sustainability Review Applications of Modified Biochar-Based Materials for the Removal of Environment Pollutants: A Mini Review Jung Eun Lee and Young-Kwon Park * School of Environmental Engineering, University of Seoul, Seoul 02504, Korea; [email protected] * Correspondence: [email protected] Received: 2 July 2020; Accepted: 28 July 2020; Published: 29 July 2020 Abstract: The biochar treated through several processes can be modified and utilized as catalyst or catalyst support due to specific properties with various available functional groups on the surface. The functional groups attached to the biochar surface can initiate active radical species to play an important role, which lead to the destruction of contaminants as a catalyst and the removal of adsorbent by involving electron transfer or redox processes. Centering on the high potential to be developed in field applications, this paper reviews more feasible and sustainable biochar-based materials resulting in efficient removals of environmental pollutants as catalyst or support rather than describing them according to the technology category. This review addresses biochar-based materials for utilization as catalysts, metal catalyst supports of iron/iron oxides, and titanium dioxide because the advanced oxidation process using iron/iron oxides or titanium dioxides is more effective for the removal of contaminants. Biochar-based materials can be used for the removal of inorganic contaminants such as heavy meals and nitrate or phosphate to cause eutrophication of water. The biochar-based materials available for the remediation of eutrophic water by the release of N- or P-containing compounds is also reviewed. Keywords: modified biochar; ZVI catalyst support; TiO2 photocatalyst support; remediation of eutrophic water 1. Introduction Biochar obtained from the pyrolysis of various biomass under the control of an oxidant has been studied because of the reduced cost, utilization of waste, and eco-friendly method [1,2]. Biochar applications can be divided into several categories, of which the most prevalent utilization of biochar might be an application as a catalyst and catalyst support for environmental remediation [3] and electrode materials or membrane in electrochemical field. Ag/biochar component was fabricated as supercapacitor electrode [4]. In order to improve removal efficiency of contaminants, biochar needs to be pretreated by activation and functionalization processes before use [5–8]. Generally, biochar can be activated physically or chemically to improve the activity by adjusting the surface area and pore volume [9–11]. In physical activation, CO2 or steam gas are commonly applied to biochar, whereas an inorganic acid, base, or neutral salts, such as HNO3,H2SO4,H3PO4, KOH, and ZnCl2 are used in the chemical activation of biochar [12,13]. Compared to physical activation, the activation of biochar by chemical reagents could necessarily require a post-process, such as neutralization, to avoid damaging the facility [12]. The functional groups attached to the surface of biochar have also attracted attention because of being able to expand its utilization [14,15]. Depending on the order of treatment process, there are two types of processes to place functional groups on the biochar surface more efficiently. One is the impregnation of biochar with functional group precursor reagents after the pyrolysis of Sustainability 2020, 12, 6112; doi:10.3390/su12156112 www.mdpi.com/journal/sustainability Sustainability 2020, 12, 6112 2 of 15 Sustainability 2020, 12, x FOR PEER REVIEW 2 of 14 biomass, and the other is the pre-coating of biomass with reagents containing functional groups prior togroups pyrolysis prior or to thermal pyrolysis processes or thermal [13]. processes In general, [13]. depending In general, on depending the components on the andcomponents structure and of biochar,structure the of physical biochar, and thechemical physical propertiesand chemical of modified properties biochar of modified show di ffbiocharerence inshow accordance difference with in theaccordance order of pyrolysis.with the order Biochar of pyrolysis. improved Biochar with multi-functional improved with groups multi-functional was used to groups oxidize was a reducible used to compoundoxidize a reducible through the compound transfer of thro electronsugh the released transfer from of aelectrons photocatalyst released or metalfrom [a16 photocatalyst]. These surface or functionalmetal [16]. groups These on surface the biochar functional are responsible groups on for th bothe biochar enhancing are responsible the removal for effi ciencyboth enhancing of pollutants the byremoval improved efficiency adsorption of pollutants capability by [ 17improved–20]. In addition,adsorption modified capability biochar-based [17–20]. In addition, materials modified can also bebiochar-based used to mitigate materials the malodourouscan also be used air to caused mitigate by the volatile malodourous organic compoundsair caused by generated volatile organic from municipalcompounds waste generated [21]. The from biochar municipal was waste also e ff[21].ective The in biochar diminishing was also the effective odorous in compounds diminishing from the livestockodorous farmingcompounds [22, 23from]. livestock farming [22,23]. WhenWhen there there is anis activean active oxygen-included oxygen-included functional func grouptional ongroup the biocharon the surface, biochar e.g., surface, carboxylic, e.g., carbonyl,carboxylic, and carbonyl, hydroxyl and groups, hydroxyl the sitegroups, of surface the site can of actsurface as an can electron act as transferan electron platform, transfer such platform, as an electronsuch as acceptoran electron or donor.acceptor Although or donor. the Although electric charge the electric distribution charge of distribution biochar surface of biochar is dependent surface on is thedependent applied feedstock,on the applied various feedstock, multifunctional various multifun groups attachedctional groups on the surface attached can on play the asurface nucleophile can play or electrophilea nucleophile in theor processelectrophile of reaction. in the Therefore,process of a re rangeaction. of applicationsTherefore, a using range biochar-based of applications materials using havebiochar-based been investigated materials for have catalytic been degradation investigated and for photolysiscatalytic degradation for the removal and ofphotolysis harmful organic for the contaminantsremoval of harmful and heavy organic metals contaminants [24,25]. Recently, and heavy beyond metals the [24,25]. application Recently, of bioenergy beyond and the biorefineryapplication processof bioenergy [26], biochar and biorefinery modifications process and [26], the biochar application modifications of metal oxidesand the as application catalyst or of supports metal oxides have beenas catalyst investigated or supports widely because have biochar-basedbeen investigat materialsed widely are sustainable because biochar-based and safe in the remediationmaterials are of contaminatedsustainable and water safe and in the soil rem [27].ediation However, of contaminated although the biochar-basedwater and soil materials [27]. However, have high although potential the inbiochar-based terms of cost, materials the application have high to large-scale potential novelin terms technology of cost, shouldthe application be developed to large-scale due to lack novel of feasibilitytechnology [28 should]. This paperbe developed reviews thedue literature, to lack of focusing feasibility on [28]. the environmental This paper reviews biochar the catalyst literature, and catalystfocusing support on the ofenvironmental iron/iron oxides biochar and titaniumcatalyst and dioxides catalyst relating support the of characteristics iron/iron oxides of biochar-based and titanium materialsdioxides torelating the activity the characteristics for the removal of of biochar-ba contaminantssed andmaterials the remediation to the activity of eutrophic for the waterremoval using of biochar-basedcontaminants and materials. the remediation Various biochar of eutrophic derived wate fromr using plant biochar-based residuals and materials. animal excrements Various biochar and pollutantsderived from removal plant by residuals modified and biochar animal were excrements described and as shown pollutants in Figure removal1. by modified biochar were described as shown in Figure 1. Figure 1. Biochar by the pyrolysis of biomass derived from various feedstocks and application of modifiedFigure 1. biochar. Biochar by the pyrolysis of biomass derived from various feedstocks and application of modified biochar. 2. Modification and Application of the Biochar as an Environmental Catalyst 2.1. Active Oxidant Species Regarding the Reaction Mechanism AOPs (advanced oxidation processes) are one of the most effective environmental remediations. In general, ozone, hydrogen peroxide (H2O2), and persulfate (PS, S2O82−) can produce active radical Sustainability 2020, 12, 6112 3 of 15 2. Modification and Application of the Biochar as an Environmental Catalyst 2.1. Active Oxidant Species Regarding the Reaction Mechanism AOPs (advanced oxidation processes) are one of the most effective environmental remediations. 2 In general, ozone, hydrogen peroxide (H2O2), and persulfate (PS, S2O8 −) can produce active radical species, such as hydroxyl radicals ( OH) and the sulfate radical anion (SRA, SO ) by UV wavelength • 4•− irradiation. These active radical species react with various functional groups