# Analysis of Influencing Characteristics of Biochars for Ammonium Adsorption
## 3 Results and Discussion  
The data collected were organized by feedstock material category and by specific feedstock material, for direct comparison of like biochar materials for ammonium adsorption capacity. The results of this comparison are shown in Figure 1.  
The materials with the highest adsorption capacity in the data collected were wood or wood-waste based or from the agricultural waste categories. Compared to the other materials in the same categories, the capacities were very high. For walnut shell, only one article was referenced; thus, these high adsorption values have not been corroborated by another test and should be carefully considered. For the mixed hardwood, two studies were referenced, but one had a high range of capacities (9.73-54.86 mgNH4\({}_{4}\)/g biochar [20]), and the other was lower (2.8-5.3 mgNH4\({}_{4}\)/g biochar [18]). The processes for both biochar creation and the adsorption testing were very different between the two studies (pyrolysis temperature, pH of solution, particle size of biochar, residence time, etc.) so it is difficult to pinpoint a specific reason for the difference in the adsorption capacities. This is a perfect example of the issues seen in the field of biochar for adsorption--a lack of consistency in testing and measurements makes drawing conclusions tricky if not impossible. In order to narrow down the characteristics reported in literature for analysis, five review papers were consulted, and their conclusions about important influencing factors for ammonium adsorption were recorded in Table 1. \begin{table}
\begin{tabular}{c c c c c} \hline \hline Influencing Factor & [55] & [56] & [57] & [10] & [58] \\ \hline Cation exchange capacity (CEC) & & & & x \\ \hline Surface functional groups & x & x & x & x \\ \hline Ambient temperature & & x & & x \\ \hline \hline \end{tabular}  
* BET—Brunauer, Emmett, and Teller method for determining surface area.  
\end{table} 
Table 1: _Cont._The characteristics chosen for analysis against adsorption capacity were based on the characteristics in Table 1 as available. Surface functional groups and ambient temperature were not characteristics reported in most articles, and therefore it had to be removed from analysis. Dosage of biochar was calculated as g biochar/mL solution, and competing ions were noted as 1 for pure solutions (experiments using only an NH4+ compound like NH4 Cl) and 0 for mixed (including mixed chemical solutions and \"real\" wastewater samples). The influencing factors listed in Table 1 were mentioned in review articles as possible mechanisms important to ammonium adsorption. As many are interrelated, singling out individual characteristics and their impact on the adsorption capacity is complicated. As an example, the pH value of the solution will control the speciation of ammonium and ammonia, thereby affecting the adsorption ability of any material. The pH thus should be kept below a value of 9 to prevent volatilization and loss of ammonia to the atmosphere. However, solutions with extremely low pH have an abundance of H+ ions that increase competition for adsorption sites and thus the adsorption of ammonium may be low. Additionally, surface functional groups can exist as protonated or deprotonated depending on the pH, which affects the availability of binding sites for NH4+[58]. Therefore, there is likely an optimal pH for the solutions containing ammonium that allow for the highest availability of NH4+ ions with low competition from competing ions and most active functional groups. Due to the binary nature of the data for pure versus mixed solution (competing ions) and washed versus unwashed biochar treatment, a Mann-Whitney test was used to determine significance in adsorption capacity. This test was chosen due to the less strict assumptions about the data distribution when compared with a standard \(t\)-test but would still provide information about whether the two groups were statistically different or the same [59].