Spectrophotometric determination of COD based on synergistic photocatalysis redox reaction using titanium dioxide nanoparticles and phosphomolybdic heteropoly acid

Chemical oxygen demand (COD) is one of the important indicators to measure the degree of organic pollution in water. In this work, a rapid spectrophotometric method for detection of COD was achieved based on the oxidation of organics in water by photogenerated holes or free radicals and the reduction of phosphomolybdic heteropolyacid by photogenerated electrons by using TiO2 nanoparticles as a photocatalyst. Taking potassium hydrogen phthalate as the COD standard, under the optimal conditions, the absorbance of reduced phosphomolybdic heteropoly acid was linear with COD in the range of 0.50-100 mg L -1. The detection limit for was COD detection was 0.171 mg L -1. The proposed methods was used for the determination of COD in real water samples, and the results were in general agreement with the national standard method. Compared with the direct photo initiated reduction of phosphomolybdic heteropoly acid without TiO2 nanoparticles, the photocatalytic reaction has better stability and higher efficiency.

Developing the sensing features of copper electrodes as an environmental friendly detection tool for chemical oxygen demand

The chemical oxygen demand (COD) of water bodies is an essential indicator of organic contaminants. The majority of current testing methods have the drawbacks of requiring multiple processes, being time-consuming, and requiring the use of harmful and hazardous reagents. In this work, a low-cost copper wire (Cu-wire) electrode was designed and fabricated to be used as a sensing electrode for the detection of chemical oxygen demand in water. The sensing features were developed by electrodeposition of copper nanoparticles (nano-Cu) that were prepared by fast-scan cyclic voltammetry (FSCV) deposition at the optimum preparation conditions. For improving the adherence and stability of the deposited nano-Cu thin layer, the Cu-wire electrode was scratched to increase the surface roughness. The surface morphology of the prepared nano-Cu/Cu-wire electrode was investigated by scanning electron microscope (SEM). Energy-dispersive X-ray spectrometer (EDX) was used for elemental analysis characterization. The non-modified and the nano-copper modified electrode were utilized and optimized for electrochemical assay of COD using glycine as a standard in 0.075 M NaOH as an electrolyte solution. The calibration curves (COD, mg L-1 vs. I, mA) were plotted from linear sweep voltammetry (LSV) and chronoamperometry (I-t) curves for a wide range of COD under the optimized conditions. It shows that the electroanalytical features of the proposed nano-Cu-based COD sensor exhibit a linear range from 2 to 595 mg L-1 and a lower limit of detection (LOD) of 2.6 mg L-1 (S/N = 3). The established electrochemical method demonstrated a high tolerance level to Cl- ions where 1.0 M Cl- exhibited a negligible influence. The sensor was employed for detecting the COD in diverse real water samples and the attained results were validated using the standard dichromate method. The obtained results could open the window toward using simple and cost effective tools in order to monitor the water quality.

Reduced graphene oxide-polyaniline film as enhanced sensing interface for the detection of loop-mediated-isothermal-amplification products by open circuit potential measurement

The development of low cost, portable diagnostic tools for in-field detection of viruses and other pathogenic microorganisms is in great demand but remains challenging. In this study, a novel approach based on reduced graphene oxide-polyaniline (rGO-PANi) film for the in situ detection of loop-mediated-isothermal-amplification (LAMP) products by means of open circuit potential measurement is proposed. The pH-sensitive conducting polymer PANi was electro-deposited onto rGO coated screen printed electrodes and tuned to be at the emeraldine state at which the pH sensitivity was maximized. By combining PANi and rGO, the pH sensitivity of the system was modulated up to about −64 mV per pH unit. This enabled the number of amplified amplicons resulting from the isothermal amplification process to be monitored. The sensor was then examined for monitoring LAMP reactions using Hepatitis B virus (HBV) as a model. This simple, low-cost, reproducible and sensitive interfacing layer is expected to provide a new possibility for designing point-of-care sensors under limited-resource conditions.

A novel disposable sensor based on reduced graphene oxide-polyaniline (rGO-PANi) for detection of loop-mediated-isothermal-amplification (LAMP) products.

Unique copper and reduced graphene oxide nanocomposite toward the efficient electrochemical reduction of carbon dioxide

The electrochemical reduction of CO₂ to useful chemicals and fuels has garnered a keen and broad interest. Herein, we report a unique nanocomposite consisting of Cu nanoparticles (NPs) and reduced graphene oxide (rGO) supported on a Cu substrate with a high catalytic activity for CO₂ reduction. The nanocomposite was optimized in terms of the composition of Cu NPs and rGO as well as the overall amount. A gas chromatograph was employed to analyze the gaseous products, whereas a chemical oxygen demand (COD) method was proposed and utilized to quantify the overall liquid products. The optimized nanocomposite could effectively reduce CO₂ to CO, HCOOH and CH₄ with a Faradaic efficiency (FE) of 76.6% at −0.4 V (vs. RHE) in a CO₂ saturated NaHCO₃ solution. The remarkable catalytic activity, high FE, and excellent stability make this Cu-rGO nanocomposite promising for the electrochemical reduction of CO₂ to value-added products to address the pressing environmental and energy challenges.

Photoelectrocatalytic microfluidic reactors utilizing hierarchical TiO2 nanotubes for determination of chemical oxygen demand

A novel and highly sensitive microfluidic device which integrated hierarchical TiO₂ nanotubes exhibited an improved detection efficiency for determination of COD.

Electrochemical tuning of the activity and structure of a copper-cobalt micro-nano film on a gold electrode, and its application to the determination of glucose and of Chemical Oxygen Demand

Micro-nano structured Cu-Co was in situ fabricated on the surface of a gold electrode via electrochemical reduction of CuCl₂ and Co(NO₃)₂. It is shown that the shape of the particles can be controlled by variation of deposition current, deposition time, pH value and the ratio of Cu(II) and Co(II) ions. If prepared under current of −200 μA in 0.1 M, pH 4.0 acetate buffer solution, the film possesses high catalytic activity towards the electrochemical oxidation of glucose at a largely increased oxidation current compared to a non-modified surface. The electrochemical activity of this sensor can be easily tuned. Glucose is a standard compound for evaluating the chemical oxygen demand (COD), and we have therefore studied the application of the sensor to the determination of this parameter. Under optimized conditions, the sensor has linear response to glucose in the 1.92-768 mg L⁻¹ concentration range, and the detection limit is 0.609 mg L⁻¹ (at an S/N ratio of 3). A large number of surface water samples was studied, and the results obtained by this method were found to be linearly correlated to those obtained by the dichromate method (r = 0.995; n = 33).

Graphical Abstract

Rapid determination of the chemical oxygen demand of water using a thermal biosensor

In this paper we describe a thermal biosensor with a flow injection analysis system for the determination of the chemical oxygen demand (COD) of water samples. Glucose solutions of different concentrations and actual water samples were tested, and their COD values were determined by measuring the heat generated when the samples passed through a column containing periodic acid. The biosensor exhibited a large linear range (5 to 3000 mg/L) and a low detection limit (1.84 mg/L). It could tolerate the presence of chloride ions in concentrations of 0.015 M without requiring a masking agent. The sensor was successfully used for detecting the COD values of actual samples. The COD values of water samples from various sources were correlated with those obtained by the standard dichromate method; the linear regression coefficient was found to be 0.996. The sensor is environmentally friendly, economical, and highly stable, and exhibits good reproducibility and accuracy. In addition, its response time is short, and there is no danger of hazardous emissions or external contamination. Finally, the samples to be tested do not have to be pretreated. These results suggest that the biosensor is suitable for the continuous monitoring of the COD values of actual wastewater samples.

Downscaling the chemical oxygen demand test

The usefulness of the standard chemical oxygen demand (COD) test for water characterization is offset to some extent by its requirement for highly toxic or expensive Cr, Ag, and Hg species. In addition, oxidation of the target samples by chromate requires a 2-3 h heating step. We have downscaled this method to obtain a reduction of up to ca. 80% in the use and generation of toxic residues and a time reduction of up to ca. 67%. This also translates into considerable energy savings by reducing the time required for heating as well as costly labour time. Such reductions can be especially important for analytical laboratories with heavy loads of COD analyses. Numerical results obtained with the standard COD method for laboratory KHP samples (potassium hydrogen phthalate) show an average relative error of 1.41% vs. an average of 2.14% obtained with the downsized or small-scale version. The average % standard deviation when using the former is 2.16% vs. 3.24% obtained with the latter. When analysing municipal wastewater samples, the relative error is smaller for the proposed small-scale method than for the standard method (0.05 vs. 0.58, respectively), and the % std. dev. is 1.25% vs. 1.06%. The results obtained with various industrial wastewaters show good agreement with those obtained using the standard method. Chloride ions do not interfere at concentrations below 2000 mg Nacl/L. This highly encouraging proof-of-concept offers a potentially alternative greener approach to COD analysis.

Spatial autocorrelation analysis of Chinese inter-provincial industrial chemical oxygen demand discharge

A spatial autocorrelation analysis method is adopted to process the spatial dynamic change of industrial Chemical Oxygen Demand (COD) discharge in China over the past 15 years. Studies show that amount and intensity of industrial COD discharges are on a decrease, and the tendency is more remarkable for discharge intensity. There are large differences between inter-provincial discharge amount and intensity, and with different spatial differentiation features. Global spatial autocorrelation analysis reveals that Global Moran’s I of discharge amount and intensity is on the decrease. In space, there is an evolution from an agglomeration pattern to a discretization pattern. Local spatial autocorrelation analysis shows that the agglomeration area of industrial COD discharge amount and intensity varies greatly in space with time. Stringent environmental regulations and increased funding for environmental protections are the crucial factors to cut down industrial COD discharge amount and intensity.

Facile preparation of platelike tungsten oxide thin film electrodes with high photoelectrode activity

Tungsten trioxide (WO(3)) thin film electrodes with platelike structures were prepared by a facile hydrothermal reaction of tungsten sheets in a dilute nitric acid solution at 100-180 °C and subsequent calcination at 450 °C. The calcination step facilitated the transformation of the crystal structure from tungsten oxide hydrates (WO(3)·H(2)O or WO(3)·2H(2)O) to monoclinic WO(3) without significant modification to the platelike structures. The photoelectrochemical performance of the thin film electrodes for water splitting that took place in a dilute sulfuric acid was strongly dependent on both temperature and the time used for the hydrothermal reaction. This suggests that the thickness of the film influences the process of photoexcited electron transport. The time required for the hydrothermal reaction under higher temperatures was reduced in the generation of thin film electrodes with high photoelectrode activity, because the crystal growth is accelerated at high temperatures and the electron transport is restricted by a relatively thick compact layer that is comprised of WO(3) nanoparticulates. The electrode exhibited sensitivity to the violet portion of the visible light spectrum due to the bandgap of 2.8 eV and high photoelectrode efficiency, as well as an incident photon-to-current conversion efficiency (IPCE) of 66.2%, for the photoelectrochemical oxidation of water.