time measurements. Commonly used methods for polymer detection include gas chromatography (GC), high-performance liquid chromatography (HPLC), mass spectrometry (MS) and combination of these methods. In addition, starch-triiodide method, viscosity, calorimetry, turbidimetry, fluorescence spectrometry, colloid titration, radioactive labeling, flow injection analysis, size exclusion chromatography (SEC), nuclear magnetic resonance (NMR) spectroscopy, fluorescence tagging, and spectrophotometric determination using cationic dyes were also used [1,6,[10], [11], [12], [13], [14], [15], [16], [17], [18], [19]]. Difficulties in reproducing measurements in complex environmental samples were reported by Dentel et al. [20].

Previous studies reported various laboratory techniques to measure polymer concentration in water and wastewater samples. Keenan et al. [21] used SEC and reported a detection limit of 20 μg/L in nanopure water and 66 μg/L in alum sludge supernatant. Becker et al. [6] employed fluorescence tagging and reported a detection limit of 10 μg/L in purified water and 100 μg/L in water samples collected from a drinking water reservoir. Gehr and Kalluri [13] used colloid titration and test suspension methods to measure residual polymer concentration in sludge filtrate and reported 0.6 mg/L for colloid titration and 0.1 mg/L for test suspension methods for the same sample. Chang et al. [1] measured the residual polymer concentration in filtrate from a belt press filter using NMR and reported that the residual polymer concentration was approximately 8 mg/L. They also reported that the polymer measurements done using charge titration, viscosimetry and NMR analyses for the same sample showed little agreement, and higher recoveries were obtained with the NMR method.

Recently, Gibbons and Örmeci [22] developed a spectrophotometric method to measure polymer concentration in water using a bench-top UV–vis spectrophotometer. UV–vis measurements are quick, simple, and accurate, and can find successful applications in research and industry. It was shown that a strong linear relationship existed between polymer concentration and sample absorbance in pure water, and polymer concentration could be determined using a calibration curve. The tested polymers strongly absorbed light in the wavelength range of 200–220 nm and the detection limits were reported to be around 0.5 mg/L at 200 nm. The researchers also noted that the detection limits may vary with different polymers, and using the specific absorbance maxima for each polymer would increase the sensitivity of the method and enhance the detection limit.

The objectives of this study were given as follows: (1) to test the performance and sensitivity of the spectrophotometric method using an in-line UV–vis spectrophotometer that is developed for water quality monitoring at water and wastewater treatment plants, (2) to determine statistically established detection limits for a range of polymers in distilled water as well as in samples collected from water and wastewater treatment plants, (3) to evaluate the effect of water quality on detection limits, and (4) to investigate the effect of dilution on detection limits. New generation of in-line spectrophotometers provide accurate measurements in real-time, and a successful method for in-line and real-time measurements of polymer concentration could find possible applications in research as well as in water and wastewater treatment.