Reviewing variables and their implications affecting adsorption of Cr(VI) onto activated carbon: an in-depth statistical case study.

Removal of Cr(VI) from the aqueous phase using numerous activated carbons (AC) has been broadly studied in the last decades. Nevertheless, the diversity of activation methods, AC properties, and adsorption conditions precludes the standardization of specific characteristics required to achieve better adsorption results. This work reviewed the pertinent literature on Cr(VI) adsorption onto AC published over the past four decades. Pearson's correlation matrix and principal component analysis (PCA) assisted in identifying the parameters and AC characteristics that have the greatest influence on the maximum adsorption capacity (q). Two hundred thirty-six adsorption assays were found reporting data on 110 ACs and different parameters. Of these, 39.8% of the studies contemplated the variables q, pH, temperature (T), surface area (S), micropore volume (V), and mesopore volume (V), and only 19.5% reported the point of zero charge (pH). Statistical analysis disclosed that S and V have a strong positive correlation with q, while V, T, and pH show little or no correlation. The difference between pH and pH (PZC) indicated a significant anticorrelation with q, thus evidencing that lower PZC values enhance adsorption. The findings are useful for all researchers that work with Cr(VI) adsorption on AC since they provide a start point concerning the required adsorbent characteristics and process conditions to be employed.