Sorption of humic acid on zeolitic tuff: a preliminary investigation

Abstract 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 + -, NH 4 + -, Ca 2+ -, Mg 2+ -, or Ba 2+ -enriched samples. Except for the Li-enriched sample, Langmuir sorption curves fitted the experimental data, allowing the calculation of the maximum HA sorbable amount, Q max , and the affinity constant between the sorbate and the sorbent, k . The Q max 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- Q max 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 Q max 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 Q max 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.