# Advances on Water Quality Detection by UV-Vis Spectroscopy
## 1 Introduction  
Since the 21 st century, with the continuous improvement in human living standards, the problems of global climate change, rapid population growth, and environmental pollution have become increasingly serious. Human activities have exerted great pressure on the environment. Among the many environmental problems, addressing water pollution is an urgent need [1]. Water is an indispensable element of human production and life, and is also related to food security, environmental protection, and human health, among other aspects. With rapid social development and the increasing use of agricultural fertilizers, an increasing number of industrial pollutants are discharged into rivers and oceans, causing eutrophication. When the original ecosystem is destroyed, the oxygen content of water is reduced, and many fish and other species are killed, leading to challenges in terms of water resource conservation [2]. As the living standards of consumers have standardized upward, demand for higher a quality and quantity of water has emerged relative to the past [3]. Scinentifically based water quality index (WQI) models then become important to measure the degree of contamination and advise whether specific water resources require restoration and to what extent [4]. The main water quality parameters include chemical oxygen demand (COD), heavy metal content, nitrate nitrogen (NO\({}_{3}\)-N), dissolved organic carbon (DOC), and turbidity. At present, the methods for determining water quality parameters mainly include chemical, biological, and physical methods [5]. As shown in Figure 1, the main chemical methods are titration analysis and electrochemical analysis, both of which determine the pollutant content in the laboratory. The instrument used is bulky and expensive, and a large number of reagents are required, leading to secondary pollution;further, the results are not real-time results. Biological methods mainly include enrichment analysis and biosensor technology; however, the detection accuracy and sensitivity are far lower than in other methods. Physical methods mainly include hyper spectral remote sensing technology and molecular spectroscopy technology. Spectroscopy is a method to identify substances and conduct quantitative determination through the emission or absorption spectra of substances, which have been frequently used in the field of rapid water quality determination in recent years [6]. In recent years, due to the advantages of high precision, high detection efficiency, nondestructive sampling, environmental protection, low cost, and portability, ultraviolet-visible (UV-Vis) technology combined with a variety of advanced technologies has developed into an excellent and effective tool for detecting pollutants in aqueous environments [7]. UV-Vis spectrum analysis is an important development direction for water quality monitoring, which evaluates the degree of water pollution by establishing a correlation model of absorbance, organic matter, and inorganic matter concentration (Section 3.3) [8]. Although some researchers have tried to summarize the field before [9], with the deepening of UV-Vis spectroscopy in this field of water quality detection, its research objects and research methods have also undergone tremendous changes. Many researchers have proposed many novel research methods for different water quality parameters. For example, [PERSON] et al. [10] used principal component analysis (PCA) and wavelet transform to calculate water quality index. Its applicability was discussed. [PERSON] et al. [11] used the modified water quality index (WQI) to evaluate the water quality of the aquaculture area. The main factors affecting water quality were determined by using PCA. Therefore, a review is very necessary to sort out the research process and methods in this field, especially in the latest application progress of ultraviolet spectroscopy technology.