Concern about water scarcity in some regions around the world is constantly increasing with the continuous pollution of existing waters in different regions. Chemical compounds that cause major pollution in the ecosystem can be listed as dyes (El Nemr 2012a; Rafatullah et al. 2010; Shakoor et al. 2017), heavy metals (Bilal et al. 2013; Chao et al. 2014; El Nemr 2011, 2012b), drugs (Cuerda-Correa et al. 2010), hydrocarbons (El Nemr et al. 2004, 2005, 2013) and pesticides (Ignatowicz 2009; Salem et al. 2013, 2014; El Nemr et al. 2016a, 2016b; Ragab et al. 2016; Asiri et al. 2020; Hassaan and El Nemr 2020). These substances are released into the environment from industrial and hospital wastewater or through domestic sewage (El Nemr 2011, 2016).
In particular, dyes are easily detected in wastewater due to their color. Synthetic dyes are the leading dyes that are widely used in paint, leather, textile and different industries (El Nemr 2012a; Lin et al. 2017). Ecological balance and human health are adversely affected by this pollution as most paints are carcinogenic, toxic and non-biodegradable (Rafatullah et al. 2010; Iqbal 2016). The amount of untreated dyestuffs discharged into water bodies, approximately 10-20%, is estimated to average (0.7-2.0) ×105 tonnes per year (Dawood and Sen 2012).
Among the synthetic dyes, azo dyes come first because of their features such as having the most color variety, being the largest and being versatile. Carcinogenic compounds are formed as a result of excessive use of these chemicals (Rauf and Ashraf 2009).
There are many techniques for the treatment of dyeing wastewater, and the main ones can be listed as chemical oxidation (Karthikeyan et al. 2012), advanced oxidations (El Nemr et al. 2017, 2018; Hassaan et al. 2017a, b), photo-degradation (Madhusudan et al. 2013; Chang et al. 2016; El Nemr et al. 2019; Helmy et al. 2018, 2021), coagulation/flocculation (Saleh and Gupta 2012), biological treatment (Gupta et al. 2015), electrochemical treatment (Felix et al. 2014) and adsorption treatment (Anastopoulos and Kyzas 2014; Eldeeb et al. 2021; Lin et al. 2016, 2017; Salama et al. 2015). Removal of dyes by adsorption method using activated carbon is one of the most preferred among these techniques due to its high efficiency (El Nemr 2012c). However, the high production and processing cost of commercial activated carbon has led scientists to seek to synthesize cheaper and more effective adsorbent materials (Abdelwahab et al. 2007; Heibati et al. 2015; Song et al. 2015). For this, the trend towards biochar production as a cheaper and environmentally friendly alternative is increasing day by day. Biochars obtained by using biomass and waste materials as starting materials also prevent the waste of scarce resources. Carbonaceous solid materials obtained by gasification or pyrolysis of biomass at temperatures above 350 oC in a nitrogen atmosphere are defined as biochar (Kołodyńska et al. 2017; Song et al. 2015). Güzel et al. (2017), in their study, found that activities for commercial activated carbon production are generally more expensive than activities for biochar production. In addition to their low cost, biochars also have advantages such as reducing secondary environmental pollution, renewability and creating high value-added adsorbents (Liu et al. 2014). In addition to these, the use of biochar as an adsorbent also reduces the amount of carbon dioxide released into the atmosphere (Ahmad et al. 2014; Abdelhafez and Li 2016). Although biochars have more functional groups on the carbonaceous surface, their surface areas and pore volumes are smaller than activated carbons (Liu et al. 2011; El-Nemr et al. 2022a, b).
In order to improve the practical applications of biochars for the removal of dyestuffs from wastewater, it is possible to further increase the number of functional groups by chemical changes on their surfaces. Modifications such as impregnation with minerals, oxidation, nanoscale formation and reduction of the biochar surface are generally efforts to increase the adsorption capacity of biochar (Wang et al. 2019). Impregnation with mineral elements takes place by adding amino groups to the pores of the adsorbent to increase the functionality of the biochar (Yao et al. 2014). In the surface oxidation method, it is aimed to increase the number of acidic functional groups by using various bases (KOH or NaOH), acids (H3PO4, HNO3 or H2SO4/HNO3) and certain oxidizing reagents (NH3.H2O, O3, H2O2, NaClO, KMnO4 or (NH4)2S2O8) (Song et al. 201; Jimenez-Cordero et al. 2015; Liatsou et al. 20164; Chang et al. 2018). In nanoscale metals assistance, biochars are loaded with nano metals, increasing the thermal stability, number of adsorption sites, specific surface area and resistance to oxidation of biochar. This increases the affinity of the biochar and helps to remove toxic substances from the water (El-Nemr et al. 2020a, 2020b, 2020c, 2021, 2022a, 2022b; Zhang and Hay 2020). Surface reduction modification occurs by bonding biochars with nitrogenous functional groups, especially primary amines, secondary amines, tertiary amines, imidazole and quaternary ammonium. The most commonly used reducing agents can be listed as Na2SO3, H2, NH3.H2O, FeSO4 and aniline (Ma et al. 2014; Sahlabji et al. 2021).
There are many studies in the literature on the adsorption of various pollutants of these adsorbents, which are obtained by obtaining activated carbon from agricultural waste biomass. Coconut husk (Foo and Hameed 2012), olive stone (Yavuz et al. 2010), watermelon peel (El-Nemr et al. 2020), gulmohar (Ponnusami et al. 2009), potato (Gupta et al. 2016; Kyzas et al. 2016), orange peel (El-Nemr et al. 2021a), mandarin peel (Koyuncu et al. 2018; Unugul and Nigiz 2020), wheat straw (Zhang et al. 2012), peanut husk (Song et al. 2011), sesame hull (Feng et al. 2011), coffee bean husks (Baquero et al. 2003), tea waste (Islam et al. 2015), rice straw (Kim et al. 2014), green algae Ulva lactuca (El Nemr et al. 2021b; Shoaib et al. 2021a), red algae Pterocladia capillacea (Shoaib et al. 2021b), sugarcane bagasse (El-Nemr et al.2021c) and Macore fruit (Aboua et al. 2015) are some of these biomass.
Mandarin is one of the temperate climate fruits belonging to a kind of citrus family. According to the data published by the United Nations Food and Agriculture Organization (FAO) in 2021, the annual production of citrus is approximately 80 million tons (FAO 2021). Countries such as China, Turkey, Brazil, Egypt, Spain, Japan, Italy and South Korea are the countries with the highest production. The peels of the mandarins used in fruit juice factories, which are thrown into the environment, constitute approximately 8-14% of their total weight. These peels are mostly used in solid fuel, fertilizer, cosmetics and animal feed industries (Koyuncu et al. 2018). It produces a large amount of fruit peel as biomass waste due to mandarin consumption (Boluda-Aguilar et al.2010). Organic carbon components such as hemicellulose, cellulose and pectin in its structure allow the production of environmentally friendly biochars from tangerine peels by pyrolysis. Thus, materials with an excellent adsorption capacity are obtained (Dhillon et al. 2004). A comprehensive study on the effect of physicochemical properties of biochars obtained from mandarin peels on the removal of dyestuffs in wastewater has not been published yet. No studies have been conducted on the adsorption performance of biochars obtained by first dehydration with H2SO4, followed by ozonation in water, and finally, amination with Triethylenetetramine (TETA), using mandarin peel waste materials as a suitable precursor for the removal of Acid Red 35 dye from wastewater. In this study, Mandarin-Biochar-O3-TETA (MBT) produced from mandarin peels, which is a low-cost agricultural waste material, by dehydration process was investigated for its efficiency in Acid Red 35 dye removal from aqueous environment. Parameters such as initial adsorbate concentration, solution pH, contact time between adsorbent and adsorbate and the effect of adsorbent dose were investigated as the removal conditions of Acid Red 35 dye from aqueous solution. Adsorption kinetics and isotherms for the removal of Acid Red 35 dye on MBT adsorbent were investigated to determine the structure and maximum adsorption capacity of the adsorption.