Sorption of humic acid on zeolitic tuff: a preliminary investigation

This work involves a study of the interaction of humic acid (HA) with zeolitic (phillipsite+chabazite) Neapolitan yellow tuff (NYT). HA sorption isotherms were carried out on untreated as well as on Li+-, Na+-, K+-, NH4+-, Ca2+-, Mg2+-, or Ba2+-enriched samples. Except for the Li-enriched sample, Langmuir sorption curves fitted the experimental data, allowing the calculation of the maximum HA sorbable amount, Qmax, and the affinity constant between the sorbate and the sorbent, k. The Qmax for the untreated sample was 8512±265 mg kg−1, with k=10.8±1.4 l kg−1. The experimental amounts of HA taken up by the Li-enriched sample were very small (183 to 2322 mg kg−1) and too scattered to be consistently fitted by any significant equation. For other cation enriched-samples, the HA-Qmax amounts ranged from 6583±205 mg kg−1 for the K-treated tuff to 22 029±1065 mg kg−1 for the Mg-treated tuff, whereas the k values ranged from 8.0±1.7 l kg−1 for the Na-treated tuff to 81.4±17.5 l kg−1 for the Ba-treated tuff. The tuff samples enriched by divalent cations took up HA in amounts larger than those observed for the untreated sample; the opposite was true for the monovalent cation-enriched samples. The HA sorption curves on monovalent cation-enriched tuff were scarcely different, within experimental variance, from those observed for the untreated sample. On the contrary, both the Qmax and the k values calculated for the HA sorption on the divalent cation-enriched samples were widely and significantly larger than those of the untreated sample, with the highest Qmax for Mg- and Ca-enriched sample, and the highest k value for the Ba-enriched one. The results demonstrate that enrichment by divalent cations enhances the ability of Neapolitan yellow tuff to take up humic acid, whereas enrichment by monovalent cations reduces it; the results also show that humic acid sorption on tuff must be regarded as a complex phenomenon, occurring as a compromise between the ability of the cations to form stable bridges with the organic matter, and, on the other hand, their specific selective sorption by the tuff.