Arsenic Occurrence and Cycling in the Aquatic Environment: A Comparison between Freshwater and Seawater

Abstract

Owing to the toxicity and adverse effects of arsenic on human health, its levels in aquatic environments are among the most serious threats to humans globally. To improve our understanding of its occurrence and cycling in aquatic environments, herein we review the concentration, speciation, and distribution of arsenic in freshwater, seawater, and sediments. Many natural processes, such as rock weathering and geothermal activities, contribute to the background arsenic concentrations in the natural environment, whereas metal mining and smelting are anthropogenic sources of arsenic in the water. The high solubility and mobility of arsenic in aquatic environments affects its global cycling. Furthermore, the biological processes in the aquatic environment are discussed, especially the possible microbe-mediated reactions of arsenic in sediments. In addition, various environmental factors, such as redox conditions, pH, and salinity, which influence the transformation of arsenic species, are summarized. Finally, the differences between freshwater and seawater with reference to the concentration as well as speciation and distribution patterns of arsenic are addressed. This review provides deep insights into arsenic occurrence and cycling between freshwater and seawater aquatic environments, which can more accurately distinguish the risks of arsenic in different water environments, and provides theoretical guidance for the prevention and control of arsenic risks.

1. Introduction

Arsenic is an element that is distributed globally and is abundant in the Earth’s crust (20th most abundant element) and seawater (14th most abundant element). It is classified as a metalloid and exhibits both metallic and non-metallic properties [1]. As a poisonous and globally distributed contaminant in the environment, arsenic was called the “king” of pollutants and is among those at the top of the United States Environmental Protection Agency (EPA) Superfund list [2]. Inorganic arsenic, which affects human skin and lungs, has also been classified as a strong carcinogen by the International Agency for Research on Cancer (IARC) [3]. Arsenic pollution in aquatic systems has caused disasters in human drinking water in more than 20 countries, including India, Bangladesh, China, the USA, and Argentina [4,5,6,7,8], and has received increasing international attention. This metalloid is present in both inorganic and organic forms and exists in four valences: –3, 0, +3, and +5 [9]. The inorganic arsenic forms mainly include arsenite (As^III) and arsenate (As^V), and methylated arsenicals are the major forms of organic arsenic compounds in the presence of monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA). The remaining organic arsenic species include arsenobetaine (AsB), arsenocholine (AsC), arsenosugars (AsS), and tetramethylarsonium salts [10]. Most recently, thiolated arsenic, which occurs in sulfidic environments, has been the focus of many studies [11,12,13].

Arsenic in the environment mainly originates from the weathering of rocks containing arsenic, volcanic eruptions, and certain biological activities [14]. In addition, the wide application of arsenic-containing herbicides, pesticides, and wood preservatives over long periods (i.e., centuries) is a major source of arsenic [15]. This metalloid does not degrade and cannot be destroyed in the environment. Instead, its high solubility and mobility in aquatic ecosystems results in its global cycling [1]. Most human drinking water is considered to be contaminated with arsenic because of arsenic global cycling originating from diverse natural sources and frequent human activities.

Once arsenic is released into water (freshwater and seawater), it undergoes various biochemical reactions involving physical spatial transportation and transformation of chemical forms. In aquatic environments, arsenic transportation and distribution depend not only on its chemical forms but also on its interactions with the environmental matrix [16], especially metal oxides such as iron oxyhydroxides and manganese oxides. Arsenic in aquatic systems can be adsorbed onto charged particles because of its oxide-like properties and can settle into the sediments as flocculated particles [17,18]. As arsenic partitioning between water and sediments is complex and relates to many environmental and biotic factors, when variations in Eh, pH, and organic matter content exist in sediments, the arsenic in sediments may be resuspended into the overlying water column [19]. In addition to physical space transportation, the arsenic speciation transformation process has been considered in many studies [20,21,22]. Oxidation and reduction occur quickly depending on environmental conditions such as pH, Eh, and biological activities [22]. Methylation and demethylation are also important components of the arsenic biogeochemical cycle, which can influence the environmental behavior of arsenicals [21]. As a result, arsenic circulates in the aquatic ecosystem through physical spatial transportation and chemical transformation. It is highly important to differentiate circulation between freshwater and seawater ecosystems.

Therefore, this study describes the sources of arsenic in aquatic environments and pays close attention to the occurrence of arsenic in aquatic environments, including freshwater, seawater, and the associated sediments. The potential differences in the behavior of arsenic in these different environments were compared. The overall aim of this study was to summarize the understanding of the behavior of arsenic in different aquatic environments, which is essential for risk evaluation and regulation of arsenic contamination.

2. Sources of Arsenic in the Aquatic Environment

Arsenic has been introduced into aquatic environments through both natural and anthropogenic sources [22,23]. Natural sources include rock weathering and geothermal activities [24], with global arsenic emissions of ~12,000 t/y [25]. Simultaneously, human activities, such as metal mining and smelting, the burning of fossil fuels, and the discharge of arsenic-based industrial wastes lead to significant amounts of arsenic entering the environment [22]. Approximately 82,000 t of arsenic is emitted into the environment each year from anthropogenic sources [26]. Arsenic from natural and anthropogenic sources eventually converges in the environment through biogeochemical cycles, including weathering reactions, atmospheric settlement, and biological activity (Figure 1). Therefore, in environmental risk evaluations of arsenic, both natural and anthropogenic sources should be considered.

2.1. Natural Sources of Arsenic

Natural sources refer to arsenic entering aquatic environments through natural geological processes, with rock weathering being the principal natural method [20]. Weathering results in elevated concentrations of arsenic in natural waters, especially in groundwater [22]. Arsenic of geological origin is found in all three rock types: igneous, metamorphic, and sedimentary rocks [27]. The arsenic concentrations in different igneous rocks tend to be comparable and low. In most metamorphic rocks, arsenic concentrations are similar (≤5 mg/kg), except in pelitic rocks, such as slates and phyllites, which have high arsenic concentrations of approximately 18 mg/kg [28]. Compared with the aforementioned two rock types, relatively higher arsenic concentrations, between 5 and 10 mg/kg, are found in sedimentary rocks. Among these, argillaceous deposits, which contain more binding matter, have been found to have a wider distribution with higher arsenic concentrations [23]. For instance, the highest concentration of arsenic (35,000 mg/kg) was found in some coal samples [29]. Arsenic in the oceanic lithosphere was also a natural source for the seawater, but the data and information on arsenic concentration were rare [30]. It was reported that arsenic concentrations ranged from 0.5 to 5.8 mg/kg in magmatic rocks from island arc and back-arc settings, which were slightly higher than basic and ultrabasic rocks [31].

Geothermal activity is another major natural source of arsenic contamination [32], with arsenic appearing frequently in geothermal fluids [33,34]. It has been demonstrated that arsenic may leach from rock or molten lava [33]. Geothermal waters, which commonly contain elevated arsenic concentrations mixed with drinking water sources, give rise to arsenic contamination, as reported in a geothermal field in Mexico, where the highest recorded arsenic concentration was 73,600 μg/L [35]. Arsenic can leach from volcanic rocks into the surrounding groundwater; in California, for example, the arsenic concentration in volcano-related groundwater is approximately 8000 μg/L [16]. Hot springs associated with volcanogenic sources carry arsenic from the mantle to the Earth’s surface [14]. The concentration of arsenic in the hot springs of geothermal fields has been reported to range from 170 to 4800 μg/L in New Zealand [26]. For the marine environment, hydrothermal fluids with variable amounts of arsenic were an important input source of arsenic [30]. Overall, arsenic discharged into the aquatic environment through natural pathways constitutes the background arsenic concentration in the aquatic environment.

2.2. Anthropogenic Sources of Arsenic

Anthropogenic sources refer to the release of arsenic-containing waste generated by human activities, including industrial and agricultural activities and domestic sewage, into rivers, oceans, and municipal water supplies [7,14,36]. Anthropogenic activities have led to significant levels of arsenic contamination in surrounding environments, with industrial activities being the most notable. Industrial processes, such as mining, smelting, ore beneficiation (tin, zinc, copper, and gold), and wood preservation, all contribute to an increase in the arsenic content of industrial wastewater [37]. In terms of marine environment alone, the major anthropogenic sources of arsenic in surface seawaters are riverine inputs, which are contaminated by various pollutants such as industrial and agricultural chemicals [23]. It has long been recognized that mining, smelting of nonferrous metals, and metal ore processing are major sources of arsenic contamination worldwide. Arsenic pollution events associated with mining activities have been highlighted in numerous studies [16,38,39]. High concentrations of arsenic have frequently been reported to reach 48,000 mg/L [16]. In addition, the nonferrous metallurgical industry emits more than 40,000 tons of arsenic each year [40,41]. At present, the disposal of smelting and mining waste has led to arsenic pollution in groundwater in many places, including the western parts of the USA, southeastern Europe, Canada, Chile, Ghana, South Africa, Thailand, and Turkey [22,42,43]. Electrical waste (semiconductors) [44] and chemical products [45] also contribute to industrial sources of arsenic.

Agricultural wastewater is another major anthropogenic source of arsenic. For hundreds of years, inorganic arsenic has been widely used in herbicides, pesticides, insecticides, and fungicides [46,47,48]. In crop cultivation, arsenic has been widely used in pesticides and herbicides since the late 1800s. Widespread usage of inorganic arsenic compounds, mainly sodium arsenite, began in the early 1900s. Consequently, organic arsenic is used as a pesticide to control insect pests in various crops (tobacco, cotton, and potatoes) and fruit trees [43]. Zinc methyl arsenate is used to prevent infestation by Corticium sasakii in rice paddies [31]. In the late 1980s, legislation prohibited the use of inorganic arsenic-based pesticides in agriculture. However, organic arsenicals, such as MMA and DMA, are still allowed owing to their lower toxicity [23]. Arsenide is widely used in wood preservation and as a food additive. For example, 4-aminoben-zenearsenic acid and 3-nitro-4-hydroxyphenylarsonic acid (roxarsone) are additives in animal food used to raise broiler chickens and cause weight gain during chicken breeding, which leads to a high arsenic content in poultry excrement [49]. The maximum arsenic content in poultry litter was 40 µg/g [50]. This arsenic-bearing poultry litter is often used as fertilizer for farmlands and pastures. Arsenic in fertilizer is mostly water soluble and may enter the aquatic environment via biogeochemical cycles [51]. In summary, there are many possible routes for arsenic transfer into the aquatic environment from both natural and anthropogenic sources.

3. Arsenic Occurrence in Freshwater

3.1. Arsenic Concentration and Forms in Freshwater

In general, the typical arsenic concentration in unpolluted freshwater ranges from less than 1 µg/L to 10 µg/L. The range of arsenic concentrations in freshwater worldwide are shown in

Table 1. Arsenic concentration in freshwater around the world (µg/L).

Water Body and Location	Arsenic Concentrations (Average or Range)
Rivers from upper Paraguay River basin, USA	0.05–1.69
Bowron Lake, Canada	<0.2
Lowhee Creek, Canada	0.2–2.0
Lake Biwa, Japan	0.6–1.7
Dordogne, France	0.7
Youngsan River, Korea	1.5 (1.3–1.7)
Lakes in Yellowknife, Canada	2–136
Rivers of Poopó basin, Bolivia	10–11,140
Humboldt River, USA	12–60
Xiaoqing River, China	13.9–58.9
Moira Lake, Ontario, Canada	22.0–47.0
Contaminated lake near Cobalt, Canada	23.6–972
Madison and Missouri rivers, USA	44 (19–67)
Lakes in the Ziwaye-Shala basin, Ethiopia	165 (2.39–566)
Ashanti, Ghana	284 (<2–7900)
Lakes in the Town of Cobalt, Canada	431 (2.2–972)
Rivers of Rio Loa basin, Chile	1400–21,000
Mono Lake, USA	10,000–20,000

. Usually, arsenic baseline concentrations range from 0.1 to 0.8 µg/L in river water. The variation in concentration, which reaches a maximum of 2 µg/L, is often a reflection of the difference in the constitution of the surface water supply from the influence of base flow and basement rock formation [23]. Richter et al. found that a river showed the lowest value of arsenic concentration (0.05 µg/L) at the basin-draining sandstone areas of the upper Paraguay River, mainly because the bedrock is arsenic-poor [52]. However, owing to natural processes and anthropogenic activities, relatively high concentrations in some rivers have been widely reported. To date, there has been an ever-increasing amount of research on arsenic contamination in rivers due to the release of geothermal fluids and the discharge of hot springs. Arsenic concentrations in rivers after mixing with arsenic-rich geothermal waters have been reported to be in the range of 10–100 µg/L or higher in arsenic-rich geothermal areas [53,54,55,56,57]. In Tatio, Chile, for example, arsenic-rich geothermal water (47,000 μg/L) mixed with water from the Loa River which provides drinking water for the local population [54]. In another example, because of the merging of geothermal spring water into the Madison River, arsenic concentrations in the river water and downstream of the Missouri River are as high as 360 μg/L [58]. In addition, elevated concentrations of arsenic are present in river water due to human activities, especially mining activities [59,60,61]. Jonnalagadda and Nenzou noticed that water discharged from mine waste into surface water contributed to an increase in arsenic levels [59]. Rango et al. and Seror-Armah et al. noted high arsenic concentrations (approximately 150–8250 µg/L) in water bodies in the mining regions of the Main Ethiopian Rift [61,62]. Recently, Mohammad and Tempel highlighted arsenic concentrations in the Humboldt River, Nevada, ranging from 12 to 60 µg/L because of the influence of nonmetallic economic deposits [60].

The arsenic content of lakes is typically similar to that of rivers, or even lower, with values commonly between 1 and 10 µg/L [63,64]. In Bowron Lake in Canada, the baseline concentration was found to be lower than 0.2 µg/L [63]. The arsenic concentration in Taihu Lake in Jiangsu, China, regarded as a vital water source area, has been reported to be in the region of 0.06–7.92 µg/L [27]. Notably, influenced by geothermal inputs and industrial discharge, elevated arsenic content has also been found in many lake systems [23]. Extremely high concentrations ranging from 10,000 to 20,000 µg/L have been reported in Mono Lake, California, USA, owing to inputs from geothermal springs and evaporation [65]. Furthermore, many lakes are affected by nearby mining activities, with arsenic concentrations ranging from 100 µg/L to 500 µg/L [66,67,68]. The mean arsenic concentration in surface waters from nine lakes directly influenced by mining near Cobalt, Ontario, Canada was 431 µg/L [68].

Inorganic arsenic is the most common form found in natural water bodies [22] and can exist in nature in diverse oxidative states. Moreover, the forms of arsenic in freshwater environments are affected by several environmental and biological factors, such as pH, redox conditions, phosphorus concentration, microbial activity, and productivity levels [69]. As a thermodynamically stable form, arsenic exists almost entirely as As^V [64,70]. Microbial reduction, especially by phytoplankton and As^V bacteria, leads to the formation of thermodynamically unstable As^III [71]. These inorganic forms of arsenic are taken up or digested by aquatic organisms and then methylated to organic forms through different elimination pathways that return the arsenic to the water column [69]. In other words, the proportion of organic forms of arsenic increase with microbial activity in water. The relatively stable forms of organic arsenic present in freshwater are MMA and DMA. The concentrations of these two forms of arsenic in freshwater systems are similar to that of inorganic arsenic in highly productive freshwater systems [69]. Recent publications have reported the occurrence of some unknown arsenicals, such as arseno-sugars, in freshwater [69,72,73]. In general, compared with the concentrations of arsenic, which varies over several orders of magnitude, arsenic speciation shows similar characteristics, but tends to be site-specific.

3.2. Cycle of Arsenic in Freshwater

Arsenic in freshwater undergoes complex chemical and physical reactions including oxidation, reduction, adsorption, precipitation, and dissociation [26]. Spatial transportation and transformation of chemical forms of arsenic occur in combination (Figure 2). Taking the occurrence of arsenic in lake water as an example, Ferguson and Gavis observed that an inorganic arsenic cycle occurred in some lakes [20]. On a spatial scale, inorganic arsenic in the upper section of the water column combines with iron hydroxides to form co-precipitates. Through co-precipitation, adsorption, and crystal epitaxial attachment, these arsenicals settle into sediments as insoluble sulfides or metallic arsenic. These processes may cause the concentration of arsenic to stratify with depth [69]. Normally, in lakes, arsenic concentrations increase with increasing depth [63]. During this process, the coexistence of matter such as

Fe, Mn oxides, and nitrates may influence the arsenic concentration because of their ability to regulate the dissolved arsenic concentration [74,75]. For instance, nitrates in water affect the occurrence of inorganic arsenic by oxidizing Fe²⁺ to arsenic-sorbing particulate hydrous ferric oxides (Fe₂O₃•XH₂O, FeOOH, and HFO) [75]. However, when the pH and redox conditions are appropriate, arsenic in the sediments can be resuspended into the water column. Thus far, arsenic in aquatic environments is cycled completely through the system, with circulation being closely linked to several environmental factors. Matschullat identified a negative correlation between arsenic concentration and pH values [43]. Hence, related to water and soil acidification, an increase in arsenic concentration may occur in lake sediments. In addition, several studies have reported seasonal variations in the occurrence of arsenic in lake water, with higher concentrations in summer than in winter [64,76,77]. These variations were due to the release and adsorption of arsenic under different redox potential conditions in summer and winter, probably related to biological productivity [64].

Oxidation and reduction of organic arsenic are the first steps in species transformation in freshwater. In the aerobic upper section of the water column, As^III tends to oxidize to As^V and then combines with iron hydroxides to form co-precipitates. However, As^III is also common in freshwater systems [71,78], and it is worth noting that microbial activity greatly reduces As^V [69]. Some species of bacteria, such as dissimilatory arsenate-respiring prokaryotes, utilize the reduction of arsenic to obtain the energy required for survival. Plankton, especially algae, also facilitate the transformation of As^V in freshwater systems [79,80]. Wang et al. reported that freshwater algae take up As^V and release As^III into water through excretion, which has been recognized as an important route of arsenic detoxification [79]. Organic arsenic species with regular formation in freshwater are methylated arsenic compounds that appear as trivalent methyl arsenic species (MMA^III and DMA^III) and pentavalent methyl arsenic species (MMA^V and DMA^V) [81]. Many studies have shown that methylated arsenic is a major form of arsenic in freshwater [69,82]. In the natural environment, Braman and Foreback first confirmed the presence of DMA and MMA in McKay Bay freshwater samples (Florida, USA), contributing approximately 68% of the total arsenic in this system [83]. Similar findings have been widely reported in the literature; however, the percentage of these species was typically lower than 10%. Recently, arsenic speciation was detected in freshwater sites from the Youngsan River; MMA and AsB were found in samples, comprising 6.3% and 2.2% of the total arsenic concentration, respectively [73].

It is widely believed that MMA and DMA formation is closely associated with various microorganisms present in freshwater [84,85]. Many studies have demonstrated that aquatic organisms have the ability to methylate arsenic to alleviate its toxicity [86,87,88], which varies depending on the species [89]. Phytoplankton tend to produce monomethylarsenic and dimethylarsenic compounds in freshwater systems [26]; the diatom Skeletonema can generate DMA [69,88], and the cryptophyte Chroomonas spp. can produce MMA [69]. Several studies on arsenic biotransformation have indicated that organisms at higher trophic levels have a greater ability to methylate arsenic and even form more complex organic arsenic such as trimethyl arsenic compounds [1,90,91,92,93]. For example, freshwater shrimp transform inorganic arsenic into less toxic organic forms, including MMA, DMA, AsB, and trimethyl arsenic (TMA), which are excreted into the surrounding aquatic environment [93]. Consequently, the occurrence of organic arsenic species in freshwater systems is strongly linked to biological productivity [88]. The ratio of methyl arsenicals is positively related to chlorophyl-a (chl-a) content and algae density [89]. To date, biological processes associated with seasonal cycles have been investigated in detail [84,87,94]. The peak concentrations of methyl arsenicals were found to occur in early summer [84] (Hasegawa et al., 1997), and Sohrin et al. also pointed out that DMA^V was the predominant methyl arsenic species, the proportion of which could reach 64% of the total arsenic in summer in the surface water of Lake Biwa [87]. In summary, the transformation of arsenic species in freshwater, which occurs due to biological activities, can be described as follows: As^V is taken up by aquatic organisms and subsequently reduced to As^III, which is then methylated to MMA, DMA, and even more complex organic forms, and released back through excretion into the water column.

4. Arsenic Occurrence in Seawater

4.1. Arsenic Concentration and Speciation in Seawater

As the 14^th most abundant element in seawater, the mean concentration of arsenic in unpolluted seawater is 1.7 μg/L, ranging from 1 to 3 μg/L [31,95]. Many studies have reported arsenic concentrations in all four oceans, especially coastal areas, which are closely related to human activities [96,97,98]. As the largest ocean in the world, the mean arsenic concentration in the open ocean region of the Pacific Ocean is approximately 1.0 μg/L [96]. Concentrations of arsenic in the central Pacific gyre, the Southwest Pacific Ocean, the Northwest Pacific Ocean, and the Western Pacific equatorial region have been reported to be lower than 1.8 μg/L [98,99]. Average arsenic concentrations in the surface waters of the East and North Indian Oceans are in the range of 0.8–0.9 μg/L [99], whereas the surface seawater of the Atlantic Ocean showed arsenic concentrations ranging from 1.0 to 1.8 μg/L [69]. Arsenic levels in several marine environments are listed in

Table 2. Arsenic concentrations in seawater and sediments around world (µg/L and µg/g).

Marine Water	Arsenic Concentration (µg/L) (Average or Range)
China Sea	0.6
Atlantic Ocean	0.6–1.6
Indian Ocean	0.8–1.1
Atlantic Ocean	0.94–1.56
Pacific Ocean	1.0
Unpolluted seawater	1.0–2.0
Atlantic Ocean	1.0–1.8
Pacific Ocean	1.2–1.6
Coastal Australia	1.3 (1.1–1.6)
Rhône estuary, France	1.3–3.7
Southern Tasman Sea	1.4
Galway Bay, Ireland	1.7
Southern Ocean	1.7–1.8
Krka estuary, Yugoslavia	1.8
Scheldt estuary, Belgium	1.8–4.9
Marine hydrothermal fluids	24.0–5850
Marine Sediment	Arsenic Concentration (µg/g) (Average or Range)
Paranagua Bay, Brazil Western North Sea	0.1–81.5 <0.2–135
Uncontaminated marine sediments	5.0–15
UK estuarine sediments Rio Lao basin, Chile Doce River mouth, Brazil	7.0–950 7–11,000 8.2–232.3
Carnon/Restronguet Estuary	9.0–5000
East China Sea	11.5 (1.7–22.1)
Baltimore Harbor, USA	25.0–41.1
Deep-sea sediments	40
French Mediterranean Estaque port	107–220
Tinto and Odiel River and Estuary	200–3000

. In general, little variation in arsenic concentrations has been observed in open oceans. However, human activities that produce arsenic-contaminated discharges may add arsenic loads to estuaries and coastal waters. Arsenic concentrations were in the range of 1.8–4.9 μg/L in the Scheldt estuary, Belgium, while a low level of 4.6 to 7.2 μg/L was detected in the outer estuarine area of the Youngsan River Estuary, Korea [73,95]. In addition, because of different river influxes, salinities, and other environmental factors, estuarine waters also show obvious divergence [22]. For instance, with the increase of salinity in sea water of the uncontaminated Krka estuary, Croatia, arsenic concentration rose from 0.13 μg/L to 1.8 μg/L [100]. In seawater, arsenic concentrations are also influenced by marine hydrothermal fluids, which elevate temperatures and potentially cause magmatic degassing. Water–rock interactions are facilitated, and large amounts of soluble arsenic are released from the hydrothermal fluid. The arsenic concentration of the East Pacific Rise, which is affected by the mid-oceanic ridge hydrothermal fluids, was approximately 80.5 μg/L. The highest arsenic concentration was approximately 24.0 μg/L in the Mid-Atlantic ridge. Extreme concentration values of 1386.0 μg/L and 5850.0 μg/L were reported, and they occurred near hydrothermal systems close to shorelines [30].

Arsenic can occur in marine environments in various chemical forms. The most common forms are As^V and As^III [72]. At normal pH, As^V is present as H₂AsO₄⁻¹ (pH ranges from 2.5 to 7) and HAsO₄⁻² (pH ranges from 7 to 12), whereas As^III is present as As[HO]₃ (pH < 9.3) [101]. Although As^V is the most abundant form under thermodynamically stable conditions, with its ratio exceeding 300:1 to As^III in open seawater (Li, 1991), As^III coexists with As^V owing to the biotic and abiotic reduction of As^V. Field investigations have suggested that the concentrations of As^III in seawater are always higher than those calculated [102]. Inorganic arsenic in seawater undergoes a series of chemical and biological processes including oxidation, reduction, and methylation, while other methylated arsenic species and more complex organic arsenical products occur in seawater. Specifically, organic arsenic species include methylated arsenicals, such as MMA^V and DMA^V, which are found frequently in seawater, and others, such as MMA^III, DMA^III, AsB, AsC, AsS, dimethylarsinoyl acetate, and dimethylarsinoyl ethanol [103]. Among these organic forms, methylated arsenic forms, which can remain in a steady state for a long period, account for approximately 20% of the total arsenic concentration [104,105]. For instance, the proportion of methylated arsenic in the shallow waters of the Northwest Pacific Ocean was 15% [99]. Another noteworthy organoarsenic, AsB, which is generally considered to be excreted by most marine organisms, has been reported in seawater [73].

4.2. Cycle of Arsenic in Seawater

Figure 2 shows the occurrence and cycle of arsenic in seawater. In marine environments, arsenic may undergo complex biogeochemical processes owing to position transfer and chemical transformation [106]. The presence of a vertical concentration distribution model of arsenic in seawater suggests the occurrence of a natural cycle [107,108]. Normally, the arsenic concentration is lower in the upper water layer than in the deep-water layer. The mean arsenic concentration in the eastern Atlantic Ocean was 1.0 μg/L; a relatively constant and higher average concentration was reported in deep water (1.5 μg/L) [109]. A similar concentration distribution pattern at depth has been found in the North Pacific Ocean [107]. Arsenic transfer in a spatial position is more obvious in continental shelf waters due to frequent mixing between sediments and the overlying water [110]. This natural cycle also shows seasonal variations, with the lowest concentration in spring and the highest in autumn [111]. Arsenic may undergo transformations in its chemical form because of changes in environmental physicochemical conditions, as well as biologically mediated reactions (biotransformation), which can strongly affect its fate [106]. The predominant environmental behavior of arsenic forms includes the interconversion between As^III and As^V, arsenic methylation, and organo-arsenic biosynthesis [112]. Biological activities play the most important role in the transformation of arsenic species. As a detoxification mechanism for marine organisms [71,90,113], it is widely recognized that As^V can be concentrated, reduced to As^III, and then converted to organic forms such as DMA, arseno-sugars, and arseno-lipids. Most notably, these organic forms are efficiently metabolized to AsB via the marine food chain and accumulate in marine animals [99]. A large proportion of organo-arsenicals transferred in continuous food chains enter the surrounding water through excretion [114] or are released by decomposing dead animals [115,116]. Because of the chemical stability of AsB, it is found in both the euphotic zone as well as in deeper water [117,118]. The fates of these complex organo-arsenicals in seawater depends on the surrounding biological activity. In contrast, the familiar organic arsenic form, AsB, is degraded by the ubiquitous presence of potential AsB-degrading microorganisms in seawater. The first step of degradation is the conversion of AsB to TMAO, then to DMA, which is an intermediate product, and finally to inorganic forms [119,120,121].

Several studies have suggested that this biological process is influenced by several key factors, such as water temperature, nutrients in water, and planktonic communities. Water temperature has been found to influence the methyl-arsenic efflux rates of organisms depending on the species [122]. As the temperature increased from 5 to 15 °C, the rate of MMA and DMA excretion by the macroalga Ascophyllum nodosum increased two-fold and more than six-fold, respectively. Moreover, DMA has been proven to be the predominant form of organoarsenic at all temperatures during the excretion of phytoplankton into ambient water [123]. DMA often reaches a maximum concentration in the water column in early summer, when As^V is at a lower level. In addition to temperature, phosphate concentration in marine water is also an important factor in regulating the biological transformation of arsenic. As^V is known to have chemical characteristics similar to those of the macronutrient phosphate. Consequently, a low phosphate concentration in seawater facilitates As^V bioaccumulation, followed by As^V reduction and As^III methylation for the detoxification of marine organisms [114]. Typically, water temperature and available nutrients in seawater have a positive influence on the concentration of methylated arsenic; that is, the intensity of the activity of organisms regulates the abundance of DMA and MMA in seawater [98]. Overall, arsenic has complex environmental behavior in marine systems, exhibiting a relatively shallow depletion, a mid-depth maximum, and a moderate deep-water enrichment [124]. Approximately 15–20% of arsenic participates in the biological cycle. Most of it is not included in the cellular composition but is rapidly metabolized and released, forming measurable concentrations of As^III and various organic species [104].

4.3. Comparison of the Differences in Occurrence between Freshwater and Seawater

Generally, given the influence of marine hydrothermal fluids, the arsenic concentration in seawater is slightly higher than that in freshwater [124]. The average arsenic concentration in seawater is approximately 1.5 μg/L, whereas in river water it ranges from 0.1 to 2.0 μg/L with the absence of significant natural and anthropogenic sources, and even lower than 1 μg/L in lake water [22]. Because of different salinities, riverine inputs, and redox gradients, the concentration of arsenic in estuarine water shows increased variation [100]. The concentrations of arsenic in seawater have been reported to be more consistent than those in freshwater [22,114]. However, the main arsenic species in both freshwater and seawater were comparable. The major inorganic forms of arsenic in freshwater and seawater are As^V and As^III [125]. Furthermore, mono-, di-, and trimethyl arsenicals have been isolated from both freshwater and seawater [125], suggesting that inorganic arsenic can be metabolized to methylated compounds in both freshwater and seawater systems. Recently, Hong et al. compared arsenic speciation in freshwater and seawater samples in the Taehwa and Youngsan River Estuary, Korea. The results showed that inorganic forms of arsenic, including As^III and As^V, were similar in seawater and freshwater. The compositions of organic forms of arsenic were more complex in seawater, where AsB, MMA, and DMA appeared [73,126]. Meanwhile, a larger portion of unidentified arsenic was found at saltwater sites in both river estuaries. These unknown arsenicals are likely to be products of aquatic organisms, such as arseno-sugars. Other studies have suggested that organic species are more abundant in seawater than in freshwater because of the more complex biological chains in seawater [127,128]. Therefore, the biological transformation of arsenic is crucial for the distribution of multiple constituents of arsenic compounds in water environments. From the perspective of biological activities, arsenicals undergo bioconversion via the aquatic food chain, which shows a greater extent of reduction and biomethylation of arsenic at higher trophic levels in both freshwater and marine ecosystems [73,129]. However, the content and speciation of arsenic in freshwater and marine organisms vary. For example, dimethylarsenic compounds in marine algae have been found to be AsS, whereas AsS in freshwater algae has not been determined [73]. Inorganic arsenicals and MMA were obtained from freshwater zooplankton, while various arsenic formations, especially organic ones, were detected in marine zooplankton, including MMA, AsB, and AsC [69,130]. Furthermore, marine fish can accumulate higher concentrations of arsenic, especially organic arsenic (AsB), than freshwater fish [131,132,133]. Interestingly, it has been reported that the most common arsenic compound in marine organisms, AsB, is rare in freshwater vertebrates and invertebrate organisms [134]. In addition, the characterization of arsenic in freshwater benthic invertebrates as mostly AsS, but not AsB, contrasts with the marine and freshwater environment where AsB is the major compound. Biotransformation by aquatic organisms produces abundant arsenic speciation in both freshwater and seawater environments. However, different organisms have different arsenic biotransformation efficiencies, suggesting that the presence of arsenic speciation is diverse.

5. Arsenic Occurrence in Freshwater and Seawater Sediments

Sediments are exposed to a multitude of pollutants and create latent origins of contamination in the overlying water column, especially those with a significant degree of anthropogenic disturbance. Overall, arsenic abundance in sediments is relatively higher than that in aquatic environments [135]. Figure 3 shows the occurrence and cycle of arsenic in the sediments. The sum of arsenic influx into the water column is 5000 t/y from sediments [136]). Arsenic concentrations are usually in the range of 5–15 μg/g in uncontaminated sediments of nearshore marine and estuarine environments [137], approximately 40 μg/g in sediments of deep-sea regions [138], and less than 20 μg/g in estuarine sediments of southeastern China [139]. In the East China Sea, for instance, arsenic concentrations range from 1.7 to 22.1 μg/g in sediments [140]. Except for the link to lithologies in the area, the sources of arsenic contamination are anthropogenic activities and sedimentary processes [141]. Sediments from estuaries accepting industrial effluents from mining activities, and aquaculture areas that contain arsenic-enriched discharges, may show expected elevated arsenic concentrations. Earlier reports have suggested that the sediments of the Carnon/Restronguet Estuary in southwestern England have total arsenic concentrations of between 9 μg/g and more than 5000 μg/g [142]. The total arsenic concentration in the sediments of the rivers of England and Wales ranged from 7 to 950 μg/g [143], with values less than 200 to 3000 μg/g in the sediments of the Tinto and Odiel Rivers, Spain [144]. Recent studies have shown that high concentrations of arsenic exist in the near-shore surface sediments of some South American countries, such as Brazil, Chile, Colombia, and Argentina [141,145,146,147]. The arsenic concentration in the bottom sediments of the continental shelf of the Doce River, Brazil, exceeded 8 mg/kg, which is attributed to noble metal exploitation over a long period [145]. Within Sepetiba Bay, Brazil, a large Zn smelter is located at the Engenho Inlet, where very high arsenic concentrations (up to 347 mg/kg) were observed in the sediments [31]. In addition to the Brazilian coastal sediments, sediments in the Loa Basin (Chile), Barbacoas Bay (Colombia), and southern Pampa region (Argentina) had varying degrees of arsenic contamination due to natural and anthropogenic activities [53,148,149].

The arsenic concentration in the sediment is influenced by several environmental conditions. Binding substances, including manganese (oxy)hydroxide, iron, and sulfur ions, which can settle the soluble arsenic from water into the sediments, are responsible for the variation in arsenic concentrations in sediments [150]. It is generally believed that there is a positive correlation between arsenic, iron, and manganese (oxy) hydroxide concentrations in sediments [151,152]. Cagnin et al. reported that reactive arsenic peaks at the sediment surface were associated with iron and manganese oxides [145]. Redox potential is another key factor that can change the mobility of arsenic in sediments. Under anoxic conditions, arsenic is likely to be mobilized from sediments to the water column due to the decomposition of manganese and iron oxides or sulfides [153]. Moreover, given that redox changes in sediments are related to water temperature, arsenic mobilization in sediments is sensitive to temperature in both freshwater and marine environments [154,155]. Arsenic concentrations in sediment samples collected during the cold and hot seasons were in the range of 107–199 μg/g and 194–220 μg/g, respectively [155]. In addition, the distribution of arsenic in sediments varies according to the depth, clay content, and total organic matter [140,155]. Arsenic concentrations are commonly higher in deeper and more fine-grained sediments.

The predominant forms of arsenic in sediments are As^V and As^III [156] in both freshwater and saltwater environments [157]. In shallow sediments, As^V is higher than As^III [158], whereas As^III is the predominant chemical species in reduced sediment layers. However, with constant biogeochemical reactions, arsenic chemical species in sediments also undergo complex transformations. It is widely known that the oxidation of As^III to As^V in the oxidized layers of sediment can be catalyzed by abundant oxidants such as iron oxyhydroxides, manganese, and nitrate [159]. Current studies also show that, in the organic and sulfur-rich layers, arsenic can bind with organic matter through thiol bonding in the form of thiol-arsenic forms, which are kinetically slow [27,160,161,162]. Wang et al. showed that thiol-bound trivalent arsenic exists in the Mekong Delta sediments [27]. Gorny et al. pointed out that the mobility of arsenic in sediments is corrected by the production of sulfides [160]. In addition, microbial transformations of arsenic are more noteworthy, composed of As^V respiratory reduction, As^V cytoplasmic reduction, As^III oxidation, and methylation. These biological activities directly influence the speciation distribution and mobility of arsenic and are responsible for biogeochemical arsenic cycling in sediments [136]. Bacterial species that appear in aerobic sediments can oxidize As^III [114]. Conversely, the reduction of As^V is mediated under anaerobic conditions [163], which has been proven to be aided by As^V-reducing bacteria [164,165]. Certain species of aerobic and anaerobic sedimentary microorganisms can concentrate As^V and As^III and biotransform them into organo-arsenicals, including MMA and DMA [86]. Thus, sediments are potential sources of organic arsenicals for the overlying water column. It has been reported that the biogeochemical cycle of arsenic in sediments of the Jianghan Plain is influenced by microorganisms through methylation [166]. In addition to organo-arsenicals directly excreted by sedimentary microorganisms into the overlying water column, primary arsenic formations that decompose from complex organo-arsenicals by these microorganisms are also released [167,168].

It is noteworthy that the mobility of different arsenic species in sediments between freshwater and marine environments also varies owing to their different compositions. Typically, arsenic tends to react with sulfides to form realgar (AsS) inclusions, which have limited solubility and mobility [114,169]. In comparison, sulfur is more abundant in marine sediments facilitated by sulfate-reducing microorganisms [114,170,171]. In other words, arsenic has stronger mobility and tends to be more bioavailable in freshwater than in seawater. Therefore, recent studies have provided a clear picture of arsenic transformation in sediments, which could affect arsenic migration, transformation, accumulation, and the fate of arsenic in aquatic environments.

6. Conclusions and Future Perspectives

Arsenic pollution of aquatic environments has attracted worldwide attention. Some water bodies contaminated with arsenic by natural and human activities have significant concentrations, which endanger the safety of living organisms and consumers. Therefore, the first section of this paper reviewed both anthropogenic and natural sources, which should be factored into evaluations when assessing the environmental risk of arsenic. Rock weathering is the most critical natural mechanism that leads to the release of arsenic into the environment. Industrial and agricultural wastewater are the largest sources of anthropogenic arsenic.

To better understand arsenic cycling in aquatic environments, the concentrations and chemical forms of arsenic in freshwater and seawater were reviewed. Arsenic may undergo transformations in its chemical form not only because of changes in environmental physico-chemical conditions but also through biologically mediated reactions (biotransformation). Therefore, the distribution of arsenic species in aquatic systems was emphasized to illustrate the arsenic biogeochemical cycle, which is affected by biological and environmental factors.

The occurrence of arsenic in freshwater and seawater was compared. The arsenic concentrations in seawater were more consistent than those in freshwater. Meanwhile, arsenic appeared in more diverse chemical formations in marine environments than in freshwater environments. In comparison, various constitutions of arsenic compounds between freshwater and saltwater sites were more obvious than the concentration discrepancy linked with the diversity in biotic community composition.

Furthermore, as a significant source of pollution in the aquatic environment, the arsenic concentration and species distribution in sediments were addressed in detail, particularly the transformation processes of arsenic. Several environmental factors, such as iron and manganese (oxy) hydroxide concentration, and temperature, are involved in altering the arsenic concentration. High rates of microbial activity in sediments affect the fate of arsenic, thereby inducing more abundant chemical forms of arsenic in sediments than in the aquatic environment.

Overall, this review provides up-to-date information on the concentration, speciation, distribution, and transformation of arsenic in freshwater and seawater environments, enhancing our understanding of the occurrence of arsenic in aquatic environments. Future studies should (1) collect more relevant data on arsenic concentration distribution in freshwater and seawater systems worldwide to assess health risks in different regions; (2) provide more information about the different forms of arsenic in freshwater and seawater to identify their bioavailability more effectively; (3) emphasize the various mechanisms involved in arsenic cycling in different aquatic environments to obtain an in-depth understanding of the arsenic transformation mechanism and biogeochemical processes.