1. Introduction
Maize is one of the most important agricultural products in terms of production, consumption, and economic importance. In addition to being a source for human food, it is used for animal feed, and as feedstock for biofuel production [
1,
2]. Its production keeps increasing, for example, according to the Food and Agriculture Organization (FAO), maize production increased significantly in the United States from 250 million tonnes in 2000 to about 350 million tonnes in 2018. Its contamination with deoxynivalenol (DON) generally occurs in countries with humid climates, such as the United States, Europe, and Canada [
3,
4]. Deoxynivalenol (DON) (also called vomitoxin due to the strong vomiting effect after consumption [
5]) is a mycotoxin substance produced by pathogens such as
Fusarium graminearum (
Gibberella zeae), and
Fusarium culmorum, contaminating crops such as maize, wheat, barley, oats, and rye [
5,
6]. DON affects human and animal health, causing food refusal, vomiting, suppression of the immune system, abdominal pain, and diarrhea [
2].
Table 1 shows DON concentrations in maize from 2011 to 2020 in Ontario, Canada. In 2018, more than half of Ontario’s corn was infected with DON due to high humidity that summer, and only 33% of corn produced that year contained less than 0.5 ppm of DON. Furthermore, 25% of corn contained vomitoxin levels above 5 ppm, which is the maximum allowable limit for pig feed [
7].
Many studies discussed different solutions for decontamination of the toxic maize and possible outcomes (
Table 2).
Figure 1 schematically illustrates the possible remediation or detoxification methods for corn grains, which can be categorized as physical, chemical, and biological treatments. In some instances, combinations of these processes can be used. Washing or steeping can be effective for water-soluble toxins such as DON.
Trenholm et al. [
9] and Sinha [
10] studied the effect of washing on the DON-concentration changes in contaminated corn, and the results show up to 67% reduction. Another study by Bullerman and Bianchini showed a 19% DON reduction in corn using sorting, trimming, and cleaning [
11]. Chemical treatments for mycotoxin corn have also been widely studied. but there are only a few implementations of this due to the complications of treated products for human consumption. Cazzaniga et al. [
12] showed that sodium bisulfite is effective at reducing DON concentrations. However, alteration of the corn’s structure makes this product suitable only for animal feed. A recent study performed in 2021 discussed the effect of ozone as a chemical treatment for removing DON. The results showed a 41% reduction after 180 min, indicative of achieving the difficult degradation of DON only after long contact times [
13]. No chemical or enzymatic treatments have been approved by the European Union for mycotoxin treatments due to the extensive need for safety testing after treatments and possible structural and chemical changes in the products [
2].
Table 2.
Summary of various techniques used for the decontamination of DON from maize.
Table 2.
Summary of various techniques used for the decontamination of DON from maize.
Process | Year | Condition | Results | Drawbacks |
---|
Chemical and physical treatment [14] | 1986 | (1) Moist ozone (1.1 mol %) in air (2) 30% chlorine | (1) 90% of DON reduction after an hour (2) Total destruction of DON after 0.5 h | - |
Washing [9] | 1992 | Washing three times with DI water, using sodium carbonate solution (1 M) | 73% of DON reduction | Expensive post-treatments such as drying after decontamination |
Cooking [12] | 2001 | Extrusion process at 180 °C, adding sodium metabisulfite (1%) | 95% of DON reduction | - |
Bt corn [4] | 2004 | Using Bt corn as a genetically modified crop | Annual loss reduced from $52 to $8 million | The majority of Bt corn is used for animal feed |
Food processing [11] | 2007 | Sorting, trimming, and cleaning | DON concentration reduction from 5.5% to 19% | These processes do not fully destroy DON structure |
Ozone [13] | 2021 | In contact with Ozone for 180 min | 42% of DON reduction | DON appeared to be quite difficult to degrade even at elevated contact times |
Biomass is currently drawing considerable attention as a renewable resource for the production of materials and energy [
15]. The environmental advantages (e.g., low carbon footprint), abundance, and continuous renewal, as well as the waste-reduction potential make it a sustainable alternative to fossil resources [
16]. Although the total replacement of fossil fuels with biomass is impossible in the near future, multiple approaches are being pursued for the production of value-added products from residual biomass and waste. Many studies are at the research and development level, but several have already achieved commercial implementation [
17]. There are several pathways for biomass conversion from various feedstocks (i.e., woody products, such as forestry wastes and bark; agricultural products, such as purpose grown crops, residues, and manure; and organic wastes, such as industrial and municipal organic fractions) to chemicals and fuels. These include biological, thermal, and mechanical (physical) processes. The most common thermochemical conversion processes are pyrolysis, combustion, gasification, and liquefaction. Pyrolysis has the advantage of being a relatively fast process, not requiring intensive feedstock pre-treatments [
18], and generating valuable products that have applications in the agricultural, pharmaceutical, and chemical industries [
19,
20]. Therefore, non-edible DON-contaminated corn was targeted as a potential biomass feedstock for the pyrolysis process.
This study represents an original contribution towards investigating the feasibility of using pyrolysis for the destruction of DON in corn grains as a potential solution for managing this seasonal waste. Through this process, waste is converted into value-added products such as bio-char, bio-oil, and non-condensable gases, which have various industrial applications. This pathway for converting this non-edible feedstock has not been explored previously.
4. Conclusions
In this study, deoxynivalenol (DON) contaminated corn was converted into value-added products (e.g., bio-char, bio-oil, and gas) using slow pyrolysis in a batch reactor. The pyrolysis process reduced the DON concentration from 5–7 ppm in the raw corn grains to zero ppm, making thermochemical conversion a promising method for industrial applications. When the final reaction temperature increased from 450 °C to 650 °C, the bio-oil yield increased from 29 to 47 wt.%, while the bio-char yield decreased from 41 to 25 wt.%. The total gas yields were temperature independent and remained approximately constant (30%). The effect of the heating rate was investigated via testing between 5 and 35 °C min−1. The result shows a maximum bio-oil yield of 46 wt.% which was achieved at the highest heating rates of 35 °C/min. Acetic acid and levoglucosan were the most significant components in the bio-oil, representing the highest GC-MS peaks, with yields of 26 and 13 g per kg of bio-oil. The pyrolysis operating temperature affected the pH of bio-char. For instance, the pH of the bio-char changed from 6.1 to 7.4 as the temperature increased from 450 to 650 °C. This is a promising result for soil amendment applications. The BET surface area of the bio-char significantly increased from 3 to 419 m2 g−1 by activation at 900 °C in the presence of CO2 for 3 h. The possible high adsorption capacity of activated bio-char could be due to the large number of pores developed during the activation process. This result was confirmed through SEM analysis. The presence of functional groups (e.g., –C=O, –C=N) on the biochar surface was confirmed with FTIR. The high pH (9) of the activated bio-char and the presence of functional groups make the activated corn bio-char a good candidate for adsorption applications. The gas product is comprised mainly of H2, CO, CO2, CH4, and C2H4, and the HHV of this stream was calculated to be 16.46 MJ/Nm3, showing its potential to be used as an energy source. The results show the potential industrial applications of bio-oil, activated bio-char, and non-condensable gases as chemicals, adsorbents, and energy resources, respectively.