catalysts Article Co(II/III) Complexes with Benzoxazole and Benzothiazole Ligands as Efficient Heterogenous Photocatalysts for Organic Dyes Degradation Martyna Szyma ´nska 1 , Włodzimierz Czepa 1,2, Cezary Hołubowicz 3, Renata Swisłocka´ 4 , Teresa Łuczak 1 , Maciej Kubicki 1 , Joanna Karpi ´nska 3 , Marta A. Fik-Jaskółka 1,* and Violetta Patroniak 1,* 1 Faculty of Chemistry, Adam Mickiewicz University, Uniwersytetu Pozna´nskiego8, 61-614 Pozna´n,Poland; [email protected] (M.S.); [email protected] (W.C.); [email protected] (T.Ł.); [email protected] (M.K.) 2 Centre for Advanced Technologies, Adam Mickiewicz University, Uniwersytetu Pozna´nskiego10, 61-614 Pozna´n,Poland 3 Faculty of Chemistry, University of Białystok, Ciołkowskiego 1K, 15-245 Białystok, Poland; [email protected] (C.H.); [email protected] (J.K.) 4 Faculty of Construction and Environmental Engineering, Bialystok University of Technology, Wiejska 45e, 15-351 Białystok, Poland; [email protected] * Correspondence: martafi[email protected] (M.A.F.-J.); [email protected] (V.P.) Received: 8 October 2019; Accepted: 30 October 2019; Published: 1 November 2019 Abstract: The problem of pollution in the current world is growing, however people’s awareness of environmental protection and ecology is also increasing. The aim of the study is to present three new Schiff base compounds with Co(II/III) ions and to assess their photocatalytic activity. The study was supported by cyclic voltammetry technique. In due course the complex 2 revealed as the most effective in AR18 degradation, even more than commercially available TiO2. The search for new photocatalysts able to decompose harmful organic dyes into environmentally friendly basic substances is becoming a new trend in the area of chemistry development. Keywords: Schiff base; cobalt(II/III) complexes; photocatalysis; cyclic voltammetry; redox processes 1. Introduction Nowadays the natural environment is becoming more and more exposed to the threat of pollution. In the past decades the growing food, textile, chemical etc., industries have led to the alarming level of the Earths’ contamination. Currently, the trend is to maintain the high efficiency of the factories and to decrease the amount of post-production and post-usage waste. Both, the big industries and consumers joined the pro-Earth movement and support the low-waste lifestyle to prevent further contaminations [1]. However, since the level of pollution is already very high, it is important to develop new and efficient ways to decrease it. Notably, over 100,000 organic dyes is used worldwide in the manufactures [2,3]. One of the challenges is the removal of the organic dyes, used in textile, paper, polymer, cosmetics, and food industry from wastewater [4]. Organic dyes, such as Reactive Blue 21 and Acid Red 18 and the products of their cleavage exhibit carcinogenic and mutagenic effects when present in high concentrations [5–7]. In fact, the list and usage protocols of textile dyes that decompose to toxic amines are not regulated yet [8]. Therefore, there is a strong need to find a way to efficiently reduce the amount of dyes already present in the environment. The significance of adsorption of noxious compounds is dropping, since it does not neutralize dyes to non-toxic compounds, but only removes it from water [9]. The challenge is to decompose Catalysts 2019, 9, 913; doi:10.3390/catal9110913 www.mdpi.com/journal/catalysts Catalysts 2019, 9, 913 2 of 16 the compounds to its harmless derivatives that can be achieved by catalytic degradation. The perfect catalysts exhibiting efficient decomposition of organic dyes should be cheap, as well as easy to obtain and to operate [10]. In particular, photocatalysis is a modern green chemistry approach which breaks down organicCatalysts 2019 pollutants, 9, x FOR PEER into REVIEW simple, non-toxic compounds such as water, carbon dioxide,2 of and16 other simple inorganiccatalysts exhibiting anions efficient under thedecomp influenceosition of organic photocatalyst dyes should and be UV cheap, irradiation as well as oreasy even to obtain visible light. It is alreadyand to shownoperate that[10]. In photocatalysis particular, photocatalysis is a useful is a and modern promising green chemistry tool for approach efficient which degradation breaks dyes from wastewaterdown organic [11 pollutants,12]. into simple, non-toxic compounds such as water, carbon dioxide, and other simple inorganic anions under the influence of photocatalyst and UV irradiation or even visible light. One of the most commonly used photocatalyst is titanium dioxide (TiO2)—a cheap, non-toxic, and It is already shown that photocatalysis is a useful and promising tool for efficient degradation dyes stable compound. Notably, TiO2 needs to be exposed to UV light to catalyze the degradation processes. UV from wastewater [11,12]. accounts only for ca. 2% of the sunlight, while visible light is ca. 45%. Therefore, a new class of catalysts with One of the most commonly used photocatalyst is titanium dioxide (TiO2)—a cheap, non-toxic, high photocatalyticand stable compound. activity inNotably, the presence TiO2 needs of visible to be exposed light is neededto UV lig andht to recently catalyze widely the degradation explored [13,14]. Dopingprocesses. of the TiO UV2 with accounts Schiff only base for complexes ca. 2% of the increases sunlight, thewhile photocatalytic visible light is activityca. 45%. Therefore, because of a thenew action in the widerclass range of catalysts of sunlight—both with high photocatalytic UV and visible activity light. in Thethe degradationpresence of visible of methylene light is needed blue in and the visible light usingrecently the modifiedwidely explored hybrid [13,14]. compounds Doping was of examinedthe TiO2 with by M.Schiff M. Momenibase complexes [15]. increases the photocatalytic activity because of the action in the wider range of sunlight—both UV and visible light. Since the d-electron metal ions may adsorb visible light, some of the complexes exhibit enhanced The degradation of methylene blue in the visible light using the modified hybrid compounds was photocatalyticexamined performance by M. M. Momeni toward [15]. organic compound degradation. Therefore, there is a number of papers presentingSince the outstanding d-electron metal effi ionsciency may of adsorb the process visible light, in the some presence of the complexes of complexes exhibit enhanced with such metal ions asphotocatalytic Zn(II), Co(II), performance Ni(II), Cu(II), toward Fe(II) organic [16– compound19]. degradation. Therefore, there is a number of Onepapers of the presenting degradation outstanding pathways efficiency bases of the on process the oxidation in the presence mechanism of complexes called withFenton such metal reaction —a ions as Zn(II), Co(II), Ni(II), Cu(II), Fe(II) [16–19]. well-known reaction utilizing H2O2 and iron ions. The homogenous catalyst under electromagnetic One of the degradation pathways bases on the oxidation mechanism called Fenton reaction—a radiation produces hydroxyl radicals providing effective decomposition of organic species [20]. This reaction well-known reaction utilizing H2O2 and iron ions. The homogenous catalyst under electromagnetic is widelyradiation used forproduces destroying hydroxyl hazardous radicals providing organics effe includingctive decomposition aromatic amines, of organic surfactants, species [20]. andThis organic dyes [21reaction–23]. Therefore, is widely suchused for an approachdestroying inspireshazardous the organics development including of aaromatic class of amines, new efficient surfactants, compounds with photocatalyticand organic dyes activity. [21–23]. An Therefore, example such could an beapproach the mechanism inspires the of development the methylene of a blueclass decolorizationof new efficient compounds with2+ photocatalytic3 activity. An example could be the2+ mechanism3 of the process in the presence of Co , HCO −, and H2O2 forming the Fenton-like Co -HCO −-H2O2 system that 2+ 3- was presentedmethylene by blue Li et decolorization al. [24,25]. Thanks process to in this,the presence organic of impurities Co , HCO could, and beH2O oxidized2 forming easily. the Fenton- Moreover, in like Co2+-HCO3--H2O2 system that was presented by Li et al. [24,25]. Thanks to this, organic impurities another case peroxide decomposition was catalyzed by Co2+ ions [26]. could be oxidized easily. Moreover, in another case peroxide decomposition was catalyzed by Co2+ Asions the [26]. degradation of organic dyes is promoted by electron transfer, to better understand photocatalyticAs behaviorthe degradation of cobalt of organic complexes, dyes is cyclic promoted voltammetry by electron (CV) transfer, was performed. to better understand CV is avaluable techniquephotocatalytic for investigation behavior of of a varietycobalt complexes, of redox processes. cyclic voltammetry Moreover, (CV) it might was performed. provide a wideCV is range of characterizationavaluable technique showing for its investigation potential in of photocatalytic a variety of redox reactions processes. including Moreover, stability it might of provide the compounds, a wide range of characterization showing its potential in photocatalytic reactions including stability of presence of intermediates during redox reactions, and reversibility of the processes [27]. the compounds, presence of intermediates during redox reactions, and reversibility of the processes Because[27]. of the previous research in our group [28], we decided to check the photocatalytic