Lignitic Humic Acids as Environmentally-Friendly Adsorbent for Heavy Metals

Abstract

Humic acids are a part of humus material, are abundant in nature, and form a substantial pool of natural organic matter. They participate in the transport of both beneficial and harmful species. Due to their structure and properties, they can interact with metal ions and, with them, form relatively stable complexes. These substances are thus responsible for the so-called self-cleaning ability of soils. Lignite as a young coal type contains a relatively high amount of humic acids which can be used as an environmentally-friendly adsorbent for heavy metals. In this work, we compared the adsorption of single Cu2+ ions with the simultaneous adsorption of several different metal ions (Cd2+, Cu2+, Pb2+, and Zn2+). The adsorption efficiency of humic acids was very high, almost 100% in the case of the single adsorption of Cu2+ ions and more than 90% for the adsorption from the mixture of metal ions. The stability of formed complexes, considered on the basis of the leaching in different extraction agents, was higher than 80%; only 9-18% was in the mobile phase. After adsorption, metal ions are contained in humic acids after adsorption in mainly strongly bonded form (60-73%) and can be liberated from their structure only in low amounts.