# Unravelling the Environmental Application of Biochar as Low-Cost Biosorbent: A Review
## 1 Introduction  
Nowadays, the environmental crisis and vulnerabilities have attracted the attention of researchers to the hunt for new green and friendly strategies for increasing resources [1]. In addition, the current linear economic system stands out diametrically from the life cycle of nature, in which there is no garbage or landfills are reused for other different stages. In this context, in recent years, the replacement of conventional sources with recoverable wastes has set the pace for the achievement of genuinely revolutionary processes from an environmental perspective [2]. The utilization of these wastes as a raw material has been proposed as one of their principal alternatives, and a brightening industrial future has been recently suggested for them, based on the concept of the circular economy. An example of the circular economy is the transformation of wastes into biochar, which, although initially prepared from non-residual biomass, in recent years have been produced from a wide variety of organic raw materials. Their availability is subject to sustainability requirements, such as not competing with the human food chain or animal and plant nutrition, and coming from a sustainable source for the environment and climate protection [3]. Thus, biochar preparation itself is a process of recyclingorganic waste with multiple benefits: waste treatment, environmental protection, and reduction in the biochar processing cost. Biochar is a carbon-rich product obtained under reducing thermal conditions by the decomposition of organic matter with a limited supply of oxygen [4]. It is comprised of particles with different sizes and has high porosity with micro-, meso-, and macropores, whose sizes range from <2 nm to 2-50 nm and >50 nm, respectively. The macropores come from the spaces of the original raw material, and micropores and mesopores are generated in the thermochemical process [5]. In the production of biochar, researchers have applied several thermochemical methods using several feedstocks, with diverse contents of cellulose, hemicellulose and lignin, leading to different physicochemical properties of biochar [6; 7]. As it was explained by [PERSON] et al. [8], the characterization of the biochar surface could explain its main properties, which are composites of aliphatic and aromatic groups and having abundant functional groups such as hydroxyl, epoxy, carboxyl, carbonyl, ether, ester, amide, sulfonic, and acyl groups as well as minerals and trace metals. Therefore, it is deemed a material that possesses the capacity to exchange electrons and high reactivity. The potential applications of biochar include as an energy source (biofuel), soil conditioner, and adsorbent [9]. For the last 20 years, the reviewing of the database Scopus(r), by evaluation of articles, reviews and books containing the keyword \"biochar\", draws a growing trend in the number of articles published per year, from 1 in 2000 to 3030 in 2019, and over 2341 by July 2020 (Figure 1). Concerning the different applications, the evolution of publications is similar and has increased over the 10 years. As can be seen in Figure 1, most of the articles published are related to the use of biochar as a soil conditioner. This tendency is observed from the beginning, followed by articles related to biofuel. However, from 2006 onwards, this pattern changes, and there is a growing interest in studies related to its usage in the remediation of polluted streams. Currently, the number of papers published so far in 2020 on this subject (1187) is higher than its application as a soil conditioner, which represents around 42% of the papers published in relation to biochar. The adsorption process is currently one of the main technologies applied in wastewater remediation. This fact is corroborated by the analysis of recent studies; thus, the search on the database Scopus(r) of the keywords \"biochar\" and \"adsorption\" reveals the exponential growth of the articles published, from 10 in 2009 to nearly 1000 in 2019.