# Analysis of Influencing Characteristics of Biochars for Ammonium Adsorption
## 3 Results and Discussion
In all such cases, these clear groups of linearly arranged points represent data from a single study, which often identifies a strong relationship between adsorption capacity and some other factor. As an illustrating example, Table 3 shows the data from the studies corresponding to the trend enumerated in points 2 and 4 above. As CEC was not reported in any of the three articles, the data were not included in the table. We can see that a particular set of experiments can be visible as a straight line in the biplot indicating a prevailing adsorption trend. However, tabulating the experiments enumerated in item 4 above shows the difficulty in comparing results between studies. Looking at BET surface area, for example, the first five entries in Table 3 differ only in initial ammonium concentration, but the adsorption capacity varies from 9.14 mgNH4\({}^{+}\)/g biochar to 37.63 mgNH4\({}^{+}\)/g biochar, corresponding to a difference of 19% of the maximum adsorption capacity. The study by [PERSON] et al. [44] used the same material, but with an  
Figure 5: PCA biplot for all data by biochar material. increase of 100 m\({}^{2}\)/g in the BET surface area, the adsorption capacity only increased by 1 mgNH\({}_{4}\)+/g biochar. In addition, the last point by [PERSON] et al. [20] had similar experimental conditions and similar BET surface area but much higher adsorption capacity. The final study by [PERSON] and [PERSON] [47] showed increases in adsorption capacity in the four points which correspond to different chemical treatments of the biochars, with the first point being the pure biochar without treatment. The different treatments increased the surface area, but the first treatment led to a decrease in adsorption capacity due to changes in the surface morphology (likely from a condensation reaction between walnut shell components and dicyandiamide). This example demonstrates that surface area alone is often not enough of an indication of surface properties to draw direct conclusions about ammonium adsorption. Comparing the data from group q, it is unclear whether the large difference in capacity is due to pyrolysis temperature, contact time, biochar dose, or some other parameter related to the method that was not recorded. The results of the PCA show that it is possible to identify extremely strong trends in literature at the level of individual studies, but that almost all of these trends become much weaker when considering the literature as a whole. Using data from a range of studies results in support for a relationship between adsorption capacity and BET surface area, CEC, ammonium concentration, and perhaps pyrolysis temperature. However, the weakness of these relationships when compared to individual studies reflects the difficulty in drawing conclusions on critical influencing factors for ammonium adsorption from a combination of studies. Prior to this study, most research articles that comment on possible adsorption mechanisms for ammonium onto biochar attempt to draw conclusions without the support of statistical analysis. It is clear from this work that there are significant challenges to performing statistical analysis due mostly to a lack of consistency in reporting different physical and chemical characteristics of the biochar. It is hypothesized that what is currently reported is based on the availability of equipment rather than the variables that are definitively contributing to the adsorption. It is difficult to make suggestions on what variables are most important and therefore which should be focused on for reporting since the missing data makes analysis complex. However, from the preliminary findings here, we may suggest that solution purity, biochar washing, biochar surface area and cation exchange capacity are important factors for adsorption capacity for some materials. Material type should be considered in the analysis of biochar for adsorption as it seems to alter the relationship of the different characteristics with adsorption capacity, though it is difficult to determine if