Distribution of Some Selected Surface Active Agents (SAAs) in the Aquatic and Global Environment with Their Toxic Impact: A Comprehensive Review

Ramsingh Kurrey, Anushree Saha, Manas Kanti Deb*

School of Studies in Chemistry, Pt Ravishankar Shukla University, Raipur 492010, Chhattisgarh, India

*Corresponding author: debmanas@yahoo.com

[Received: 30 November 2019; Revised version: 21 September 2020; Accepted: 21 September 2020]

Abstract. Surface active agents (SAAs) are a class of compounds, which find various applications in different fields of human activities. Surfactants are generally amphiphilic molecules, which are strongly adsorbed at interfaces between the phases. Surfactants windily used as detergency, emulsion, stabilizing and dispersing agents have led to the discharge of highly contaminated wastewaters in aquatic environment. Once reached in the various compartments of the environment such as rivers, lakes, soils, and sediments, surfactants can undergo aerobic or anaerobic degradation. Concentrations of surfactants in wastewaters, river waters, and sewage waters can range milligrams in maximum cases, while it reaches several grams in sludge, soil and sediments in environments. The environmental facts of SAAs and concentration in surface waters, soils or sediments are reviewed in details. This review provides information on levels of surface-active agents in various environmental samples including soil, sediments, sewage wastewater, river wastewater and aerosols.

Keywords: Surface active agents (SAAs), toxicity, distribution, aquatic and global environment.

Introduction

Surface Active Agents (SAAs) or surfactants belong to a group of chemicals that are well known for their cleaning properties with strongly adsorbed at interfaces between the two phases (Kurrey et al., 2019; Olkowska et al., 2012). The SAAs molecule is part of non-polar hydrophobic and polar hydrophilic, Fig. 1. Generally, the hydrophobic part is known as tail, which is usually an elongated hydrophobic alkyl moiety. The properties, which surfactant exhibit in aqueous medium are associated with the nature of polar head or hydrophilic group (Holt, 2000; Thiele, 2005; Myers, 2005; Sinha et al., 2015; Zembrzoska et al., 2016). On the basis of this chemical characteristic behavior SAAs are classified into the following classes. Cationic surfactants: e.g., cetyltrimethylammonium bromide (CTAB); Anionic surfactants: e.g., linear alkylbenzenesulfonates (LAS), sodium dodecyl sulfate (SDS); Non-ionic surfactants: e.g., nonylphenol (NP); Zwitter ionic surfactants: e.g., methyldodecyleaminepropane sulphonate (MDAS); Gemini surfactants: i.e., butanediyl-1, 4-bis (dimethylcetylammonium bromide) (BDAB) (Table 1).Their excessive use as ingredients in care products such as  shampoos, body wash and in household cleaning products like dishwashing detergents, laundry detergents, hard-surface cleaners. These all product containing SAAs has led to the discharge of highly contaminated water system in aquatic and global environment. The report segments the market by application as household detergents, personal care, industrial and institutional cleaners, food processing, oilfield chemicals, agricultural chemicals, textiles, emulsion polymerization (plastics), paints and coating, construction and other. Surfactants long persistence is observed in environment due to their toxic and ionic in nature, maximum concentration is found as pollutants in sewage water, ground water, sediments, sediments sludge, soil and river water etc. The increasing use of SAAs in household and industries has lead to monitor their levels and their toxic impact on environment (Beers and Fisher, 1992; Fernandez et al., 1996; Heinig et al., 1999; Naylor et al., 2002; Singh et al., 2002; Bao et al., 2009; Zembrzuska et al., 2016; Jackson et al., 2016). The different types of physical and chemical changes, which surfactants undergo in environment, can either lead to the positive or negative outcome. The positive consequence may be stated as that the chemical change has lead to death of bacteria, fungi and other micro-organism (Fernandez et al., 1996; Heinig et al., 1999; Naylor et al., 2002; Singh et al., 2002; Bao et al., 2009; Jackson et al., 2016). Table 2 shows the different types of important commercial and industrial surfactants. Its toxicity is mainly attributed to a function of the capability of the surfactant to adsorb and enter into the biological cell membrane and protein; it causes the disturbance in biological activity and cell lyses or even death (Kurrey at al., 2019, Bidari et al., 2010).The interaction of surfactants with cellular membrane of different biota also depends on surface activities as the interaction or reaction with membrane gets facilitated on living of surface tension (Carballo et al., 2007). SAAs load in surface water has lead to various types of diseases like: skin and eyes irritation and cancer and needs frequent monitoring with suitable measures (Li et al., 2003).The amphiphilic properties of these surfactants allow their transportation in dry and wet deposition to surface and runoff water (Bennie et al., 1997; Marcomini et al., 1998;Lang et al., 2002; Bruno et al., 2002; Merino et al., 2003; Carballo et al., 2007; Clara et al., 2007; Andreu et al., 2007; Chen et al., 2008; Silva et al., 2009;Li et al., 2009;Olkowska et al., 2011; Jardak et al., 2016). Moreover, surfactants are reduced of the surface tension in water at very low concentration (Naylor et al., 2002). Another property of the surfactants is ability to association in solution and formation of micelles. The contamination of various wastewater resources by SAAs is a growing concern and relatively poorly understood compared to other freshwater resources of the environments.

Figure 1.­­ Schematic diagram of surface active agent exhibiting polar head and non-polar tail in the molecule

Wastewater provides the most reliable perennial source of freshwater on Earth. It maintains flows and levels in rivers and lakes, is essential for the health of wastewater-dependant ecosystems, and in many parts of the world is the most important source of drinking water (Zembrzuska et al., 2016). Wastewaters are the main sources of surfactants in the environment and surface waters contain the greatest loads of surfactants. Groundwater are also generally thought to contain a much greater diversity of compounds compared to wastewater and surface waters, although this may be simply a function of the capability of various analytical methods and the limited number of groundwater studies rather than actual environmental occurrence. It is now established that these compounds enter the environment from a number of sources and pathways: wastewater effluents from municipal treatment plants, septic tanks, hospital effluents, livestock activities including waste lagoons and manure application to soil, subsurface storage of household and industrial waste as well as indirectly through the process of groundwater-surface water (GW-SW) exchange (Carballo et al., 2007; Clara et al., 2007; Andreu et al., 2007; Chen et al., 2008; Silva et al., 2009; Li et al., 2009; Olkowska et al., 2011; Jardak et al., 2016).. There has not been a systematic review of published studies focussing on the occurrence of surfactants in water. Current gaps in our understanding regarding surfactants in various wastewaters are highlighted as well as possible areas for future research.

In the present review an overview on the concentration reported in various solid, liquid and air samples at different study sites in the world has been incorporated. The measurements of surfactant in different sources i.e. ground water, sewage water, industrial water, soil, and aerosol on the human daily life, agriculture, plant, and animal have been described. In this review, we investigate several issues regarding the toxicity of surfactants in the global and marine environment, different ecological behaviors, and the different monitoring procedure in order to reduce the effects and level of surfactants on aquatic and global environments. Also the probable future prospects of research in the field of surfactants have been highlighted concerning the increasing toxicity of surfactants. Abbreviations have been given for various chemical species.

Occurrence of SAAs in the environment and their toxic effect

The major sources of surfactants in water bodies

Surfactants are a significant anthropogenic component of the aquatic environment, being used in a large number of household and industrial products, often typified by down the drain disposal. The major components of the surface active agent stream are discharged into surface water (Zembrzoska et al., 2016). Detergents, which contain surfactants, can seriously damage of the living organism in the environments. It has been reported that LAS concentration in the range 0.02-1.0 mg/L can cause excessive secretion of mucus resulting in damage to fish gills and minimize settling rate, break swimming patterns in blue mussel larva and decrease respiration in the human body (Olkowaska et al., 2011). Table 3 shows the toxicity of SAAs is a most important parameter (LC₅₀ and EC₅₀), which determines the extent of their applicability as reported by Olkowska and others (Beers et al., 1992; Naylor et al., 2002; Singh et al., 2002; Olkowaska et al., 2011; Jackson et al., 2016). SAAs are also responsible for chemical process in rivers water and liquid effluents of treatment plants and reduction of water quality parameters (Mungray et al., 2008).

     According to the literature review, the concentration of surfactants as according to Bureau of Indian Standards (BIS 1991) and Indian Standard Drinking Water Specification (ISDWS 2012), the concentration of anionic surfactants has to be less than 1.0 mg/L (BIS, 1991). The water quality of Ganga Canal at various sites, are drastically increasing due to the industrial discharge from Haridwar and nearby city Roorkee, India (Seth et al., 2013; BIS, 1991). The river Ganga is of utmost religious and natural aesthetic importance in India. However, the river canal is under threat of loads of pollution due to the performance of various sacraments like mass bathing, waste disposal and various ritual practices. The toxic influence of these human activities was monitored by measuring the levels of surfactants (Seth et al., 2013). The Har Ki Pauri, Singhdwar, Piran Kaliyar and Old Bridge, Roorkee sites have been investigated for determination of surfactant concentrations in the range 1.18-2.5 mg/L, which were found to be more than permissible limit (1.00 mg/L). Also surfactants’ concentration crossed the desirable limit of BIS during the period of analysis (Davi et al., 1999; Krogh et al., 2003; Sublayrolles et al., 2009). 

Anionic surfactants (AS) in sewage were found as a result of the consumption of consumer products like detergents, shampoo, cleaning and washing agents, and personal care products. The group of AS is linear alkylbenzene sulfonate (LAS). The LAS concentrations of sewage water were reported upto 3-21 mg/L(BIS, 1991; Davi et al., 1999; Krogh et al., 2003;Mungray et al., 2008; Sublayrolles et al., 2009; Bidari et al., 2010; Seth et al., 2013). These levels depend on the removal of LAS in WWTPs and STPs(Scott and Jones,2000; Holt et al., 1995; Paul et al.,1988; Tabor and Barber1996; Mcavoy et al.,1993; Othman et al.,2007; Feijtel et al.,1995; Waters and Feijtel, 1995; Gandolfie,2000; Nishiyama et al.,2003;Holtet al.,1998; Longwell and Maniece,1995). Even though, the anionic surfactants were reported in treated sewage while studying risk assessment to aquatic environment by Mungray and Kumar. In India, per capita consumption of detergents rose to over 4000 mg/L in 2005 (Seth et al., 2013). Surfactants concentration increased can be the result of the worse dissemination rate of the sewage treatment and waste water treatment system in India(Sun et al.,2003; Waters et al.,1976; Petrovic and Barceló,2004; Osburn, 1982). We suggest according to information of Water Pollution Control Board, Bureau of Indian Standards and Indian Standard Drinking Water Specification, the higher coverage of wastewater treatment plants facilities including sewage contribution to the decrease of the concentration of various surfactant (Din, 1989; Hellmann and Fresenius,1980; Hellmann and Fresenius, 1983; Schroder, 2001). It is generally considered that concentration of surfactants in river and surface water depending on the volume of characteristics of wastewater flowing into the river and surface water system, and is mostly related to the degree of pollution in the environment. The main reason behind increased of the concentration of surfactants and water pollution levels that the number of sampling site and frequency of monitoring are limited, analysis of data from the monitoring programs together with other information not easily available in comparison other countries(Hellmann, 1978).

Many researchers have reported the occurrence of SAAs and their mechanistic biological route to its degraded products in surface and wastewater samples (Szymanski et al., 2001; Polish Standard PN-EN 903). The total concentration of anionic surface active agents in the river effluents of Polish zone of the Baltic Sea was determined by meteorological and water management institute of Gdynia in years 1990-1999(Pastewski and Mędrzycka, 2003). The Warta River and its tributaries were selected for the quantification of anionic surfactants in year 1997-98, Poland. The high levels of analyte were observed during winter season by Pastewski and Mędrzycka(2003). The concentrations observed in the Wisla River and Reda River was0.015-0.082 mg/L and 0.008-0.066 mg/L, respectively asreported by Pastewski and Medrzycka (2003). Latif et al. 2012have investigated the concentration of surfactants in the 12 inter-connecting water bodies of Lake Chini, the second largest natural Lake in Malaysia (Othman et al., 2007; Latif et al.,2012). According to the studies, the concentration of SAAsas MBAS and DBAS in the Lake surface microlayer was reportedin the ranges0.0467- 0.13mg/L and 0.0526, 0.0584 mg/L, respectively without therecord of temperature variability parameter (Latif et al., 2012).Concentrations of anionic surfactants between 0.005 and 0.15 mg/L were established in the analyzed water streams (Chitikela et al., 1995; Pedraza et et al., 2007). The main reason behind increased the concentration of surfactants that the low coverage rate of the sewage treatment plant process in many other countries. This review indicated that distribution of different classes of surfactant is the presence of various sources e.g., sewage sludges, sediments, soil in solid samples, intake water, river water, natural or untreated water, sewage water, and tap water inliquid samples and aerosols.

The major sources of surfactants in sewage sludges, sediments, soil

Majorities of the studies have been reported for SAAs level in sediments sludges from river and lake water of coastal area, especially in North America, Europe and Asia (Petrovic et al., 2002). Several papers have been currently published for quantitative analysis of alkylphenolic group of compound (Li et al., 2009). Levels of alkylphenolic group of compound found upstream of point origin of pollution were generally lower than 0.0002 mg/L. And many investigators also found higher concentration of alkylphenolic group of compounds in upstream pollution of STPs. Concentration was ranging generally from 0.000075 to 0.00055 mg/L and 0.00001 mg/L in highly industrialized area (Kloepffer, 1996). Petrovic and Bercelo (2004) give an overview of the concentration of surfactants in sludge, sludge amended soil and sludge in agricultural product (Petrovic and Barcelo, 2004). Majority of investigatorshavefound higher concentration of anionic surfactant (LAS) compounds in sewage sludge and soil samples, and concentrations were more than permissible limit in the region. The higher concentrations of another class of surfactants, alkylphenolic group of compounds have been found in work done by Hosselsoe and co-workers(Hesseloe,2001).Similarly, a major proportion of surfactants studies have been done in sewage sludge and soil, which shows the high quantitative levels of anionic surfactants (LAS) as reported byPetrovic and Barcelo, 2000;Scott and Jones,2000) has reported that the LAS concentrations up to 100–500 and 5000 to 15,000 mg/kg in sewage sludge that had been aerobically and anaerobically treated, respectively(Scott and Jones,2000;Latorre et al.,2003). In the same way, higher concentrations of AFEOs detected in anaerobically treated sludge ranged from 900 to 1100 mg/kg. According to Jensen et al., 1999, high concentrations of LAS were reported in anaerobically digested sludge ranged 3000–30,000 mg/kg compared to lower amounts of LAS reported in aerobic stabilized sludge ranged up to 100–500 mg/kg or untreated sludge up to 400–14,000 mg/kg(Jensen,1999).The worldwide concentrations of various surfactants are a presence in solid environmental samples. The majorities of researchers have been reported for the concentration of surfactants for various solid environmental samples (Hennes and Rapapost,1989; Vejrup and Wolkoff,2002; Munoz et al.,2016; Breen et al.,1996; Hellmann,1981; Waters,1989).The higher concentration of the non-ionic surfactants e.g. NP (15%) and OPEO (11%) are a presence in solid samples in comparison with other surfactants as shown in Fig. 3.d

The major composition of surfactants in atmospheric aerosols

Surfactants in the atmosphere may act as cloud condensation nuclei (CCN), with a potentially negative impact on the environment (Gill and Graedel, 1985, Seidl and Hanel, 1983; Becagli et al., 2011; Sukapan and Brimblecombe, 2002; Radke, 2005; Wahid, 2011). The use of surfactants in motor vehicles, lubricants and diesel fuel, biomass burning, road dust and construction, which is the reason behind the increase the concentration of atmospheric surfactants in aerosols (Radke, 2005; Andrews and Larson, 1993; Frka et al., 2012; Loglio et al., 1985; Halim et al., 2010). The highest concentration of surfactants may influence the state of the gas-liquid interfaces of atmospheric particles and droplets (Latif et al., 2011). Many research group, have suggested that models of cloud formation based organic and inorganic compounds provide a decrease in surface tension (Latifand Brimblecombe, 2007; Latif et al., 2005; Roslan et al., 2010; Srivastava et al., 2008; Awuku et al., 2007). Surface tension is one of the most important factors that control the vapour pressure of small droplets and minimize the surface area of the liquid (Vejrup and Wolkoff, 2002). Olkowska et al. have been investigated the major group of surfactants in presence of tropospheric aerosol, which can affect the formation and development of clouds (Karsa and Porter, 1995; Wahid, 2011; Frka et al., 2012; Loglio et al., 1985; Halim et al., 2010; Latif et al., 2011). The presence of SAAs in atmospheric aerosol samples have also been observed and reported that anionic surface active agents (MBAS) have high levels in the atmosphere i.e. 59 pmol  m^-3 in comparison with  level of cationic surface active agents (DBAS) concentration upto 36 pmol  m^-3 by Latif and Brimblecombe (Latifand Brimblecombe, 2004). According to Bascom and co-workers anionic surface active agents (MBAS) were dominant in fine mode atmospheric aerosol samples, ranging between 35 and 40 pmol  m^-3 and cationic surface active agents (DBAS) was  also found to be dominant in the coarse mode atmospheric aerosol samples, and ranged between 22 and 23 pmol  m^-3.

Figure 2.The concentration of surfactants for various solid environmental samples

The major source of these surfactants has been attributed to industrialization in the sampling vicinity(Bascom et al.,1996).Many other research groups, including Sukapan et al. 2002, Andrews et al. 1983, and Loglio et al. 1985 have found the higher concentration of surfactants in various size fraction aerosols. Comparative studied, the result indicated that the highest level of surfactants e.g. MBAS and DBAS in aerosols was recorded the various monsoons like: summer season, autumn season and winter season (Loglio et al., 1985). The source apportionment analysis for the concentration of MBAS and DBAS for various size fractions of aerosols like: Course modes, fine mode, TSP, PM2.5, PM10 and PM1 have been reported(Sukapan and Brimblecombe, 2002; Radke, 2005; Wahid, 2011;Radke, 2005; Andrews and Larson, 1993; Frka et al., 2012; Loglio et al.,1985; Halim et al., 2010; Latif et al., 2011;Latifand Brimblecombe,2007; Latif et al., 2005; Roslan et al., 2010; Srivastava et al., 2008; Awuku et al., 2007;Vejrup and Wolkoff,2002;Bascom et al.,1996).The world wide concentration of anionic and cationic surfactants of higher in various monsoons like: summer season (PM₁₀=36.6% for MBAS and PM₁₀=25.8% for DBAS), autumn season (PM_2.5=48% for MBAS and PM_2.5=28.4% for DBAS) and winter season (PM_2.5=29.4% for MBAS and PM₁=40.9% for DBAS)are distributed in aerosols in comparison with otherclasses of surfactants as shown in Fig. 3.

Figure 3.The source apportionment result for the concentration of MBAS and DBAS for various size fractions of aerosol

The major composition of surface active agents in liquid environmental samples

Majorities of research groups have been investigating the total concentration of various surfactants in liquid environmental samples as shown in Fig.5.

Intake and River water

Tap and surface water sample in several countries were found to be contaminated with surfactants e.g. perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA) (Hanif et al., 2012). These compounds were detected globally in the tissues of fish, marine mammals and bird, but their concentrations ofSAAs in animals from industrialized areas were higher in comparison with less populated in remote areas. Anionic and nonionic surface active agents are toxic in the 0.1 mg/L concentration level for living organismas reported by Vejrup and Wolkoff, 2002.

Natural or Untreated water

In the industrialized world the different type of surfactant will pass through a sewage treatment plants as well as waste water treatment plants. In these processes the surfactants are exposed and released into natural water, therefore, generally surfactants based are aerobic biodegradation and biodegradation process generally depend on the limit or enough solubility of surfactants in water (Barekat and Afzali,2013).The usage of household cleaning detergents and other domestic use might appear to be the major source of anionic and non-ionic surfactants. The usage of elevated amounts of detergents and washing products result in higher concentration in river, and seaside. Determination of surfactants in river water, raw water supply and drinking water are important since surfactants have become a threat to our water supply network. Many researchers have detected town area having a higher concentration of surfactants in wet season (MBAS = 0.262mg/L, DBAS = 0.01169 mg/L) (Chitikela et al.,1995;Janssens, et al.,1997; Bolong et al., 2009; Fujii et al., 2007). The concentration of surfactants at other areas is lower in comparison to town areas. River water along with the town area may receive a variety of organic wastes generated from domestic waste discharged by nearby restaurants. Furthermore, in town areas, other anthropogenic sources might as well supply to the high level of surfactants. Street dusts mainly consist of particles emitted by vehicles and industries, are believed to contain large amount of surfactants, which might be dissolved in runoff water and ultimately enter the river water (Hellmann and Fresenius, 1980; Grisp et al.,1983). Comparative studies the concentrations of SAAs were drastically higher in trafficked road dust in comparison with residential road dust (Sanemasa et al., 2006). Meanwhile, during the dry season, rural areas provide the highest concentration of both surfactants (MBAS = 0.462mg/L, DBAS = 0.035mg/L) (Hanif et al., 2012).

Sewage, River, Tap and Waste water

Municipal and industrial waste waters are one of the most important pollution sources affecting the quality of ground as well as surface water adversely in many developed countries by Koparal et al. 2006(Alaton and Erdinc,2006). According to many researchers including, Arslan-Alaton et al. 2006, Gomez et al. 2011, Huang et al. 2012, Olmez-Hanci et al. 2011 the surfactants are regarded as one of the major pollutants detected in the aquatic environment(Gomez et al., 2011; Huang et al.,2012; Hanci et al., 2011; Idouhar and Tazerouti, 2008). The average concentrations have been reported at around 40.6 and 82.6 mg/Lto 48.3 and 98.3 mg/L, in sea water by Ross and Liao (2015) (Akyuz, 2007).According to Norberg and co-workers, the average concentration of surfactants has been investigated at 2–7 mg/L in waste water.Majority of researchers have found average concentration of ionic and non-ionic surfactants in sewage water as well as waste water samples, and concentrations were more than permissible limit in the region (Ferrer and Barcelo,2001; Lara etal.,2006).Koga et al. 1995 have been reported the concentration of anionic and cationic surfactants e.g. DBAS was 22-17 mg/Land 47-40 mg/ Lin river water(Koga et al.,1995). According to international and domestics guideline the tolerance limit for NS concentration are not prescribed in tap water, the higher concentration of anionic surfactants was found 0.2 mg/L and 0.5 mg/L in comparison with NS by the Ministry of Health and Labor in Japan and U.S.E.P.A. respectively(Munoz et al., 2014; Moldvan et al., 2011; Chen et al., 2014; Borghi et al., 2011; Ferella et al., 2013; Rao et al.,1995).The higher concentration of anionic and cationic surfactants e.g. LAS (14.2%) and CTAB (11%) are presence in liquid samples in comparison with another classes surfactants as shown in Fig.4.Rivera-Utrilla et al. 2012 have been reported the average concentration of surfactants is ranging between 1 and 10 mg/L in domestics waste waters (Utrilla et al., 2012). In previous research work finding the LAS concentration ranging from 2 to 21 mg/L, 400 to 14,000 mg/L in raw waste water and untreated sludge, respectively  by Fountoulakis and co-workers (Fountoulakis et al.,2009). Majorities of research groups including, Holt et al. 1998; Ying et al. 2006 were also detected the LAS concentration (up to 0.05 to 400 mg/L and 1090 to 1100 mg/L) in sewage water, wastewaters and surface waters(Holt et al., 1998; Ying,2006). Eschauzier and co-workers have been reported that PFOA and PFOS removal from drinking water as well as surface waters were using granular activated carbon and that their concentrations ranged below 4.2 ng/L(Eschauzier et al.,2012).In the UK, total concentrations of AP1EOs at 45±16 mg/L and up to 3970 mg/L were detected in WWTP effluents and river sediments, respectively by Montgomery and others (Bennie et al.,1997; Clara et al., 2007). Isobe et al. 2001 have been carrying out the total concentrations of NP in river water ranging from undetectable to 17.5 mg/L in Spain, Japan, Germany, USA, and Canada (Céspedes et al.,2009). Majorities of authors have been reported concentrations of NP in river, vicinity of contaminated rivers water and agricultural activities ranging from undetectable to 17.5 mg/L, 0.1– 0.8 mg/L and 0.16– 0.38 mg/L in Spain, Japan, Germany, USA, and Canada (Barber et al., 1988; Ahel etal.,1994, Rudel et al.,1998). NP have been also found in various sources such as lake water, waste water, surface water sewage, water river water etc at relatively high concentrations. The following Table 4and Fig.5shows the levels of different types of surfactants detected in the various environmental samples from all over the world (Olkowaska et al., 2012; Andreu et al., 2007; Li et al., 2009; Olkowaska et al., 2011, 40-45,53, 63-110,112).

Figure 4. The concentration of various surfactants in liquid environmental samples

       Figure 5. The earlier concentration of some surfactants in wastewater samples.