A Review on Adsorbable Organic Halogens Treatment Technologies: Approaches and Application

Abstract

Halogen-containing organic substances have a detrimental and toxic impact on the environment and human health due to their high stability, carcinogenic effects, and ability to accumulate when ingested. The production and release of these substances have significantly increased in recent decades, resulting in a lack of effective treatment technologies. Adsorbable organic halogens (AOX), a specific parameter used to monitor pollution, represents the total amount of chlorinated, brominated, and iodinated organics that can be adsorbed on activated carbon from various environments. This paper provides an overview of selected articles from the past three decades (1990–2023) focusing on the primary natural and industrial sources of AOX. It also evaluates different determination techniques and a variety of removal approaches based on biological, physical, chemical, and combined processes. Additionally, the limitations and efficiency of these approaches are briefly characterized. While biochemical and physical methods have been limited by financial constraints and reduced efficiency, biological, chemical, and physicochemical techniques have shown significant potential in improving water quality. This knowledge can be valuable for the development of alternative water treatment techniques and underscores the importance of sustainable water usage.

1. Introduction

On the way to a sustainable future, water quality is one of the most important factors needed to maintain a healthy environment and society. Since the implementation of the 2030 Agenda for Sustainable Development, two of the seventeen goals (Sustainable Development Goals—SDGs) have been related to sustainable water management: Goal 3—Good health and well-being and Goal 6—Clean water and sanitation [1]. The successful achievement of these SDGs consists in constant control of water quality and pollution level. Among the various parameters used for pollution degree monitoring, the presence of adsorbable organic halogens (AOX) is informative and important [2]. The criterion specifies the total amount of chlorinated, brominated and iodinated organic halogen compounds that can be adsorbed on activated carbon from different environments (water, soils, sludge, sediments and industrial discharges). AOX includes simple volatile halogenated compounds (chloroform) as well as complex organic molecules (furans, dioxins, syringols, vanillins etc.) [3,4]. Most of them are toxic, mutagenic and carcinogenic at high concentrations, have long half-life period and high lipophilicity and can accumulate in food chains [5,6]. Indeed, in 1976, several adsorbable organic chlorines (AOCl) were included by the European Commission on the ‘black list’ of toxic compounds. The Stockholm Convention defines 23 AOCl compounds as persistent organic pollutants [7]. However, there are no common regulating norms that determine the permitted AOX content. For example, the limits in pulp and paper industry discharges in China are settled at 0.72 kg·ton⁻¹, in Canada at 0.25 kg·ton⁻¹ and in the USA at 0.27 kg ton⁻¹ [8].

It has been estimated that adsorbable organic bromine (AOBr) and iodine (AOI) possess more toxic nature than their chlorinated analogs [9]. This fact causes the necessity of their separate determination to evaluate the contribution of each of these parameters to environmental contamination. There are several regulations that describe AOX determination in different matrices. Thus, in the European Union, the ISO 9562:2004 and U.S. EPA Method 1650 are applied [10,11]. In documents, the microcoulometric titration is used as a reference method. The results express the total AOX content as chloride amount in the sample in µg·L⁻¹ [10]. The considerable disadvantage of this method is the interfering effect of the high concentration of inorganic chloride present in water samples. In the concentration range higher than 1 g·L⁻¹, the dilution of the sample is required. Additionally, several non-standardized methods for AOX determination can be used, such as photometry, combustion ion chromatography (C/IC), combustion ion chromatography titration, ultrasonic extraction-high temperature combustion absorption-ion chromatography, neutron activated ion analysis (NAA), inductively coupled plasma mass spectrometry (ICP/MS), inductively coupled plasma atomic emission spectroscopy (ICP/AES), gas chromatography coupled with mass spectrometry (GC/MS), liquid chromatography coupled with mass spectrometry (LC/MS), precursor ion scan of triple quadrupole mass spectrometry (PIS of QqQ MS), and flow injection analysis (FIA). The limitations of the methods depend on the sample matrix and AOX content [12,13,14].

The following paper provides an overview devoted to adsorbable organic halogens (AOX) as environmental pollutants. It covers their sources in nature and industry, standardized and non-standardized methods of AOX determination, as well as possibilities of their removal and treatment using various biological, physical, chemical, physicochemical, electrochemical, and biochemical methods. The methods are briefly characterized to provide a comprehensive understanding of their applicability.

2. Methodology

Google Scholar, Scopus, Web of Science, ScienceDirect and ACS Publications were used to provide the literature overview concerned with the selected topic. The following keywords were chosen during the search: adsorbable organic halogens (AOX), organohalogens, organic halides, halogenated organic compounds, chlorinated organic compounds, brominated organic compounds, iodinated organic compounds, AOX determination, AOX removal, AOX treatment and terms related to various industries (AOX in chemical wastes, AOX in steel industry wastes etc.), which are potential AOX generators.

3. Adsorbable Organic Halogens: Origin, Classification, and Determination

3.1. Sources and Formation of AOX

Adsorbable organic halogens (AOX) comprise four groups of substances: small-molecule organic halogenides, macromolecular organic halogenides, halogenobenzenes, and halogenophenols. These compounds can be categorized into two groups based on their origin: naturally formed and anthropogenically produced. Naturally formed AOX exist as a background resulting from environmental processes [15]. Over 3650 organohalogen compounds, generated by living organisms or natural abiogenic processes such as volcanoes, forest fires, and other geothermal activities, have been identified [16]. AOX can also be generated in the atmosphere through smoldering biomass burning in the presence of trace amounts of halogens [17]. Moreover, natural processes produce approximately 8 million tons of CH₃Cl annually, while man-made emissions amount to 26 thousand tons. Small quantities of AOX (1–30 µg·L⁻¹) have been detected in snow and rain samples [18]. In soils, AOX are produced through biotic vegetation processes (such as plants’ mineral nutrition and decaying roots) and the degradation of organic matter by bacteria and fungi, particularly Basidiomycetes [19,20]. These processes predominantly give rise to aliphatic halogenated organic compounds [21]. Around 50% of fungal strains produce a high content of AOX, indicating that AOX production is a common characteristic among most microorganisms. Many of these fungal species hold significant ecological importance. Basidiomycetes are considered the primary source of natural organohalogens in forest ecosystems. Water supplies can become contaminated with AOX from groundwater, which contains chlorinated fulvic and humic acids [22], chloroform, and other halogenated low-molecular-weight acids [12,15,23,24]. Surface water typically contains 10–50 µg·L⁻¹ of AOX [25]. It is worth noting that adsorbable organic brominated compounds (AOBr) can also be present in surface water, potentially attributed to decay processes occurring in eutrophic lakes [26]. Plants also contribute significantly to the AOX content [27,28].

However, the majority of anthropogenically produced AOX are generated during different industrial processes. Based on the performed literature review, the following sources of anthropogenically produced AOX can be highlighted:

• The pulp and paper industry (oxidation of lignin and hemicellulose during the bleaching processes) [29,30,31,32];
• The pharmaceutical and dyeing industries (chlorinated drugs and dyes degradation products) [7,33,34,35];
• Landfills [36,37,38];
• Chemical wastewaters [39];
• Chlororganic pesticides (dichlorvos, trichlorfon, chlorpyrifos etc.) [40,41].
• Chlorine-based water disinfection and municipal wastewater [42,43];
• Steel industry [44,45,46];
• Petrochemical branch (cyclic halogenated organics), biofuel production and palm oil mill effluent (POME) [35,47];

The pulp and paper industry is considered to be the main source of man-made AOX. The pulp and paper industry discharges can contain up to 1.5 kg·ton⁻¹ of AOX [48]. The main processes responsible for AOX formation are pulping and bleaching, during which liquid, solid and gaseous waste fractions are generated [49,50]. Solid fractions consist of reject fibers, wastewater treatment plant sludge, scrubber sludge, lime mud, green liquor dregs, boiler and furnace ash. Polychlorinated dibenzo-p-dioxins (PCDDs), polychlorinated dibenzofurans (PCDFs), polychlorinated biphenyls (PCBs), chlorophenols and chloroguaiacols are present in these fractions [51,52]. The gaseous components are sulfur and nitrogen oxides (SO_x, NO_x) and volatile organic compounds (VOC) such as carbon disulfide, methanol, methyl ethyl ketone, phenols, terpenes, acetone, alcohols, chloroform, chloromethane, and trichloroethane [53]. The liquid wastes can contain up to 500 diverse chlorinated organic compounds including chloroform, chlorinated hydrocarbons, phenols, catechols, guaiacols, furans, dioxins, syringols, vanillins etc. [53,54,55,56]. To reduce AOX content in outgoing wastewaters it is necessary to remove the main source of AOX [57] or use bleaching agents that do not cause AOX formation, for example the elemental chlorine free (ECF) process [58,59,60]. According to [31], the concentration of AOX in pulp and paper wastes is in the range of 2.8–83.0 mg·L⁻¹.

The wastewater from dyeing industry often possesses some amounts of AOX, mainly AOCl and AOBr, as a result of electrolytic treatment, application of advanced oxidation processes, for instance the Fenton system [61], or the use of organohalogen supplements [62]. The high concentrations of inorganic halides inhibit dye degradation, especially in the acidic medium [6]. Some chlorinated aromatic compounds, including 3-chloroisocoumain, 2-chloro-7-hydroxynaphthalene, 1,3,5-trichloro-2-nitrobenzene, tetrachlorohydroquione, 4-chlorophenol, 2,4-dichlorophenol, 4-chlorophthalic acid and 2,4,6-trichlorophenol [63] have been determined by GC/MS as treatment products. The AOX content in wastewaters from the dyeing industry depends on chloride concentration and reaction time and is near 3.5 mg·L⁻¹ [64].

In the pharmaceutical industry, halogenorganic compounds are widely used as solvents in pharmaceutical processes or raw materials for the fabrication of different medicines, and some of them are also produced by pharmaceutical production activity [7,65,66]. For example, wastewater from the production of epichlorohydrin contains more than 10 mg of AOX per liter [33,67]. The estimated amount of AOX in this type of waste is in the range of 3.0–5.0 mg·L⁻¹ [66].

Landfill wastes can also be a source of necessary precursors of halogenated organic compounds, which with rainwater form leachates [68]. Landfill leachates rich in organics may flow into the water supply systems [69], easily react with active chlorine water disinfectants, and, therefore, generate halogenated compounds [70,71]. Moreover, leachates contain some quantities of inorganic iron and lead salts, which can predetermine the chemical reduction of AOX (with efficiency near 40%) [72]. In [36], authors elucidated that the AOX amounts in landfill leachates can vary from 2.2 to 4.4 mg Cl L⁻¹.

Chemical wastes are received from the production of photographic chemicals, dyes, phosphorous organic compounds, phenol derivatives, chemicals for plastics manufacturing, caoutchouc, explosives, processing of polyvinylchloride, chlorination of aromatic compounds for fuel additives, processing of mineral oil, diverse aromatic compounds, inorganic substances and basic materials for detergents, and can accommodate from 0.7 to 1.3 mg·L⁻¹ of AOX [35,39]. Specifically, chlorobenzenes, chloronitrobenzenes, chloroanilines, chlorinated nitroanilines and halogenophenols have been identified in chemical wastewaters generated during dye synthesis [73].

Agrochemistry, especially the production and extensive application of pesticides, contributes greatly to the release of AOX into the environment. Indeed, insecticides and herbicides are chlorine-containing compounds and their chlororganic metabolites can be more toxic. Some pesticides are very resistant and have high bioaccumulative ability. For example, dichlorodiphenyltrichloroethane DDT was banned in the early 1980s, but even now it can be found in the livers of penguins [74]. In [40], diuron was shown as a source of AOX contamination. It is interesting to note that non-persistent pesticides have a great influence on human health and can cause different diseases, for instance diabetes [41].

The smaller amounts of adsorbable organic halogens (0.02–1.0 mg·L⁻¹), mainly chloroform and chloramines, are formed during the routine chlorination of drinking water, swimming pools [75], cooling waters, and processed waters in laundries [42,76] as a result of the use of water treatment agents such as chlorine, calcium hypochlorite, and sodium hypochlorite [77]. Therefore, measured AOX content includes nearly 20–50% of disinfection by-products (DBPs) [78]. The results of domestic wastewater (sewage) analysis have revealed the presence of 87 organochlorines generated by sodium hypochlorite treatment [79]. Raw hospital wastewater and effluents are also considered important sources of AOX in municipal wastewater. The wastes consist of various halogenated solvents (CH₂Cl₂, CHCl₃ etc.), pentachlorophenol [80] and other chemical substances, for example ethidium bromide utilized in laboratory tests [81,82]. Furthermore, the high level of AOI in municipal wastewaters caused by the presence of special X-ray contrast agents based on iodinated organics (diatrizoate and iopromide) has been identified in [43]. Recent research on contrast media topics has shown that these compounds remain stable after release into wastewaters and can be determined in relatively high concentrations of up to 10⁻⁴ g·L⁻¹ [83].

The peculiarity of steel industry wastes is their acidic character that causes the higher stability of AOX. The main source of AOX in steel industry wastes can be considered fly ash, obtained from coke as a result of metallurgical processes. Fly ash contains PAHs (polycyclic aromatic hydrocarbons), which can be exposed to gaseous hydrogen chloride which further leads to AOX formation [44]. Additionally, fly ash possesses increased metals content, especially copper, which can work as a catalyst for chlorination reaction in the presence of oxygen [45]. The sintering plants of the steel industry can also produce AOX [84].

In the case of petrochemical branch, biofuel production and palm oil mill effluent (POME), their wastes contain high concentrations of phenols, benzene and other cyclic organic compounds that can be precursors of halogenated organics [35,47]. It should be pointed out that coking wastewaters possess refractory organic pollutants, which during treatment in the presence of halide anions can generate a high amount of AOX (up to 1.6 mg·L⁻¹), especially brominated organic compounds with higher stability and toxicity [85].

3.2. Classification of Organically Bounded Halogens (_OX)

The presence of organohalogens (OX) can be expressed through several specific parameters, with their characterization depending on their occurrence in various environments.

Table 1. Types of organically halogens.

Name	Abbreviation	Definition
Adsorbable Organically Bound Halogens	AOX	The organically bound halogens, chlorine, bromine and iodine (but not fluorine), contained in a sample that can be adsorbed on activated carbon
Bound Organic Halogen	BOX	The organically bound halogens, chlorine, bromine and iodine (but not fluorine), contained in sediment
Extractable Organically Bound Halogens	EOX	The organically bound halogens (but not fluorine) that can be extracted with a non-polar organic solvent by liquid/liquid or liquid/solid extraction. EOX and BOX are connected by equation EOX = 0.3 × BOX
Leachable Organically Bound Halogens	LOX	The organically bound halogens (but not fluorine) contained in sediment samples (soil leachates)
Purgeable Organically Bound Halogens	POX	Volatile organically bound halogens (except fluorine) contained in water sample
Total Organically Bound Halogens	TOX	The organically bound halogens (except fluorine) as a sum of all kinds of organic halogens present in sediments and soils, including polyvinylchloride (PVC)
Volatile Organically Bound Halogens	VOX	The organically bound halogens (but not fluorine) contained in the soil’s air (mainly, chlorinated Cl− and C2-compounds)

provides a summary of the different types of organically bound halogens. Among these, AOX are the most comprehensive and commonly monitored parameter. However, the choice of organohalogen type depends on the nature of the samples.

3.3. Determination of AOX

The determination of AOX is regulated by several standard guidelines described in ISO 9562:2004 (AOX, SPE-AOX in water samples) [10], ISO 11480 (AOX-pulp, paper, board) [86], EPA 9020.B (AOX/TOX, USA, drinking and ground water) [87], EPA 1650 (AOX, USA, water and waste water) [11], DIN 38414-18 (AOX, Villingen-Schwenningen, Germany) [88]. Briefly, the procedure of AOX analysis includes three main steps:

• Adsorption of AOX on activated carbon;
• Washing of the activated carbon by nitrate solution to remove inorganic halides, especially chlorides;
• Combustion of the loaded carbon in an oxygen steam followed by microcoulometry (argentometric titration). The result is expressed as microgram per liter Cl (µg·L⁻¹ Cl) [89,90].

The main disadvantage of this reference method is the interfering effect of the high concentration of inorganic chloride and organic matter (chemical oxygen demand (COD) parameter) and the absence of halogen differentiation possibility. Additionally, the polar and hydrophilic halogen-containing compounds can remain in solution and are not adsorbed on activated carbon which leads to incorrect results [91]. Since AgF possesses high solubility in water, the determination of AOF becomes a difficult task; therefore, AOF is not included in the AOX parameter [92,93]. To automate and simplify the analysis of adsorbable organic halogens, special equipment has been developed: AOX/TOX Analyzer Multi X^®2500 (Jena, Germany), Heraous AOX-MT200, Mitsubishi Chemistry AOX/TOX analyzer, TOX-10S (Tokyo, Japan), XPLORER AOX/TOX, POX and EOX analyzer, etc. The equipment includes all steps of AOX analysis in one system except sample preparation [94]. The analysis of saline wastes, industrial wastewater or brine water is performed within the modified ISO 9562 procedure including solid phase extraction (SPE) using Multi X^®2500 in combination with APU 28 SPE system [95]. SPE allows removing interferents before AOX adsorption by polymer resins.

Among other alternative methods of AOX determination, spectroscopy with plasma emission detector (PED) [96], gas chromatography coupled with mass spectrometry (GC/MS) [97], ion chromatography (IC) [98], combustion ion chromatography (C/IC) [99], combustion-microcoulometry, combustion ion chromatography titration [100], ultrasonic extraction-high temperature combustion absorption-ion chromatography [101], neutron activation analysis (NAA) [102,103], inductively coupled plasma mass spectrometry (ICP/MS) [104], inductively coupled plasma atomic emission spectroscopy (ICP/AES) [105], liquid chromatography coupled with mass spectrometry (LC/MS) [106], precursor ion scan of triple quadrupole mass spectrometry (PIS of QqQ MS) [79] and flow injection analysis (FIA) [107,108] have been utilized. The wide application of NAA, GC/MS, LC/MS, ICP/MS and ICP/AES is not regulated by legislation because of the high cost and skill level required. Besides, LC/MS “suffers” from adducts formation in the mobile phase. The limitation of FIA methods is the low robustness and reproducibility of obtained results. Regarding GC/MS, it is relatively easy to use; however, there are several problems related to detected substances. Firstly, the volatile and unstable AOX can be lost. Organic bromine and iodine compounds may decompose to elemental bromine or iodine during the combustion and act as oxidizing agents. Secondly, effective chloride removal is required, the presence of which causes overestimation of AOX values. Thirdly, the reduction of adsorbable organic halogens on activated carbon is possible. Fourthly, there is no possibility of AOBr and AOI determination (only AOCl) [109]. The most reliable, cheap and easy-to-use analytical tool for AOX analysis, except for the standard method, is ion chromatography. This method allows determining all types of adsorbable organic halogens, i.e., AOCl, AOBr, AOI and even AOF. The main problem of IC is the interfering effect of chloride, chlorate, perchlorate and sulfate that leads to overestimation of obtained results [110]. To eliminate the interfering effect, ultrasonic extraction-high temperature combustion absorption-ion chromatography can be applied with the limit of quantification of 0.1–0.5 mg·kg⁻¹ [101]. It is of interest to note that an advanced technique PIS of QqQ MS has overcome the limitation related to the screen analysis of organohalogens, especially organobromine compounds [79].

The important factor affecting the adsorbable organic halogens analysis is the sample matrix, which defines the sample pre-treatment step. For instance, if the sample contains oxidizing agents, sodium sulfite, previously tested for active chlorine presence, is added to eliminate them. Natural objects (natural water, soils etc.) usually include microorganisms or algae, which can lead to a high positive bias of AOX level as a result of their chloride content. To avoid this, the analysis is performed at an acidic medium with pH < 2 adjusted by HNO₃. The reason for underestimated values of AOX content is the sample preparation procedure. For example, centrifugation and drying are used in the analysis of sludge as treatment procedures. In [97], the authors demonstrated that these procedures have a significant influence on the elimination of AOX because near 50–80% of them remain in the sludge, which means that 20–50% are extracted from the sample.

All methods used for analytical determination should have standard reference materials that contain exactly known concentrations of determined substances. The absence of such materials is one of the main problems of analytical chemistry in general. In the case of AOX analysis, there is no commonly used reference sample, caused by the fact that AOX is a sum of halogenorganic compounds. The choice of a standard substance depends on the method of analysis and experimental conditions [13]. Thus, 4-chlorophenol and 2,4-dinitrochlorobenzene are the reference substances for the determination of AOX on AOX/TOX analyzers and in the GC/MS technique. In combustion ion chromatography, the standard solution consists of halogenide anions. This makes the analysis of AOX easier in comparison to other methods [99]; however, the method suffers from the interfering effect of anions on AOX determination. Furthermore, 2,4,6-trichlorophenol, 2,4,6-tribromophenol and 4-iodophenol have been used as standards for AOCl, AOBr or AOI calibration procedures in online ion chromatographic determination, respectively [111].

4. Removal and Treatment of AOX

Due to the high stability, lipophilicity, and toxic nature of AOX, as well as their potential to accumulate in food chains, it is crucial not only to determine their presence but also to develop effective and sustainable approaches for their removal and treatment. This is necessary to prevent further increases in pollution levels and impact on human health and the environment. Various technologies for AOX removal and treatment have been established and described in [52,112,113,114]. These approaches are based on different biological, physical, and chemical processes, either individually or in combination. Since AOX compounds are highly stable and resistant to degradation, the most commonly used methods focus on eliminating the main precursors of AOX. For instance, in dye wastewater treatment, the concentration of dyes in the waste is reduced, while in the paper and pulp industry, the removal of cellulose, hemicellulose, and lignin is the primary objective. Interestingly, a bio-electrochemical reactor (BER) can be utilized for the direct degradation of AOX [115]. The range of cleaning technologies includes biodegradation, biosorption, bioremediation, volatilization, air stripping, advanced oxidation processes (AOPs), UV degradation, adsorption on carbon materials, ultrafiltration, reverse osmosis, electrodialysis, and more [30,55,116,117,118]. However, it is important to note that many of these methods are not highly selective.

Table 2. The summary of methods of AOX removal and treatment.

Method	Principle of the Method	Application	Ref.
Biological	Fungi
	Lignocellulosedegrading fungi—Basidiomycetes, Hypholoma fasciculare and Mycena metata	Wastes	[19,20]
	Nematophagous fungus—Paecilomyces sp.	Pulp and paper industry wastes	[119]
	White rot fungi—Trametes versicolor, Phanerochaete chrysosporium, Inonotus dryophilus	Pentachlorophenol	[120,121]
	Fungus Trametes versicolor	Paper industry wastes	[122,123]
	Fungus Actinomycetes	Paper mill effluents	[124]
	Bacteria
	Bacteria strains Pseudomonas aeruginosa Bacillus megaterium	Pulp mill effluents	[117]
	Bacteria strains Clostridium, Acetobacterium woodii, Shewanella	Pulp and paper industry wastes	[55]
	Bacteria strains Bacillus sp. Serratia marcescens	Pulp and paper industry wastes	[125]
	Bacteria activated sludge system	Household bleaching wastes, pulp and paper mill effluents, bleached kraft mill effluents	[126,127,128,129,130]
	Modified activated sludge system—aerobic granules (GAS)	Pulp and paper industry wastewaters	[131]
	Bacterial biofilm Rhodococcus erythropolis	1,3-dichloropropene from industrial wastes	[132]
	Facultative stabilization basin (FSB)	Bleached kraft mill wastewater	[128]
	Aerated stabilization basin (ASB)	Bleached kraft mill wastewater	[128]
	Upflow anaerobic filter (UAF)	Pulp and paper industry wastes	[30]
	Anaerobic treatment	Paper and pulp mill effluents	[133]
	Sequencing batch activated sludge reactor (SBR)	Landfill leachate	[134,135]
	Aerated lagoon treatment	Wastewaters from paper and pulp industry	[118]
Chemical	Catalytic wet oxidation (CWO) reactions on CeO2-SiO2 mixed oxide catalyst	Landfill leachate and heavily organic halogen polluted industrial wastewater	[71]
	Catalytic wet oxidation (CWO) reactions on two types of catalyst: pure CeO2 and a SiO2-doped ceria	Landfill leachate, pulp and paper bleaching liquor and heavily organic halogen polluted industrial wastewater	[70]
	Cobalt/peroxymonosulfate (Co/PMS) advanced oxidation process	Dye wastewater, 2,4,6-trichlorophenol	[64,136]
	Fenton process	Pharmaceutical wastes, textile dyeing	[7,62]
	Inhibition of AOX formation by peroxymonosulfate/base/Cl− oxidation system	Dye wastewater	[137]
	Micellar-enhanced ultrafiltration (MEUF)	Rinsing water	[138,139]
	Chemical coagulation/flocculation technologies	Textile wastewaters	[140]
	Chemical reactor	Wastes	[67]
	Coagulation by anionic polysaccharides	Paper and pulp mill effluents	[141]
	Fe(II)/peroxymonosulfate (PMS) process	Degradation of maleic acid	[142]
	Ozone oxidation	Paper and pulp mill effluents	[143,144]
	NiSO4/KBH4 in alkaline medium	Pharmaceutical wastes (diclofenac)	[46]
	Peroxydisulfate (PDS)/CuO coupled process	Contaminated groundwater	[145]
	C@Cu–Ni/peroxymonosulfate (PMS)	2,4,6-trichlorophenol	[146]
	Fe-Ag and Fe-Pd	2,3,4-tribromodiphenyl ether	[146]
	Chlorine dioxide	Pulp and paper industry wastes	[57]
Electrochemical	Electrocoagulation and electrooxidation of dye using carbon steel anode	Dye wastewater	[147]
	Electrocoagulation using aluminum and stainless-steel electrodes	Dye wastewater	[148]
	Bio-electrochemical reactor (BER)	Pharmaceutical wastes	[115]
	Electrocoagulation	Wastewaters from paper and pulp industry	[149]
	Electrooxidation on Ti/TiO2–RuO2–IrO2 electrode	Phenol–formaldehyde resin manufacturing, oil refinery and bulk drug manufacturing industries wastes	[150]
	Electrooxidation on Ti/SnO2/PdO2/RuO2 (SPR)	Tannery wastes	[151]
	Electrooxidation on RuO2 coated titanium electrode	Paper and pulp mill effluents	[152]
	Electrogenerated hydroxyl radicals	Pesticides	[153]
	Direct cathodic reduction of the azo-chromophores	Dye wastewaters	[154]
	Electrochemical oxidation of reverse osmosis concentrate on boron-doped diamond anode	Wastewaters, tramadol wastes	[111,155,156,157,158]
	Photo-assisted electrochemical degradation	Textile wastes	[159]
	Boron-doped diamond electrode and others	Municipal wastewaters with AOI	[83]
Physicochemical	UV/TiO2 oxidation	Dye wastewaters, pulp and paper mill wastes	[6,160]
	UV/ozone oxidation process	Wastewaters	[161]
	Adsorption on activated carbon	4-chlorophenol, 4-bromophenol, 4-iodophenol, 2,4,6-trichlorophenol, landfill leachates	[162,163,164]
	Biochar remediation	Degradation of halogenated organics	[165]
	Al(III) coagulation/carbon adsorption process	Dye wastewaters	[166]
	H2O2/UV oxidation	Pharmaceutical wastes	[65,76,167]
	Mechanochemistry	Wastes	[168,169]
	Thermally activated persulfate by microwave heating (S2O82–/MW)	Secondary waste sludge	[170]
	Anionic exchanger DEAE-cellulose	Drinking water	[171]
	Supercritical water oxidation (SUWOX)	Industrial wastes	[172]
	Magnetically re-extractable nanoscale Pd-on-magnetite catalyst (Pd/Fe3O4)	Wastewaters	[173]
	Multi-barrier treatment	Municipal wastewaters	[174]
	Photo-Fenton process UV/H2O2/Fe2+	Pulp mill effluents	[40]
	UV/persulfate oxidation process	Saline wastewater	[175]
	UV/TiO2/RuxSey oxidation system	Paper industry wastes	[123]
	Pd/AC catalyst	Kraft pulp bleaching wastes	[176]
	UV/TiO2/granular activated carbon	4-halogenophenols	[177]
	UV/H2O2/TiO2 system	Pulp and paper wastes	[178]
	UV/sodium hypochlorite	Phenol’s degradation	[179]
Physical	Hot water extraction of hemicellulose	Paper and pulp industry wastewaters	[58,180]
	Distillation and membrane processes	Pharmaceutical wastes	[34]
	Distillation	Chemical wastes	[181]
	Centrifugation and drying	Sludge	[97]
	Nanofiltration	Paper and pulp industry wastes	[182]
Biochemical	Xylanase and laccase enzymes (obtained from Bacillus pumilus and Ganoderma sp., respectively)	Wastewaters from paper and pulp industry	[183,184]
	A lab-scale granular activated carbon sequencing batch biofilm reactor (GAC-SBBR)	Paper mill effluents	[116,185,186]

presents one possible classification of AOX removal and treatment methods. This classification is based on the nature of the processes involved, including biological, chemical, physical, physicochemical, biochemical, and electrochemical approaches. Among these technologies, the most commonly utilized methods are biodegradation, physical methods, and chemical methods [114].

The principle behind biological methods involves optimizing nutrient ratios and utilizing specifically selected microorganism strains with relevant degradation capabilities [119]. Biological treatment is recognized for its effectiveness in reducing the organic load and mitigating the toxic effects of craft mill effluents, with achievable efficiency rates surpassing 90%. Biological techniques are well suited for treating mildly polluted wastewater in the pulp and paper industry. Common methods include anaerobic and aerobic lagoons, stabilization ponds, activated sludge processes, or their modifications tailored to local conditions. In the past, the activated sludge system was widely employed for AOX treatment, but it is less commonly used today due to certain drawbacks. These drawbacks include excessive sludge production, fluctuations in removal efficiency for recalcitrant compounds, and the interfering effect of chloride, among others. Biological methods are inadequate for highly polluted wastewater from pulp and paper mills, as these wastes typically contain high concentrations of lignin, a substance that is difficult to biodegrade. The microorganisms present in biological treatment systems lack the capability to break down lignin and other high-molecular-weight substances. Therefore, biological methods alone cannot achieve complete treatment of such wastes.

The practical application of the mentioned approach has been described in [187]. The authors studied the possibility of enhancing the efficiency of treating pulp and paper wastewaters by combining aerobic and anaerobic conditions. Initially, separate anaerobic and aerobic approaches were applied to polluted wastewaters, achieving productivities of 44% and 57%, respectively. Notably, the sequential anaerobic–aerobic treatment altered the performance, resulting in an 81% elimination rate. Subsequently, the phytotoxic effect of the treated wastewaters was examined to determine their potential for agricultural use. It was found that the remediated wastewaters had reduced phytotoxicity and could be safely used for irrigation. This approach enables the recycling of industrial wastewaters, aligning with the requirements of sustainable water management and contributing to the achievement of Goal 3 and Goal 6 of Agenda 2030.

Physicochemical methods include the combination of physical and chemical processes, which can enhance the effectiveness of pollutant treatment. These techniques are commonly employed for the treatment of highly polluted wastewater from industries such as dye manufacturing, pharmaceuticals, municipal sources, and saline waste. In physicochemical treatment, oxidation processes can be initiated through physical actions such as UV radiation, heating, or mechanical agitation. Coagulation, anion exchange, and ultrafiltration processes, followed by physical treatments, are also utilized. For the treatment of saline wastewater, the UV/persulfate oxidation process is suitable due to its stability at high chloride concentrations and the independence of oxidation rate on chloride levels [175]. Moreover, mechanochemistry has proven to be an effective destruction process for halogenated organic compounds. Mechanochemical treatment initiates the dehalogenation of organohalogens, enabling full mineralization and making it one of the simplest and fastest methods available [169]. Carbon materials, particularly their adsorption properties [8,164] and redox properties [165] influenced by surface heterogeneity, play a significant role in the remediation of halogenated organic compounds. The discovery of pyrogenic carbon, such as biochar, has prompted extensive research into eco-friendly utilization of environmental resources and the development of sustainable water treatment agents [165]. These materials can be easily modified, for example, by depositing inorganic moieties on their surface to enhance their remediation performance [176,177].

In [188], the authors presented an interesting case of treating adsorbable organic bromine (AOBr) in ozonated municipal wastewaters using sunlight irradiation. It was observed that solar irradiation reduced up to 74% of AOBr within 8 h, significantly improving the decontamination performance. Toxicity tests confirmed the effectiveness of sunlight treatment, primarily through the photolysis of smaller AOBr molecules, such as tribromomethane. The treated wastewater could be reused without causing toxic effects. The proposed method can be considered a sustainable technique for water remediation.

Chemical methods entail the addition of specific chemicals, such as oxidation agents, coagulants, catalysts, and inhibitors, into the polluted system. The selection of chemicals depends on the type of waste being treated. Oxidation agents such as ozone, hydrogen peroxide/Fe(II) system (Fenton’s reagent), Fe(II)/peroxymonosulfate mixture, persulfate, etc., can be employed. However, the Fenton oxidation process is not widely used in industrial applications due to its high operating costs. Coagulating agents, such as anionic polysaccharides and anionic/non-ionic surfactants, are effective for AOX removal since they have little or no impact on neutralizing the negative charge of particles [138,141]. Catalysts such as bare CeO₂, CeO₂-SiO₂ mixed oxides, TiO₂/Ru_xSe_y, and others can also be utilized [70,71,176]. Interestingly, increasing the chlorine dioxide concentration during bleaching led to a 45% reduction in AOX generation [57].

Electrochemical approaches are environmentally friendly and convenient, offering high efficiency without the need for additional chemicals in the treated system. The efficiency of these approaches depends on factors such as electrode material, current density, pH, chloride concentration, flow velocity, and temperature [189]. Additionally, the use of different electrical charges enables the separate electrochemical oxidation of AOCl and AOBr in wastewaters [111]. In a review [83], electrochemical redox treatment was highlighted for its potential as a sustainable technology for wastewater treatment, particularly for the elimination of highly stable iodinated organic compounds. To enhance AOX removal performance, a combination of electrochemical and biological methods can be applied to pharmaceutical wastewater with high AOX concentrations. The treatment mechanism involves the direct reduction of adsorbable organic halogens on the electrode surface, generating H₂ through water electrolysis, and the fermentation of organic pollutants, which serves as the ultimate electron donor in biological reactions [115]. It is worth noting that the efficiency of biological methods alone for this type of waste was only 50% [33].

Among various physical methods, hot water extraction, distillation, centrifugation and nanofiltration are commonly employed [180]. However, it should be noted that the principle of these methods is focused on the separation of AOX from effluents rather than their degradation, making them non-selective. Additionally, these methods are costly and generate chemical sludge, which is not suitable for environmental disposal [30]. As for biochemical methods, their limited usage can be attributed to the high cost of enzymes and their short storage time. Furthermore, they often exhibit low efficiency, reaching only around 20–34% [183].

5. Conclusions

A review of selected papers has been conducted, focusing on adsorbable organic halogens (AOX), including their sources in nature and industry, standardized and non-standardized methods of AOX determination, and the possibilities of their removal and treatment using various biological, physical, chemical, physicochemical, electrochemical, and biochemical methods.

AOX can originate from both natural and industrial sources, with industrial emissions significantly surpassing natural production. Standards regulating the determination of AOX, such as ISO 9562, ISO 11480, EPA 9020B, and EPA 1650, have been discussed. Additionally, alternative analytical techniques, including spectroscopy with plasma emission detector, gas chromatography coupled with mass spectrometry, ion chromatography, combustion ion chromatography, combustion-microcoulometry, combustion ion chromatography titration, ultrasonic extraction-high temperature combustion absorption-ion chromatography, neutron activation analysis, inductively coupled plasma mass spectrometry, inductively coupled plasma atomic emission spectroscopy, liquid chromatography coupled with mass spectrometry, precursor ion scan of triple quadrupole mass spectrometry, and flow injection analysis, can also be used for AOX determination. The selection of a specific method depends on the sample matrix and composition, such as the presence of oxidizing agents, high chloride content, microorganisms and algae, as well as the availability of reference materials.

Relevant information on the AOX removal and treatment processes described in the reviewed papers has been reported. A possible classification of these methods based on the nature of the processes has been proposed, including biological, physical, chemical, physicochemical, electrochemical, and biochemical approaches. The high efficiency (up to 90%), relative simplicity, and sustainability of biological methods have made them one of the most utilized techniques. Physicochemical and chemical methods are advantageous for highly polluted wastewaters, while physical and biochemical methods are less commonly used due to their economic and operational shortcomings. Electrochemical techniques possess high selectivity, as they allow for the adjustment of electrical charge to decompose even highly persistent AOI compounds. Therefore, considering the advantages and disadvantages of the described techniques, novel effective methods can be developed that prioritize the sustainable management of water resources and environmental protection.