N-Doped Animal Keratin Waste Porous Biochar derived from Trapa Natans Husks

Animal-keratin-wastes (AKWs), horns (HN), hair (HR), puffed waterfowl feathers (PF), hydrolyzed waterfowl feathers (HF), hydrolyzed fish meal (HM), crab meat (CM), feathers (FR), shrimp chaff (SC), fish scales (FS), and waste leather (WL) were used as modifiers to prepare animal-keratin-wastes biochars (AKWs-BC) derived from Trapa natans husks (TH). AKWs-BC have a well-developed microporous structure with a pore size mainly below 3 nm. Due to the doping of AKWs, the surface chemical properties of AKWs-BC (especially N functional groups) were improved. The utilization of APWs not only realizes the resource utilization of waste, but also can be used to prepare high-performance biochars.


Introduction
At present, strong oxidants, nitrogen compounds, metal salts, and other chemical modifiers are mainly used for the modification of surface chemical functional groups of biochar [1,2]. However, chemical modifiers have the disadvantages of being high-cost and having secondary pollution. More than 5 million tons of animal-keratin-waste (AKWs) is produced each year in the world [3]. AKWs are single-fiber crosslinked structural proteins (S-S) with intramolecular and intermolecular disulfide bonds (S-S) [4]. AKWs represent renewable biopolymers that can be better utilized. However, there is a relative lack of information about the possibility of using AKWs as bio-modifiers to prepare biochar. During pyrolysis, the formation of novel radicals is produced by AKWs decomposition, which can react with precursors to produce certain new ester salts and promote the formation of new functional groups. Therefore, we used ten common AKWs (Table 1 and Figure 1), including horns (HN), hair (HR), puffed waterfowl feathers (PF), hydrolyzed waterfowl feathers (HF), hydrolyzed fish meal (HM), crab meat (CM), feathers (FR), shrimp chaff (SC), fish scales (FS), and waste leather (WL), as modifiers to obtain environmentally sustainable biochar with a high surface chemistry.

Experimental Materials and Procedures
Preparation method: All chemicals used were of analytical grade. Trapa natans (TH), containing a large amount of lignin and cellulose, is high-yield waste. TH-based biochar has well developed pore structure and specific surface area. TH (Taihu in Jiangsu, China) and ten groups of AKWs (from one farmers' market in Shanghai, China) were crushed into particle between 0.35 and 1.0 mm. TH was mixed with 10 groups of AKWs, respectively, and the mixed mass ratio was 99:1 (g TH/g AKWs). Each mixture was impregnated with 85% phosphoric acid (g phosphoric acid/g TH = 2.2:1) for 10 hours at room temperature. To improve the degree of impregnation, series of pretreatments were used to treat the AKWs: the samples were first soaked in PPA (phosphoric acid, 85 wt.%) for 30 min, then irradiated with ultrasound for 20 min, and finally heated at 100 • C and 1.5 MPa for 30 min in a vertical automatic electrothermal pressure steam sterilizer. Each of the impregnation slurries was heated to 500 • C (heating rate = 100 • C/min) in a tube furnace and maintained at this temperature for 1 hour under nitrogen protection. Each carbon solid was washed several times with clean water until the pH of the filtrate was near neutral (about 7). Each carbon solid was dried, ground, and sieved (160-200 mesh). Finally, 10 sets of biochar were obtained: HN-BC, HR-BC, PF-BC, HF-BC, HM-BC, CM-BC, FR-BC, SC-BC, FS-BC, and WL-BC, which were collectively called AKWs-BC. Pure TH-based biochar (TH-BC) was prepared by the same method in Table 1 and Figure 1.
Characterization methods: surface area and pore size distribution were determined by N 2 adsorption-desorption at 77 K with a surface area analyzer (Quanta Chrome Corporation, Mahwah, NJ, USA). Sur-face area (S BET ) was measured by the BET (Brunauer-Emmet-Teller equation) method. Pore size distribution was determined by the density functional theory (DFT) method. Micropore volume (V mic ) and micropore surface area (S mic ) were calculated using the t-plot method. The total pore volume (V tot ) was deduced from the manufacturer's software by the BJH theory. The contents of C, H, O, N, and S of the biochar were measured by a Vario EI III Element Analyzer (Mahwah, NJ, USA). Boehm titration method was used to quantify the acidic and basic functional groups of the biochars. XPS (X-ray photoelectron spectroscopy) analyzer (Nico-let-460, Thermo Fisher, Mahwah, NJ, USA) was conducted to determine the binding energy between electrons and characterize the elements on the surface of biochars.

Experimental Results and Discussion
Thermogravimetric analysis (TGA) and differential thermogravimetric analysis (DTG) curves for the pyrolysis of AKWs-BC and TH-BC were obtained by TGA-50 analyzer ( Figure 1). As shown in the Figure 1, the pyrolysis process of each sample is roughly divided into three decomposition stages. In stage 1 (0-140 • C), AKWs-BC's weightlessness was greater than that of TH-BC, which can be explained by the first metamorphic decomposition of keratin [5]. Stage 1 also involved elimination of water vapor and other volatile substances. In stage 2 (140-500 • C), the new radicals produced by keratin decomposition formed different salt and esters with the thermal hydrolysate of TH and the heat activator, thereby promoting the formation of functional groups on the surface of the biochar. In stage 3 (500 • C), the weightlessness of each sample was not significant, indicating formation of the basic structure of biochar.
The pore size distributions ( Figure 2a); N 2 adsorption/desorption isotherms ( Figure 2b); and textural properties of S BET , S mic , V mic , and V tot (Table 1) of AKWs-BC and TH-BC were obtained by automatic specific surface area and pore size analyzer (GEMINT VII 2390, Mahwah, NJ, USA). As shown in the Figure 2a, both APWs-BC and TH-BC have a narrow pore size distribution (2-3 nm). As shown in the Figure 2b, the isotherms of AKWs-BC and TH-BC were mixture of types I and IV (IUPAC), with small hysteresis loop indicating the presence of well-developed mixed micro-mesopores structure. The variation coefficients in the S BET , S mic , V mic , and V tot groups were analyzed by SPSS software ( Table 1). The low dispersion state of the data within each group indicates that the pore structure characteristics of AKWs-BC and TH-BC are similar. BET surface area (SBET), micropore surface area (Smic), micropore volume (Vmic), total pore volume (Vtot).  Figure 4: (N 6, Pyridine nitrogen and amino group) -CONH-/ N-H at 398.7 ± 0.5 eV or (N 5, pyrrolelike nitrogen) -CONH-/-NH2 at 399.8 ± 0.5 eV, while -NO2 represents nitrogen in nitro group (403.6-405.1 eV) [6,7]. The number of oxygen-containing and nitrogen-containing functional groups of AKWs-BC is higher than that of TH-BC, which is consistent with the results of Boehm's titration  [6,7]. The number of oxygen-containing and nitrogen-containing functional groups of AKWs-BC is higher than that of TH-BC, which is consistent with the results of Boehm's titration results and element composition. Overall, there is strong evidence that APWs promote the formation of functional groups on the surface of the AKWs-BC.

Conclusions
The results show that it is feasible to use AKWs as a biochar modifier. AKWs-BC exhibits a developed microporous structure and high chemical functional groups. The use of APWs can not only realize the resource utilization of waste, it can also provide new ideas for biochar preparation modification.
Author Contributions: Writing-original draft, W.Y.; Conceptualization, W.Y. and Y.X.; Software, Z.Z. and S.X.; Data curation, T.L. and Z.Z.; Formal analysis, T.L.; Supervision, J.X.; Investigation, S.X. and J.X. All authors have read and agreed to the published version of the manuscript.
Funding: This research received no external funding.