A re-analysis of NH₄⁺ sorption on biochar: Have expectations been too high?

Sorption of nutrients such as NH₄⁺ is often quoted as a critical property of biochar, explaining its value as a soil amendment and a filter material. However, published values for NH₄⁺ sorption to biochar vary by more than 3 orders of magnitude, without consensus as to the source of this variability. This lack of understanding greatly limits our ability to use quantitative sorption measurements towards product design. Here, our objective was to conduct a quantitative analysis of the sources of variability, and infer which biochar traits are more favourable to high sorption capacity. To do so, we conducted a standardized remodelling exercise of published batch sorption studies using Langmuir sorption isotherm. We excluded studies presenting datasets that either could not be reconciled with the standard Langmuir sorption isotherm or generated clear outliers. Our analysis indicates that the magnitude of sorption capacity of unmodified biochar for NH₄⁺ is lower than previously reported, with a median of 4.2 mg NH₄⁺ g^-1 and a maximum reported sorption capacity of 22.8 mg NH₄⁺ g^-1. Activation resulted in a significant relative improvement in sorption capacity, but absolute improvements remain modest, with a maximum reported sorption of 27.56 mg NH₄⁺ g^-1 for an activated biochar. Methodology appeared to substantially impact sorption estimates, especially practices such as pH control of batch sorption solution and ash removal. Our results highlight some significant challenges in the quantification of NH₄⁺ sorption by biochar and our curated data set provides a potentially valuable scale against which future estimates can be assessed.