Assessing the influence of humic acids on the weathering of galena and its environmental implications

Galena weathering often occurs in nature and releases metal ions during the process. Humic acid (HA), a critical particle of natural organic matter, binds metal ions, thus affecting metal transfer and transformation. In this work, an electrochemical method combined with spectroscopic techniques was adopted to investigate the interfacial processes involved in galena weathering under acidic and alkaline conditions, as well as in the presence of HA. The results show that the initial step of galena weathering involved the transformation Pb2+ and regardless of whether the solution was acidic or alkaline. Under acidic conditions, S degrees and Pb2+ further transform into anglesite, and HA adsorbs on the galena surface, inhibiting the transformation of sulfur. HA and Pb (II) ions form bridging complexes. Under alkaline conditions without HA, the sulfur produced undergoes no transformation, whereas Pb2+ will transform into PbO. The presence of HA changes the galena weathering mechanism via ionization effect, and Pb2+ is ultimately converted into anglesite. Higher acidity in acidic conditions or higher alkalinity in alkaline conditions causes galena corrosion when the electrolyte does not contain HA. Conversely, higher pH always accelerates galena corrosion when the electrolyte contains HA, whether the electrolyte is acidic or alkaline. At the same acidity/alkalinity, increasing the concentration of HA inhibits galena weathering. Galena will release 134.7 g m(-2).y(-1) Pb2+ to solution at pH 2.5, and the amount decreases to 28.09 g m(-2).y(-1) in the presence of 1000 mg/L HA. This study provides an in situ electrochemical method for the assessment of galena weathering.