1.4.2. Acid Pretreatment

Of the numerous chemical pretreatment procedures, acid pretreatment is among the most widely utilized. Acid hydrolysis will boost enzymatic hydrolysis performance and increase the energy conversion efficiency of lignocellulosic biomass in MFCs. Concentrated mineral acid (CA), dilute mineral acid (DA), and dicarboxylic acid has been utilized to pretreat agro wastes. CAs like H2SO<sup>4</sup> (Figure 2) and HCl are especially useful for agro wastes. These acids, however, are acidic, corrosive, and dangerous, necessitating the use of specialized reactors that can withstand corrosion. Meanwhile, lignocellulose hydrolysis of agrowastes displayed a strong reaction rate after pretreatment with dilute sulfuric acid. Initially, high temperature (T > 160 ◦C) and low temperature (T < 160 ◦C) dilute acid hydrolysis pretreatment methods were created [21]. In contrast, a high temperature throughout the DA hydrolysis is ideal for cellulose hydrolysis due to sugar decomposition. In an MFC inoculated with pure-culture, Wang et al. (2017) used diluted sulfuric acid pretreated corn straw as the substrate for direct power production. The maximum PD provided by this MFC was 17.2 <sup>±</sup> 0.3 mW/m<sup>2</sup> , demonstrating the viability of biomass hydrolysate as a source of power production in MFC. A high PD of 660 mW/m<sup>2</sup> from the hydrolysate with a pure-culture of *Shewanella oneidensis* MR-1 could also be obtained by integrating electrode alteration and electron shuttle attachment [25]. Ionic liquids have also been stated to be beneficial due to their thermal stability, low hydrophobicity, low toxicity, and increased electrochemical stability [26]. Ionic pretreatment of farm straw biomass substantially solubilizes cellulose and may recover 100% of the utilized liquid with high purity under moderate conditions. Straw biomass is pretreated with 1-ethyl-3-methylimidazolium acetate in an ionic liquid (IL) at 120–140 ◦C (EmimAC). The materials are then washed with anti-solvent for a certain number of hours, resulting in cellulose regeneration. The cellulose is then separated, the lignin precipitated, and anti-solvent recycle, and IL is developed (Figure 3) [27]. Due to the self-evident intra-structure modifications and the disparity in crystallinity characteristics, the generated cellulose precipitate has a strong enzymatic digestibility compared to the rudimentary cellulose from straw waste [28].

**Figure 2.** Schematic representation of sulphuric acid pretreatment.

**Figure 3.** Schematic representation of ionic liquid pretreatment.
