Desalination and Water Treatment ISSN: 1944-3994 (Print) 1944-3986 (Online) Journal homepage: http://www.tandfonline.com/loi/tdwt20 Influence of salinity on COD measurements in coastal water management Zhaoran Li, Yanqing Sheng, Wenjing Shi, Qiyao Sun & Robert J.G. Mortimer To cite this article: Zhaoran Li, Yanqing Sheng, Wenjing Shi, Qiyao Sun & Robert J.G. Mortimer (2015): Influence of salinity on COD measurements in coastal water management, Desalination and Water Treatment, DOI: 10.1080/19443994.2015.1091996 To link to this article: http://dx.doi.org/10.1080/19443994.2015.1091996 Published online: 21 Sep 2015. Submit your article to this journal Article views: 19 View related articles View Crossmark data Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=tdwt20 Download by: [Tulane University] Date: 03 April 2016, At: 20:59 Desalination and Water Treatment (2015) 1–8 www.deswater.com doi: 10.1080/19443994.2015.1091996 Influence of salinity on COD measurements in coastal water management Zhaoran Lia,b, Yanqing Shenga,*, Wenjing Shia, Qiyao Suna, Robert J.G. Mortimerc aResearch Center for Coastal Environment Engineering Technology of Shandong Province, Yantai Institute of Coastal Zone Research, Chinese Academy of Sciences, Yantai 264003, China, Tel. +86 535 210 9265; Fax: +86 535 210 9000; emails: [email protected] (Z. Li), [email protected] (Y. Sheng), [email protected] (W. Shi), [email protected] (Q. Sun) bCollege of Research and Environment, University of Chinese Academy of Science, Beijing 100049, China cSchool of Animal, Rural and Environmental Sciences, Nottingham Trent University, Brackenhurst campus, Southwell, Nottinghamshire NG25 0QF, UK, email: [email protected] Received 14 March 2015; Accepted 1 September 2015 ABSTRACT Chemical oxygen demand (COD) is an important parameter in water quality assessment. COD values by different determination methods were investigated in coastal rivers. The results show that there is no clear relationship between CODCr and CODMn in coastal areas. CODCr does not reflect the degree of pollution of coastal waters. As salinity increased, CODCr and acidic CODMn increased significantly, but there was little/no change observed for alkaline CODMn. Coastal zone water quality standards should be proposed to solve connection problems between marine quality standard and surface water quality standard. Keywords: COD; Salinity; Coastal waters; Water quality standard 1. Introduction oxidizing most organic compounds in water. Generally, COD is applied in surface waters (e.g. Chemical oxygen demand (COD) is one of the Cr lakes and rivers) and wastewater, and COD is most widely used parameters for water quality moni- Mn Downloaded by [Tulane University] at 20:59 03 April 2016 applied in lightly polluted waters, e.g. reservoir, tap toring and assessment. It provides information about water, and seawater. For the determination of COD in the readily oxidized fraction of the organic load or coastal waters (river, estuary, and near shore bay), reduced compounds in waters, indicating the degree there is no commonly adopted method. of water pollution [1]. COD therefore plays an Rapid industrialization, urbanization, and popula- important role in water pollution control and surface tion growth lead to pollution of rivers and estuaries water management [2,3]. According to the species of and degradation of ecosystems; therefore, the water oxidizing agents, COD can be divided into COD and Cr quality in the coastal zone is receiving more and more COD , depending on whether potassium dichromate Mn attention [5–7]. COD is the most commonly used (K Cr O ) or potassium permanganate (KMnO )is 2 2 7 4 indicator to evaluate water quality. However, because used as the oxidizing agent in the process of COD COD measurement is influenced by high concentra- analysis [4]. K Cr O has a stronger oxidation Cr 2 2 7 tions of chloride or bromine ions [8,9], and COD capability than KMnO , and is capable of completely Mn 4 does not reflect the actual degree of pollution, neither method is really applicable to brackish and saline waters. Previous studies on COD have mostly focused *Corresponding author. 1944-3994/1944-3986 Ó 2015 Balaban Desalination Publications. All rights reserved. 2 Z. Li et al. / Desalination and Water Treatment on how to improve the measurement accuracy e.g. prepare reagent solutions. Concentrations of CODCr [1–4,10], but relatively few have looked at how to and CODMn were measured with the method specified select an appropriate method for a particular salinity in the standard methods for examination of water and [11–13]. In the coastal zone, estuaries vary from brack- wastewater [APHA, 1998] [14]. Dissolved oxygen ish to fully saline, and due to the high concentrations (DO), pH, temperature (T), salinity, and oxidation of chloride and bromine (which can be oxidized by reduction potential (ORP) were measured using a YSI K2Cr2O7 under acid conditions), the CODCr values are Professional Plus Handheld DO and Temperature Sys- elevated. Similarly, heavily polluted river water results tem purchased from TechTrend International Limited, in higher CODMn than actual values because of multi- USA. The relative standard deviation of the measure- ple dilutions before analysis. In this study, we applied ments was <8% based on triplicate water analyses. a systematic testing of both COD methods in three rivers and a reservoir in Yantai (China) in order to determine if there is an optimum approach to measur- 2.3. Design of modeling test for influence of salinity on ing COD within the coastal zone, or whether all the COD measurement existing methods are inappropriate. Based on salinity variations in coastal rivers, waters of different salinity were prepared with 2. Methods and materials freshwater (sampled from upstream of the Yuniaohe River) and seawater (offshore area of the Yuniaohe 2.1. Samples collection and handling River estuary). Three groups of experiments were set Water samples were collected from a total of 20 up to determine the different COD values (CODCr, sampling sites in Menlou Reservoir, Jiahe River, acid CODMn, and alkaline CODMn): forty-five 500-ml Guangdanghe River, and Yuniaohe River in Yantai, beakers were divided into three groups of 15 beakers. China (Fig. 1). All sampling equipment and storage For each group, different volume ratios of freshwater containers were cleaned with distilled water before and seawater were added to these beakers to give a use. Samples from ~20 cm below the water surface final salinity of 0.5 (100% freshwater), 1, 2, 3, 4, 5, 6, 7, were taken using 500-mL polypropylene vials. Samples 8, 9, 10, 12, 18, 24, and 31 PSU (100% seawater), were collected without disturbing the sediment–water respectively. Then, the different COD were measured interface. The vials were fully filled with water, sealed to investigate the influence of salinity. with gas-tight screw-caps, and immediately kept in an adiabatic box with ice until further analysis. Before 3. Results and discussion analysis, all samples were shaken in a sealed condition. 3.1. COD in different coastal rivers and their correlations 2.2. Measurements and methods COD values (average of triplicate tests) for different sampling sites in the Guangdanghe River Reagents used were all of analytical grade or and the Yuniaohe River are presented in Table 1. The − Downloaded by [Tulane University] at 20:59 03 April 2016 above, and deionized water (milli-Q) was used to 1 concentrations of CODCr were 15–423 mg L in − the Guangdanghe River and 31–1,312 mg L 1 in the Yuniahohe River. The concentrations of acidic CODMn −1 and alkaline CODMn were 6–31 and 5–20 mg L in − the Guangdanghe River and 6–24 and 5–25 mg L 1 in the Yuniaohe River. The highest values of CODCr were recorded in the estuarine sections of both rivers (sites P and T). − The standard electrode potential of Cl/Cl − (1.36 V) and Br/Br (1.087 V) is much lower than the 2À 3+ − electrode potential of Cr2O /Cr (1.55 V), so Cl − 7 and Br in estuarine water can be oxidized by K2Cr2O7 (Eqs. (1) and (2)) during the CODCr analysis process, resulting in elevated measurement values [8,15]. Furthermore, the organic matter in all water samples can be oxidized by acidic K2Cr2O7 with the help of the silver sulfate catalytic effect (Eq. (3)). In Fig. 1. Schematic graph showing sampling sites. this process, organic matter first oxidizes to organic Z. Li et al. / Desalination and Water Treatment 3 (a) 12 (b) 18 11 y=0.1566x+2.1225 16 10 2 ) ) R =0.7087 -1 -1 9 14 mg L ( mg L ( 8 12 Mn Mn 7 10 COD 6 COD 8 5 6 Acidic 4 Acidic 4 3 2 0 1020304050 0 100 200 300 400 500 -1 -1 ( ) COD ( mg L ) CODCr mg L Cr (c) 11 (d) 11 10 10 ) ) -1 9 y=0.1079x+3.2672 -1 9 R2=0.3153 mg L mg L ( 8 8 ( Mn 7 Mn 7 COD 6 COD 6 5 5 Alkaline 4 4 Alkaline 3 3 01020304050 0 100 200 300 400 500 -1 -1 ( ) COD (mg L ) CODCr mg L Cr (e) 11 (f) 11 10 10 ) ) -1 y=0.8986x+0.6409 -1 9 9 R2=0.7570 mg L ( mg L 8 ( 8 Mn Mn 7 7 COD 6 COD 6 5 5 Alkaline 4 Alkaline 4 Downloaded by [Tulane University] at 20:59 03 April 2016 3 3 3456789101112 2 4 6 8 10 12 14 16 18 -1 -1 Acidic COD (mg L ) Acidic COD (mg L ) Mn Mn Fig. 2. Relationships among CODCr, acidic CODMn, and alkaline CODMn in the Jiahe River. (a), (c), and (e) were fitted by data excluding the highly saline point, and (b), (d), and (f) were fitted by the data of all sampling sites.