# Mitigation of Gaseous Emissions from Swine Manure with the Surficial Application of Biochars
## 4 Discussion  
### Effect of Biochar Treatment on Ammonia and Hydrogen Sulfide  
Biochar addition had a mitigating effect on NH\({}_{3}\) emissions. However, the effect was temporal, opening up the possibility of biochar reapplication in subsequent research. On the first day of trial 1, HAP and RO biochars showed as much as 40% (\(p<0.0001\)) and 52% (\(p<0.0001\)) reduction of NH\({}_{3}\), respectively. After that, the reduction rate started to decrease over time (Figure S8, Table S4). The last statistically significant reduction by comparing the same day measurements between the control and treatment was on day 15, where it was 17% (\(p=0.04\)) and 27% (\(p<0.0001\)) for HAP and RO biochar, respectively (Figure S8). During trial 2, HAP and RO started with 34% (\(p<0.0001\)) and 33% (\(p<0.0001\)) reduction of NH\({}_{3}\) emissions on day 1 and then gradually decreased to 15% (\(p=0.01\)) and 12% (\(p=0.04\)) on day 26. In trial 3, HAP biochar reduced 41.8% (\(p<0.0001\)) of NH\({}_{3}\) emission. However, from day 5, it showed no NH\({}_{3}\) reduction; in fact, it increased NH\({}_{3}\) emissions on some days. RO biochar started with 70.4% of NH\({}_{3}\) reduction (\(p<0.0001\)) on day 1, and then the reduction rate gradually decreased to 22.2% on day 29 (\(p<0.0001\)). Even though manure pH changed for all treatments, there were no significant differences in pH between the control and biochar-treated manures after each trial, so it cannot be stated that biochar inhibited NH\({}_{4}\)\({}^{+}\) from transformation to NH\({}_{3}\) during the entire Trial. ZPC of biochar could be a parameter that was responsible for NH\({}_{3}\) reduction. Because ZPC of biochars was below biochars pH,their surface was mostly negatively charged, which was favorable to attract NH4\({}^{+}\) cations and keep them on the biochar surface. During all three trials, manure pH changed from 7.47-8 to 8.75-9.07, and this increase in pH forced more NH4\({}^{+}\) ions into gaseous NH3 form. Thus, NH3 emissions reduction gradually decreased from day 1 to day 29. Interestingly, [PERSON] et al. reported that the oak biochar has the NH3 sorption capacity of 4-8 mg-g-1 of biochar [47]. Assuming this sorption capacity, the RO biochar would be fully saturated by day 7-14. Clearly, the RO biochar in this research was effective longer, e.g., still showing a significant reduction on day 29 in trial 3. Thus, the most probable explanation is that biochar created a semi-porous physical barrier that slowed mass transfer from manure to headspace. Emissions of H2S were more challenging to mitigate with biochars, with some interesting temporal effects (Figure S9, Table S5). Trial 1 showed no statistically significant reduction of H2S due to the effect of RO. Similarly, HAP treatment was not significant, even though it had 100% (\(p\) = 0.07) of reduction starting on day 19 until the end of the Trial. In trial 2, HAP biochar significantly reduced emissions by 24% (\(p\) = 0.001) and 39% (\(p\) < 0.0001) on day 1 and 8, respectively. RO biochar was reducing H2S emission for the first \(\sim\)12 days, where the highest reduction was 49% (\(p\) < 0.0001) on day 5. During trial 3, HAP biochar showed a significant reduction of 25.4% (\(p\) < 0.0001) till day 8. RO biochar had a significant reduction of 26.2% (\(p\) < 0.0001) until day 22. However, on day 15, even though RO reduced 43.7% of H2S, it was not statistically significant (\(p\) = 0.6221). [PERSON] et al. claimed that tested biochar had a range of sorption capacity from 1.2-121.4 mg-g-1, and the most conducive pH for H2S sorption was the pH higher than pKa of H2S, which is 7 [48]. [PERSON] et al. also stated that FTIR peaks at 3420 and 1730 cm-1, which correlated to OH stretching and COO- stretching respectively, were crucial for H2S adsorption [48]. [PERSON] et al. studied that the mechanism of H2S sorption is driven by the size of biochar pores [49].