Characteristics, Concentration, Toxicity of ECs in Water Bodies: Comparison
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Please note this is a comparison between Version 2 by Conner Chen and Version 1 by Cristina Villamar.

Emerging contaminants (ECs) are causing negative effects on the environment and even on people, so their removal has become a priority worldwide. ECs are organic, pseudo-persistent, and unregulated “new” contaminants detected in water/wastewater in trace concentrations (ng/L–µg/L).

  • conventional/non-conventional adsorbents
  • nanocomposites
  • lipophilic contaminants

1. Introduction

Emerging contaminants (ECs) are organic, pseudo-persistent, and unregulated “new” contaminants detected in water/wastewater in trace concentrations (ng/L–µg/L) [1]. Pharmaceutical and personal care products, hormones, pesticides, and microplastics, among other chemical substances, are some examples of ECs. They reach the environment through effluents from municipal wastewater treatment plants (WWTPs), septic tanks, hospital effluents, livestock activities, and subsurface storage of household and industrial wastes [1,2][1][2]. In fact, several antibiotics, such as azithromycin, amoxicillin, and ciprofloxacin, have been found in influents wastewater from Asian, European, and North American countries at concentrations (ng/L) between 3 and 303,500, 0.4 and 13,625, and 6.1 and 246,100, respectively [3,4][3][4].
After reaching the environment, or even inside the WWTPs, some ECs should be degraded/biodegraded to metabolites or gases. An environmental degradation route of ECs is photochemical transformation, which can occur directly by solar UV radiation absorption [5] and indirectly by photosensitized species reaction [6]. For example, diclofenac has demonstrated being photochemically degraded to 1-(8-chlorocarbazolyl) acetic acid and carbazole. ECs’ biodegradability depends on their bioavailability, being a more viable process for ECs with octanol-water partitioning coefficients (log Kow) between < 1 and > 4 and high polarity (pKa > 0.5) [5,6][5][6]. Ibuprofen, natural estrogens, bisphenol A (BPA), and triclosan are some examples of biodegradable ECs, which generate transformation products or metabolites (e.g., triclosan to 2,8-dichlorodibenzo-p-dioxin under UV light), achieving degradation percentages greater than 50% [6]. However, since ECs with Henry’s constant values from 10−2 to 10−3 mol/m3Pa do not degrade but rather volatilize, ECs such as dibutyl phthalate, di(2-ethylhexyl) phthalate, and nonylphenol have been found in air samples (0.031–0.055 ng/L) [5,6,7][5][6][7]. Another important factor controlling the fate/behavior of ECs (removal/bioavailability/degradation/volatilization/transport) is their ease of being sorbed–desorbed. Sorption–desorption processes are related to the partitioning soils/sediments coefficient measured by Kd [8]. Some antibiotics, hormones, biocides, and artificial sweeteners with low Kd (300–500 L/kg MLSS) have showed insignificant sorption into sludge [1].
Differences between properties/characteristics/behavior/mobility of different ECs have turned them into substances with potential environmental and human risk, despite their low concentrations. Figure 1 shows the main routes of entry of ECs into the environment, their behavior in it, and the possible effects they could cause in different living beings. Conventional WWTPs do not perform effectively for the removal of ECs. Moreover, conventional wastewater treatment processes/systems/technologies are generally not available or efficient for developing countries, and thus it is necessary to search for options that fit each place [9].
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Figure 1.
Routes of entry of ECs into the environment, its behavior, and effects.
Adsorption has proven to be the most effective, sustainable, renewable, and selective method with low cost and energy consumption [10]. It allows the removal of contaminants such as heavy metals (metal/adsorbent: nickel, lead/nano polypyrrole-polyethyleneimine [11,12][11][12]; zinc/modified fruit peels, graphene oxide + magnetite [13,14][13][14]) and dyes (dye/adsorbent: methylene blue, acid orange 10/activated carbon from oil palm waste; methylene blue and crystal violet/Bauhinia forficata residual fruit) [10,15][10][15]. Activated carbon is the most used adsorbent, but the high costs of this adsorbent, added to the loss of efficiency in the regeneration process, have made it necessary to search for other alternatives [16].

2. Characteristics, Concentration, and Toxicity of Some ECs in Water Bodies

2.1. Pharmaceuticals and Personal Care Products (PPCPs)

There are more than 3000 substances (50–150 g/inhab) used as analgesics/anti-inflammatories, antibiotics, contraceptives, antidepressants, and pressure regulators, among others [17]. Meanwhile, around 2000 chemical compounds are used as personal care products, which include fragrances, preservatives, disinfectants, and sun protection agents, among others [18]. Indeed, the PPCPs production has reached about 20 million tons/year [19]. PPCPs have been found in surface water (up to 10,000 mg/L), groundwater (up to 100 mg/L), influents (75–73,730 ng/L), and effluents (24–4800 ng/L) of WWTP, sludge, sediments, and even in living beings (e.g., triclosan in fish at 2100 ng/g) [20,21][20][21]. In general, PPCPs can be made of several complex molecules with different structures and shapes, which vary widely according to their molecular weight (88.5–> 900.0 g/mol) [4,14][4][14]. Moreover, they are polar molecules with ionizable groups, i.e., in the solid phase, they have different adsorption mechanisms (e.g., ion exchange). Most of them are lipophilic [4]. However, PPCPs have log Kow values in a wide range (−2.4–13.1), from acid to basic substances under environmental conditions (pKa = 0.6–13.9) [4,22][4][22]. PPCPs reach Kd values between 1.9 and 39,000, are partially/completely soluble in water (0.02–3.12 × 105 mg/L, T = 20 °C) [4[4][19],19], and have dissipation times between <3 and up to 300 days [18]. Recurrent discharges of PPCPs could cause endocrine problems, genotoxicity, aquatic toxicity, and resistance in pathogenic microorganisms [23]. For instance, diclofenac at concentrations between 5 and 50 mg/L can increase the plasma vitellogenin in fish. It has even caused effects on steppe eagles and vultures [1,18][1][18]. Meanwhile, ciprofloxacin, tetracycline, ampicillin, trimethoprim, erythromycin, and sulfamethoxazole can increase the antibiotic resistance of E. coli and Xanthomonas maltophilia [18,19][18][19]. Personal care products, such as benzophenone-3 threaten coral reefs and 4-methylbenzylidene camphor, have been demonstrated to generate embryonic malformations in frogs. Additionally, triclosan produces adverse effects on the first stages of the frog’s life [21,24,25][21][24][25]. However, PPCPs also affect humans, as their presence has been detected in breast milk, blood, and urine of children [21]. Furthermore, benzyl paraben and benzophenone-4 were found in the placenta, which could indicate a transfer from mother to fetus [26].

2.2. Pesticides

Until 2020, a use of around 3.5 million tons/year of pesticides was estimated, but only less than 0.1% was used for plants [27]. Some pesticides, such as glyphosate or its main metabolite (amino methyl-phosphonic acid), have been found in surface waters (0.02–6.0 g/L), soil (15.9–1025.5 kg/kg), deep waters (0.1 µg/L), and sediments (0.1–100 mg/L). Moreover, pesticides with high vapor pressure (1.51 × 10−7–1.29 × 10−1 Pa)/high volatility (e.g., pentachlorophenol) are released to the air during their application (between 5 and 90% of them), moving long distances and even reaching pristine areas [28,29,30][28][29][30]. They have been found in rainfall, e.g., methyl parathion, and also near agricultural sites where they were not applied (~23 µg/L) [28]. According to their molecular structure, pesticides have different chemical properties, reaching weak to strong acid character (pKa = 0.7–9.1), medium to high solubility in water (2.0–1.2 × 106 mg/L, T = 20–25 °C), very low to very high bioaccumulation (log Kow = −4.6–8.0), and high persistence (7 days–>5 years) [28,30][28][30]. Pesticides mainly affect non-target organisms, e.g., atrazine (concentrations in water >200 ng/L) [29] causes sex change in male frogs and affected/altered the reproductive system and fertility of mice, fish, and humans [28,29,31][28][29][31]. Another widely used herbicide is glyphosate, which affects the entire food chain, delaying periphytic colonization and reducing the abundance of aquatic organisms such as Pseudokirchneriella subcapitata and Lemma minor (EC50-7d = 11.2–46.9 and EC50-4d = 64.7–270.0 mg a.i./L, respectively) [30,31][30][31]. Paraquat produced neurotoxicity and systemic and pulmonary inflammation (inhalation for 16 days) in rats [32]. Furthermore, its use in edible crops is related to antibiotic resistance in humans, even it was classified as a carcinogenic by the World Health Organization [33]. The byproducts/metabolites of pesticides also cause negative effects [28]. For example, aminomethylphosphonic acid produced acute toxicity in Vibrio fischeri at concentrations between 50 and 167 mg/L [30].

2.3. Steroid Hormones

The human population generates about 30,000 and 700 kg/year of natural and synthetic estrogens, respectively. Natural estrogens are excreted by the adrenal cortex, testes, ovaries, and placenta of humans and animals (e.g., estrone/E1, 17β-estradiol/E2, estriol/E3) [34]. Meanwhile, synthetic hormones are synthesized from cholesterol (e.g., 17α-ethinylestradiol/EE2) [34,35][34][35]. In specific, synthetic estrogens come from oral contraceptives. However, the estrogen generated by cattle is much higher, amounting to 83,000 kg/year in the US and the European Union alone [36]. WWTPs or feedlot effluents are the main pathways for hormones entering soil, surface water, sediment, and groundwater. They have been found within WWTP influents (>802 ng/L), WWTP effluents (>275 ng/L), surface waters (0.04–667 ng/L), groundwater (5–> 1000 ng/L), drinking water (up to 3500 ng/L), and livestock waste (14–533 ng/g) [3,4][3][4]. Steroid hormones are characterized by having a molecular weight between 242 and >296 g/mol, being poorly soluble in water (8.8–441.0 mg/L, at 20 °C), with low to moderate hydrophobicity (log Kow = 1.6–4.7), weak acid character (pKa = 10.3–18.9), and non-volatile (vapor pressure= 9 × 10−13–3 × 10−8 Pa) [34,36][34][36]. Despite their low persistence (thalf-life = 0.2–9.0 days in water/sediments) compared with other ECs, they cause negative effects on ecosystems and humans (cancer and infertility) by their endocrine-disrupting character [4,36][4][36]. Chronic exposure of fathead minnow to EE2 (5–6 ng/L) led to feminization of male fish by the development of the ovary cavity, and impaired oogenesis in females [4,34][4][34]. E2 at 1 ng/L induced changes in vitellogenin production in males, and produced the feminization of some species of male fish (1–10 ng/L) [34,35][34][35]. Levonorgestrel at 6.5 ng/L inhibited spermatogenesis, reduced fish egg production and reproduction, increased female weight and length, and promoted female masculinization [34]. Likewise, EE2 at 10 ng/L affected cardiac function in bullfrog tadpoles [34], reducing the fish biomass and interrupting the aquatic food chain [36].

2.4. Micellaneous ECs

The use/consumption of different products such as fire retardants, food additives, plasticizers, solvent stabilizers, surfactants/detergents, industrial additives (fluorinated organic compounds), and corrosion inhibitors is considerable worldwide [3,37][3][37]. In fact, the worldwide production of surfactants was projected to 24.2 million tons for 2022 [38], and plasticizers such as BPA had a production of 2.0 million metric tons/year [5]. Moreover, around 159,000 metric tons/year of synthetic non-nutritional sweeteners are consumed [39,40][39][40]. The physicochemical characteristics of these substances are very varied due to the differences between them, which also makes their behavior in the environment different. For instance, artificial sweeteners present between medium and very high solubility (4–1000 g/L, 20 °C), from low to high acid character (pKa = 1.9–11.8), and very low bioaccumulation (log Kow = −1.61–0.91) [6,39][6][39]. However, other substances such as BPA (plasticizer), nonylphenol (detergent). and Tris(2-chloroethyl) phosphate (TCEP, fire retardant) have from low to high bioaccumulation with log Kow values of 3.2, 4.4, and 1.8, respectively [5,41][5][41]. Moreover, BPA and phthalates are semi-volatiles, so they easily move into the environment. They have a short half-life (5–18 days) in air because they could be photodegraded [5]. Likewise, fluorinated organic compounds have longer chains exhibiting persistence, from moderately solubility (e.g., perfluoro octane sulfonate = 1.08 g/L) to high solubility (e.g., perfluorooctanoic acid > 20 mg/L) in water, long-distance mobility, high to very high bioaccumulation (e.g., log Kow of perfluoro octane sulfonate = 4.5–6.9), and toxic effects [42,43][42][43]. In turn, plasticizers are commonly found in surface water (<1–12,000 ng/L), runoff (50–2410 ng/L), and other water sources [5,7][5][7]. They include bisphenol type -A/-S/-F and phthalates [5,7,41][5][7][41]. Fluorinated organic compounds have been detected in surface water (0.09–578,970 ng/L), groundwater (0.17–8.54 ng/L), WWTP influents (65–112 µg/L) and WWTPs effluents (43–78 µg/L), runoff (~2 ng/L), and drinking water (~54 ng/L) [5,41][5][41]. Furthermore, traces of fluorides (0.023–>1600 ng/g) have been found in some species of fish, amphibians, crustaceans, seals, whales, and polar bears [41]. Artificial sweeteners were found in drinking water (36–2400 ng/L), surface water (0.03–9600 ng/L), groundwater (non-detected–33,600 μg/L), seawater (200–393 ng/L), and lakes (non-detected–780 ng/L) [6,39][6][39]. Ethoxylated alcohol (surfactant) has been reported to affect fish and invertebrates. In fathead minnows, it affects egg production/larval survival, with a non-observed effect concentration (NOEC) ~0.73 mg/L. In species such as bluegills, it affects survival and growth at concentrations ~5.7 mg/L [44]. Non-ionic surfactants and nonylphenol ethoxylates have exhibited acute toxic effects (EC50 = 1.1–25.4 mg/L) on tadpoles of four Australian and two exotic frogs [39,44][39][44]. In turn, artificial sweeteners such as aspartame (2000–50,000 mg/L) have been reported to cause cancer in rats and headaches, nausea, and vomiting in humans. Meanwhile, 5% sucralose produced thymus shrinkage and migraine in rats and humans, respectively [39].

2.5. Microplastics

Microplastics (size = 1–5000 nm) are classified as primary (microbeads from personal care products and cosmetics) and secondary (degradation/fragmentation/leaching of plastics) [45]. Approximately 4130 tons/year of microbeads are used in different personal care products in EU countries plus Norway and Switzerland [45]. In total, plastic waste has reached values around 6300 million tons (oceans = 1.6–4.1%) between 1950 and 2015 [45,46][45][46]. More than 400,000 tons microplastics/year (95% from WWTPs) could enter the environment [45]. They were found in lakes and rivers (0.05–320 particles/L) and sediments from shore, water, and benthic at concentrations of 75–1300 particles/m2, 2.5–25,800.0 particles/m3, and 6.2–980 particles/kg, respectively [45]. A problem associated with microplastics is that those with low density (910–2200 kg/m3) could move hydrophobic contaminants long distances [47]. Dichlorodiphenyltrichloroethane (DDT), polycyclic aromatic hydrocarbons, chlorinated benzenes, polychlorinated biphenyls (PCBs), organo-halogenated pesticides, and endocrine disruptors have been found in microplastics such as polyethylene, polypropylene, PVC, and polystyrene [47]. BPA, polybrominated diphenyl ethers and phthalates, and microplastic additives also have been found in plastic debris at concentrations from 0.1 to 700 ng/g, up to 990 ng/g, and up to 3940 ng/g, respectively [48]. Microplastics produce digestive and locomotion problems, and changes in metabolic profiles in organisms, such as planktonic, crustaceans (e.g., Daphnia magna), fish (e.g., zebrafish), turtles, and whales [47,49][47][49]. These ECs could even compromise human health since they have been found in consumer species such as brown shrimp [47,48][47][48]. The surface area/shape/size/texture of microplastics is also related to toxicity. Particularly, the smallest microplastics in the form of fibers and a greater surface area have been reported to generate greater toxicity [48]. High-density polyethylene (0–80 μm) accumulated in the lysosomal system of blue mussels after 1.0 h of exposure. In addition, earthworms of the species Lumbricus terrestris exposed to 450 and 600 g/kg of polyethylene increased their mortality rate between 8 and 25%, respectively. There was also a decrease in their growth and negative effects on the construction of burrows [42]. Polypropylene fibers were more toxic than polyethylene spheres for Hyalella Azteca [45]. Among the most used methods to eliminate microplastics from aqueous medium are physical (sedimentation, membrane filtration), chemical (photocatalytic oxidation, coagulation, ozonation), and biological (conventional activated sludge systems, activated sludge systems + membrane bioreactor, microorganism biodegradation) [44,47,48,49,50][44][47][48][49][50]. The use of materials is also essential in the elimination of this type of EC, mainly through magnetic separation. Nanomaterials such as magnetic carbon nanotubes and magnetite have been used efficiently [51]. Shia et al. (2022) removed polyethylene, polypropylene, polystyrene, and polyethylene terephthalate (size: 200–900 μm), reaching efficiencies between 62.8 and 86.9% [52]. Table 1 shows the features of some ECs and their concentrations for different water sources. It is observed that there is a wide range of ECs found in different water bodies in their common concentrations (0.01 ng/L–>6010 mg/L). The EC characteristics are also very varied (log Kow = −3.4–13.9, water solubility = 0.49–21,600 mg/L), which could be a complication when it comes to removing them from water. Lipophilic substances (high log Kow) and low water solubility such as carbamazepine (anticonvulsant), triclosan (disinfectant), EE2 (hormones), and musk xylene (fragrance) suggest higher removal efficiency through adsorption processes. Meanwhile, hydrophilic substances (low log Kow) and high-water solubility, such as caffeine (stimulant), clofibric acid (lipid regulator), atenolol (beta blocker), and glyphosate (herbicide), are more difficult to remove through adsorption processes. This was verified in the research carried out by León et al. [53], in which triclosan (48.6–76.4%) was more easily removed than caffeine (40.1–67.4%) using agro-industrial residues, under the same operating conditions. Therefore, lipophilic ECs can be removed more easily than hydrophilic by adsorption process.
Table 1.
Physical and chemical characteristics of some ECs and their concentrations in water bodies.
EC Type EC Subtype Contaminant Water Solubility

[mg/L]		 Log Kow Concentration Found

[ng/L]		 Type of Water Reference
PPCPs Stimulant Caffeine 21,600 −0.07 753,500–1.0 × 106 Surface water [22,54,55][22][54][55]
20–23,970 Ground water
500–5000 Drinking water
Anti-inflammatory Ibuprofen 21 0.35 13.5–89,500 Effluent WWTP [19,22,54,56][19][22][54][56]
Anticonvulsant Carbamazepine 112–236 13.90 589–3.5 × 108 Influent WWTP [19,57,58][19][57][58]
1200–6.6 × 107 Effluent WWTP
Lipid regulator Clofibric acid 214,650 2.88 nd–420 Influent WWTP [19,59][19][59]
Antibiotic Ciprofloxacin 650 0.28 2200–14,000 Influent WWTP [19,60,61][19][60][61]
1100–44,000 Hospital wastewater
Diagnostic Contrast Media Iopromide 23.8 −2.1 780–11,4000 River water [19,62][19][62]
1170–4030 Urban effluents
Antidepressant Diazepan 50 3.08 nd–100 Surface water [19,63][19][63]
nd–100 Influent WWTP
Beta blocker Atenolol 300 0.16 90–255 Influent WWTP [19,64][19][64]
Fragrance Musk xylene 0.49–1.0 4.40 200,000–400,000 Drinking water [19,65][19][65]
Disinfectant Triclosan 10.00 4.76 200–1854 Influent WWTP [24,[25,2454]][25][54]
Pesticides Herbicide Glyphosate 15.70 −3.40 up to 6.01 × 109 Surface water [30,66][30][66]
Insecticide Acetamiprid 2950 0.80 0.08–249 Surface water [27,29][27][29]
Clothianidin 304 0.70 1.0–740 Surface water [27,29,67,68][27][29][67][68]
Thiamethoxam 4100 −0.13 1.0–914 Surface water
Steroid hormones Natural E2 13 2.45 3.8–188.0 Influent WWTP [19,58,60,69][19][58][60][69]
E1 13 3.43 12–196 Effluent WWTP [23]
17α-estradiol 13.3 4.01 6.4–12.6
Synthetic EE2 4.8 3.67 0.59–5.6
Industrial and chemical substances Anticorrosive Methylbenzotriazole 366 2.72 nd–2900 Effluent WWTP [29]
Plasticizer BPA 120 2.2–3.4 0.01–8800 Bottled water [5,70][5][70]
140–12,000 Surface waster
Microplastics - - - - 8.3–3.1 × 105 * Effluent WWTP [45,49][45][49]
0.27–30 * Lake water
0.05–320 * River water
Notes: Units for data with * = million pieces/m3.

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