12 Up to 84,000 tons of dyes can be lost in water and 90 million tons of water are attributed 13 annually to dye production and their application, mainly in the textile and leather industry, 14 making dyestuff industry responsible for up to 20% of the industrial water pollution. The 15 majority of dyes industrially used today are aromatic compounds with complex, reinforced 16 structures, with anthraquinone dyes being the 2 nd largest produced in terms of volume. Despite 17 the progress on

more »

... urisation and degradation of azo dyes, very little attention has been given 18 to anthraquinone dyes. Anthraquinone dyes pose a serious environmental problem as their 19 reinforced structure makes them difficult to be degraded naturally. Existing methods of 20 decolorisation might be effective but are neither efficient nor practical due to extended time, 21 space and cost requirements. Attention should be given to the emerging routes for dye 22 A very important factor of our life is water, as it is not only vital for our physical existence but 30 it is also necessary for numerous activities in domestic and industrial fields, varying from 31 cleaning and agriculture to cooking and product formation 1 . Unsustainable exploitation and 32 uncontrollable contamination are currently the "hot issues" regarding water management. The 33 limited water resources need to be adequately distributed and carefully used to fulfil the 34 constantly rising agricultural and industrial demand due to population growth 1 . The main 35 strategies to address water scarcity are prevention, demand management and revalorization of 36 water 2 . Following that perspective, industrial wastewater should be recycled and reused. 37 The dye sector and the sectors relevant to dye applications (textile, tannery, paper) are 38 recognized among the most polluting industries, based on both the volume and the composition 39 of effluents 3, 4 . Effluents released in the water bodies create aesthetic and environmental issues 40 5, 6 with a high societal unacceptance. Furthermore, they can cause pipe corrosion, blockages 41 and bioaccumulation 7 , and result in the production of hazardous sludge 7 . The presence of 42 dyes in effluents makes their reuse difficult, as the presence of colourand other substances -43 affects consecutive dyeing cycles 7 . 44 Awareness of environmental protection has increased and minimization of water usage and 45 wastewater production is required, in addition to the limitation on the amount of pollutants 46 released to the environment. There are legislations regulating and monitoring the dyeing 47 industry in Europe and the United States 8, 9 , however these are not clearly defined and not 48 comparable across countries in regards to the colour intensity of the discharged effluents 9 . 49 These issues make the monitoring of coloured effluents released in the environment quite a 50 challenge. The problem of the dye contaminated water is especially evident in Asia, which 51 contributes to about 50 % of textile exports and more than 50 % of world's consumption of 52 dyes. However, many of the countries involved lack sufficient legislation about environmental 53 protection relevant to textile industries 10 . Having said that, there have been efforts for colour 54 restrictions to be included in legislation 7 . 55 Although currently the relevant legislation might be vague and not properly applied 11-16 , it is 56 clear that not only the volume of discharged effluents needs to be minimized, but the quality 57 of industrial effluents discharged in the environment needs to be fully monitored as well. 58 The dyeing and textile industry is responsible for dye discharge in the effluents, as well as for 59 a plethora of other hazardous and potentially hazardous substances. Such substances, mostly 60 surfactants and persistent organics, are used to accentuate dye stability/fastness or colour 61 intensity, to assist the process of dyeing and to give specific characteristics to textiles among 62 others 3, 17 . 63 It is difficult to quantify the amount of dyes lost during production or during application on 64 textiles, as the available figures from the literature are based on estimations, or are 65 representative of very specific types of dyes or applications. Nevertheless, it is important to 66 discuss those data to understand the importance of the problem. Dye production may vary 67 between 10,000 8 and 770,000 18 tons per year and the losses are estimated around 2 % during 68 production and around 10 % during application 7 , with wastewaters being discarded directly 69 into the environment in developing countries 19 . Based on the data from 2013, the annual 70 production of textiles was around 30 million tons, increasing every year 18 . Each ton of textile 71 requires around 30 tons of water for the dyeing process 10 , while each ton of dye production 72 requires an average of 200 tons of water 5, 20 . That means a total of 80 million tons and 90 73 million tons of water respectively is attributed to dye production and textile dyeing process of 74 per year. Taking into account the amount of contaminated water (2 % and 10 % respectively 75 during their production and application), a staggering sum of about 11 million tons of water is 76 polluted per year, making the dyestuff industry responsible for about 20 % of the total industrial 77 water pollution 21 . It is thus evident that water pollution from dyes is an existing and growing 78 problem that demands attention. 79 The majority of dyes industrially used today are aromatic compounds with complex, reinforced 80 structures, leading to difficult degradation 18 . Of the industrially important dye categories 81 (Figure 1), the most common "azo" dyes are making up of almost 60 % of the synthetic dyes 82 used industrially, followed by "anthraquinones" (15 %), and indigoids in respect of the 86 Black 1 (azo dye) and Reactive Blue 4 (anthraquinone dye). 91 chemicals) had a staggering 5,000 % increase (from 500 to 25,000 tons) 23 . Given the increase 92 in production volume of dyes, it is safe to assume that the production of AQ dyes increased as 93 well; a rough estimation of about 100,000 tons of AQ dyes per year can be made. 94 The specific chemistry of the anthraquinone group is based on the anthracene and consists of 95 three fused benzene rings (basic anthracene structure) with two carbonyl groups on the central 96 ring (highlighted in Figure 1 ). This structure is naturally colourless, but substitution of the 97 aromatic rings gives colour and controls its intensity 24 . Colour gets deeper with increased 98 basicity of the substituents, for an aniline-based substituent (NHC6H5) is used, the maximum 99 absorption length rises to from 327nm (case of H) to 508nm 24 . 100 The difference from azo dyes is that in the anthraquinone structure, the carbonyl group acts as 101 an electron acceptor, thus requiring an electron donor to react and break their structure 5 . This 102 combined with resonance effects among the anthracene structure leads to higher difficulty in 103 AQ dyes degradation compared to azo dyes 19, 25 and makes the choice of an appropriate 104 degradation/decolorisation method challenging 26 . The majority of the industrially important 105 AQ dyes are derived from anthraquinonesulfonic acids, using sulfonation or nitation 27 , and 106 research has shown that presence of sulfone groups in dye structure can reduce their 107 degradability 28 . Due to their highly stable structure, AQ dyes are known for their great fastness, 108 stability and brightness 24 . 109 2. AVAILABLE METHODS FOR DECOLORISATION 110 2.1. Industrially available methods 111 112 The most known and extensively applied methods in the industry are adsorption, coagulation, 113 membrane filtration, as well as various oxidative processes 10, 29 . Regarding biological methods, 114 aerobic and anaerobic processes are currently widely applied for general water treatment, 115 offering distinct advantages compared to physicochemical methods (e.g. products of added 116 value, environmentally friendlier), but also facing challenges regarding their efficiency (e.g. 117 sensitivity, long contact time) 29 . There are many examples of papers reviewing the current and 118 future industrial methods for general dye decolorisation 3, 8, 30-34 . Every method has advantages 119 and disadvantages related to the following criteria: efficiency under various conditions, 120 practicality, requirements of pre-and post-treatment and environmental impact; ultimately 121 relating to the cost. Given this complexity, a single method can rarely satisfy these demands 122 simultaneously 5 , hence, typically, a combination of available and under-development methods 123 is preferred, maximizing their strengths and compromising their disadvantages 17, 29 . 124 Despite the significant amount of research about the decolorisation and degradation methods 125 applied for azo dyes 6, 35-39 , not much research is available on AQ dyes, with only two reviews 126 available, both discussing AQ dye decolorisation mainly by biological methods 40, 41 and 127 comparably fewer research papers compared to those available on azo dyes a . What is worth 128 mentioning, is that there is a review paper focusing on the degradation of a specific AQ dye, 129 reactive Blue 19, covering various methods and research examples dated up to 2011 42 . 130 2.2. Physical, chemical and biological methods applied for AQ 131 dye removal 132 133 The most common physical methods for treatment of dye house effluents are adsorption, and 134 filtration (using membranes and reverse osmosis). As there are numerous research studies on 135 dye removal assisted by adsorption, we have summarised the best performing literature 136 findings on AQ dye removal and degradation in Table 1, with associated comments, while 137 below we discuss selected examples. Best performance was arbitrarily evaluated based on the 138 amount of dye removed per litre, per hour, assuming continuous use of the system described at 139 the optimal state identified by the researchers. This arbitrary metric allows for a comparison 140 a about 500 papers on anthraquinone dye degradation compared to about 8,700 for azo dyes, according to Web of Knowledge search engine over the period of 1975 to 2019. between results found in literature, as given the lack of a consistent approached followed, 141 superficial comparison of results does not produce valuable conclusions. 142 2.2.1.Physical methods 143 Amongst many adsorbents explored such as activated carbon, peat, silica-based adsorbents, 144 zeolites or other naturally derived substances, activated carbon is widely studied for dye 145 adsorption. It is also the dominant adsorbent in industry, based on its great adsorption ability, 146 high surface area, stability and homogeneity 43 , which compromise the high cost of production 147 and regeneration and the possibility of decreased efficiency due to material loss during 148 regeneration 8, 44, 45 . A few examples using activated carbon for AQ dye adsorption showed that 149 uptake was higher for acidic solutions 46-48 , and that pore structure of the materials could 150 facilitate 46, 48 or hinder 47 adsorption. Another frequently discussed option in the area of 151 adsorption are the abundant in nature zeolites, with substantially lower adsorption capacity and 152 again facing high regeneration costs 49, 50 . Silicon based materials have been studied extensively 153 for pollutants adsorption as well 51, 52 . Their interesting properties such as ability for a wide 154 range of pore size and surface areas, durability, ease of functionalisation and relatively cheaper 155 regeneration compared to activated carbon, have made them excellent candidates for water 156 treatment with many examples on dye adsorption 53-56 . However, issues such as manufacturing 157 and regeneration cost as well as diffusional limitations arising from high throughput in 158 industrial scale applications, have prevented them from being widely applied in water treatment 159 yet, although research is showing positive signs on their industrial implementation 57 . Newer 160 trends in adsorption, with application for AQ dyes, include the use of agricultural waste 58, 59 . 161 As per filtration, the usually encountered textile effluent treatments include nanofiltration (pore 162 diameter up to 10 nm) and reverse osmosis 60 , but there was no example of their application on 163 AQ dyes found in literature. 164 Major issues about the application of physical methods for dye removal are the relatively high 165 required contact time, hence large spaces required, as well as the need for adsorbent (or 166 membrane) regeneration, issues that are not usually addressed in literature, but are of great 167 importance for industrial implementation. 168 2.2.2.Chemical methods 169 2.2.2.1. Coagulation -Flocculation 170 The most common chemical treatment methods applied to textile effluents are chemical 171 coagulation and oxidation processes, while electrochemical methods are gaining attention as 172 well (Table 1). Chemical coagulation or flocculation is being replaced by newer methods or 173 used in combination with other methods, in order to reduce the effect of some major drawbacks 174 such as potentially toxic sludge production and need for further treatment of the effluent 61 . 175 The principle of coagulation and flocculation methods is the opposite charge between the 176 soluble pollutant (e.g. dye) and the usually aluminum, iron, or most recently polymeric 177 coagulant, that makes the pollutant become insoluble 61 . The factors of importance during 178 coagulation are the type and dose of coagulant needed and the size and "sturdiness" of the 179 floccs (coagulated pollutants), which dominates their ease of removal 62 . Table 1 summarises 180 some distinct examples of AQ dye treatment using chemical methods, while we elaborate on 181 relevant research below. When degradation of Reactive Blue 19 and 49, individually and in a 182 mixture, was attempted using active chlorine, it was shown that degradation was much faster 183 for individual dyes 63 . Contradicting these findings, decolorisation of Disperse Blue 3 via 184 coagulation with magnesium chloride or ferrous sulphate, as individual dye or in mixture with 185 azo dyes, showed that there is a synergistic effect. Dye removal increased from 68 % 186 (individual dye) to up to 90 % (mixture with azo dyes) in presence of ferrous sulphate, whereas 187 for magnesium chloride the decolorisation percentage was maintained very high, at 93 %, 188 regardless the presence of other dyes 64 . 189 2.2.2.2. Advanced Oxidation Processes (AOP) 190 The available oxidative methods include Fenton's process with or without external energy 191 supply, or ozonation, and they operate via the production of active OHradicals that non-192 selectively oxidise dyes 65 . Their application in water treatment has been illustrated recently 28, 193 34, 65, 66 gaining much attention over the last few years. Selected stellar examples of AOP applied 194 for AQ dye degradation are shown in Table 1 and some of them are discussed below. Studies 195 on Reactive Blue 19 conducted by different groups, using the same starting concentration (100 196 mg/mL) but different combination of AOP, showed highly different results. It was shown that 197 using a combination of methods such as Fenton reaction coupled with adsorption on pyrite ash 198 67 , or ozonation coupled with UV radiation 68 can be much more efficient compared to ozonation 199 only 69 , based on the . Using the same dye on a much higher starting concentration (about 2,000 200 mg/L) and examining its decolorisation by Fenton's reaction, photocatalysis and UV radiation, 201 as single methods or combined, Radovic resulted in generally very high dye removal (above 202 90 % for a combination of Fenton reagent coupled with photocatalysis) 70 . This shows that 203 combination of AOP methods does work synergistically, and usually better than single 204 methods. What is worth highlighting about AOP when applied in AQ dye degradation, is the 205 very short reaction times required, usually few minutes, their very good efficiency and 206 mineralisation of dye, but also their high cost, which poses difficulties on their consideration 207 for scale-up 71, 72 . 208 Combination of methods 209 Emerging combinations of the once very popular chemical coagulation with newer dye removal 210 methods are implemented, in order to reduce the effect of some major drawbacks such as sludge 211 production and need for further treatment of the effluent 61 . Electrochemical coagulation 212 producing in-situ coagulants based on aluminium or iron, showed great dye removal potential 213 (Reactive Blue 19 was used as a representative AQ dye, but other dyes were studied as well) 214 and associated time 73 . That work also presented an economic evaluation of some the timescale of the decolorisation is within a few minutes, the assumption of decolorisation ability over continuous use for 1 h is made.