# Mitigation of Gaseous Emissions from Swine Manure with the Surficial Application of Biochars
## 4 Discussion
### Effect on Volatile Organic Compounds  
The average % \(R\) (Table 4) ranged from 41 to 99% for all phenolic VOCs (Figures S13-S16; Tables S9-S12). Also, both biochars had a statistically significant average % \(R\) of \(p\)-cresol & skatole, ranging from 71-97% & 47-93%, respectively (Figures S14 and S15). HAP biochar was consistently mitigating indole (average % \(R\) ranging from 73-93% (Figure S16 & Table S12).  
[PERSON] et al., reported that VOC sorption was mainly governed by surface area and non-carbonized organic matter; however, none of the VOCs from this experiment were tested [55]. In addition, [PERSON] et al., studied that surface area and pore volume were the main contributors in the sorption of gaseous phenol and toluene by activated char [56]. Moreover, according to [PERSON] et al., phenol adsorption decreased with increasing pH of an aqueous solution [57]. In the case of Trial 3, HAP biochar with larger pores showed a higher % \(R\) of VOC, especially phenol.  
The abundance of volatile fatty acids was significantly lower and that could be a reason for a low % \(R\) for all three trials for the isobutyric acid. For isobutyric acid, both types of biochar did not statistically reduce the emission from deep-pit manure (Trials 1 & 2, Table S13). However, RO biochar reduced the isobutyric acid emission by 26.2% (\(p=0.0002\)), and HAP biochar reduced \(\sim\)47% (\(p=0.003\)) in Trial 3 (Table S13), which treated the manure from outdoor storage instead of a deep pit barn.  
Finally, RO biochar showed \(\sim\)22% reduction (\(p=0.008\)) in the average odor concentration of the three trials throughout the 1-month testing period (Table S14). HAP biochar showed an average reduction of 17.5% (\(p=0.019\)) in the first week, decreased to 15% (\(p=0.019\)) reduction in the second week, and showed generation (\(p>0.05\)) of odor concentration in the last 2 weeks (Table S14).  
The research is needed to develop and test practical technologies for the mitigation of gaseous emissions from stored swine manure. Clearly, the need is still there as the performance data for marketed products is nearly non-existent [4; 5]. In a recent publication, [PERSON] et al., (2020a) reported on a comprehensive study to evaluate gas emissions data from swine manure treated with different commercial additive products [58].  
Most recently, [PERSON] et al., (2020b) [59] reported on the use of HAP and RO biochar as being very effective in mitigating acute releases of H\({}_{2}\)S from agitated manure on a pilot-scale. Up to \(\sim\)80% reduction of short-term H\({}_{2}\)S emissions were reported, showing a novel biochar application pathway for potential practical application immediately prior to manure pump-out and seasonal application to the land. However, more testing and scaling up research for both long- & short-term biochar application to manure should be planned, and different types of biochar and effects on targeted gases should be examined.  
The agronomic effects of the proposed biochar treatment of manure were recently reported by [PERSON] et al. (2020)[PERSON] et al. (2020). There is a potential for improving the sustainability of the swine and crop production systems. First, biochar can be used to mitigate emissions from manure while adsorbing nutrients from manure and, at the same time, preventing losses to the atmosphere. [PERSON] et al., (2020)[PERSON] et al. (2020) reported that using biochar and manure mixture significantly increased the organic matter, total carbon (C), and total N of the soil. Therefore, we propose to use biochar treatment to improve air quality inside barns, improve worker and animal safety during manure agitation (e.g., [PERSON] et al., (2020)[PERSON] et al., (2020), and to valorize biochar-treated manure. The overall effect would improve N &C cycling in the swine-crop production systems ([PERSON] et al., 2020)[PERSON] et al. (2020).