*Review* **Surface Modification of Biochar for Dye Removal from Wastewater**

**Lalit Goswami , Anamika Kushwaha , Saroj Raj Kafle and Beom-Soo Kim \***

Department of Chemical Engineering, Chungbuk National University, Cheongju 28644, Korea; lalitgoswami660323@gmail.com (L.G.); kushwaha.anamika@gmail.com (A.K.); 100sarojraj@gmail.com (S.R.K.) **\*** Correspondence: bskim@chungbuk.ac.kr

**Abstract:** Nowadays, biochar is being studied to a great degree because of its potential for carbon sequestration, soil improvement, climate change mitigation, catalysis, wastewater treatment, energy storage, and waste management. The present review emphasizes on the utilization of biochar and biochar-based nanocomposites to play a key role in decontaminating dyes from wastewater. Numerous trials are underway to synthesize functionalized, surface engineered biochar-based nanocomposites that can sufficiently remove dye-contaminated wastewater. The removal of dyes from wastewater via natural and modified biochar follows numerous mechanisms such as precipitation, surface complexation, ion exchange, cation–π interactions, and electrostatic attraction. Further, biochar production and modification promote good adsorption capacity for dye removal owing to the properties tailored from the production stage and linked with specific adsorption mechanisms such as hydrophobic and electrostatic interactions. Meanwhile, a framework for artificial neural networking and machine learning to model the dye removal efficiency of biochar from wastewater is proposed even though such studies are still in their infancy stage. The present review article recommends that smart technologies for modelling and forecasting the potential of such modification of biochar should be included for their proper applications.

**Keywords:** post-processing modification; surface-engineered biochar; dye removal; machine learning; artificial neural network

## **1. Introduction**

Unprecedented rising globalization, industrialization, urbanization, and anthropogenic human activities have led to a worldwide shortage of clean water [1–3]. In this regard, wastewater contaminated by water-soluble dyes is one of the prime environmental issues [4–6]. Nowadays, numerous dyes and additives are being utilized enormously in industrial applications such as textile, paper, printing, paint, laundry, cosmetics, carpet, leather, food, and rubber [4]. Globally, more than 10,000 different types of natural and synthetic dyes are produced annually, weighing in the range of 7 <sup>×</sup> <sup>10</sup>5–1 <sup>×</sup> <sup>10</sup><sup>6</sup> tons [7]. The chemical complexity, stability, and poor biodegradability of these dye-contaminated wastewaters are of prime concern and continue to limit the clean water resources available. The rising demand for dyes is simultaneously leading to the malefactors of inadvertent discharge of dye-contaminated wastewater into water streams; it directly affects the life of aquatic flora and fauna along with the food chain and is indirectly deleterious to human health [8–10]. Henceforth, it is highly significant to develop sustainable remediation solutions to remove soluble dyes and other contaminants from water.

It typically costs about \$1 billion annually to treat 640 million m<sup>3</sup> of textile and dyeing wastewater [11]. Several conventional dye-contaminated wastewater treatment technologies are used for dye removal from effluents, such as chemisorption, electrochemical oxidation, ozonation, ion exchange, membrane filtration, anaerobic lagoons, sedimentation, oxidation ponds, coagulation flocculation, photocatalytic degradation, and gamma irradiation [4,11–13]. Among these treatments, adsorption is one of the most sustainable

**Citation:** Goswami, L.; Kushwaha, A.; Kafle, S.R.; Kim, B.-S. Surface Modification of Biochar for Dye Removal from Wastewater. *Catalysts* **2022**, *12*, 817. https://doi.org/ 10.3390/catal12080817

Academic Editor: John Vakros

Received: 2 July 2022 Accepted: 22 July 2022 Published: 26 July 2022

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and cost-effective techniques because other technologies require a lot of chemicals, high energy, and are not cost effective. Carbonaceous materials possessing a high specific surface area (SSA) are widely used as adsorbing agents in wastewater treatment for dye removal [14,15]. Inexpensive adsorbents, preferably derived from natural materials and industrial wastes/by-products, such as biochar, cellulose, aerogels, activated charcoal, bentonite, fly ash, and silica, can be utilized for wastewater treatment [16–20].

Biochar (BC) is a highly stable carbonaceous material that is aromatized and amorphous in nature. It is usually formed after thermochemical conversion of organic matter and wastes at temperatures of 350–750 ◦C under limited oxygen conditions [21–23]. Its high SSA, pore volume, hydrophobicity, etc., enable its use as an efficient biomaterial for carbon sequestration, soil improvement, climate change mitigation, catalysis, wastewater treatment, energy storage, and waste management [4]. In addition, it has been well recognized as a potentially highly efficient, low-cost, and eco-friendly adsorbent for the removal of organic and inorganic pollutants, particularly heavy metals and dyes from wastewater.

The physicochemical properties and primary composition of BC are considerably altered according to the biomass feedstock, carbonization procedure, degree of pyrolysis, activation, and functionalization techniques [23]. Indulging in modification techniques, BC depicts multiscale porous structures, extensive surface functional groups, and high surface areas, utilizing various organic feedstocks. In addition, inherent functional sites (hydroquinone, defects, etc.) and minerals (silica, transition metals) add BC as a promising ingredient to tailor heterojunctions/composites. The facile metal impregnation, gas activation, sulfonation, and ionic liquid grafting of BC enable many advantages in catalytic processes owing to enhanced accessibility to active sites, excellent π–π interactions, high active surface area, and enhanced charge transfer [24].

In this review, we focused on surface modification and alteration of BC to enhance the efficiency of dye removal from wastewater. A network visualization of terms associated with biochar and dye with a minimum number of 10 occurrences of 3589 associated keywords is represented in Figure 1 (assessed 30 June 2022). It represents the current trends in research related to the application of biochar in association with dye in the Web of Science. Here, the various colors of the nodes represent different clusters, while the size of each bubble depicts its frequency of occurrence. A literature review of BC reports that thermochemical conversion often exhibits low reactivity and selectivity for dye removal [25]. To eliminate these limitations, BC is tailored via numerous techniques to achieve the desired selectivity and reactivity to enhance surface sites and hydrophobicity for regulating the dye removal kinetics [26–28]. Factors affecting BC properties and various post-processing modifications, functionalization, and BC activation for dye removal are included. Literature review reports on surface modification of BC for dye removal are very scarce. The application of smart digital technologies such as machine learning and artificial neural networks (ANN) is also covered to further enhance dye removal via BC.

**Figure 1.** Network visualization of terms associated with biochar and dye. **Figure 1.** Network visualization of terms associated with biochar and dye.
