Retention Effects of Soil Humic Substances on the Diffusive Transportation of Metal Ions During Sediment Porewater Membrane Dialysis Sampling

In situ sampling techniques such as membrane dialysis are widely applied to the monitoring of heavy metal contamination in water-saturated environments. In this study, batch experiments were conducted to investigate the influence of humic substances (HS) on the diffusive transportation of metal ions across sampling membranes. The presence of HS substantially slowed the transportation of metal ions by 22.2, 20.3, 17.4, 16.6, and 14.9 % for Zn2+, Cu2+, Ni2+, Co2+, and Mn2+, respectively, in the 100-mg/L HS-amended treatments relative to the treatments without HS under acidic conditions (pH=5.0). The retention effect of HS on the metal ions appeared to be relieved at a higher pH of 8.5. However, HS also slowed the transportation of metal ions (Cu2+, Zn2+, Ni2+ and Co2+) into the sampling cells when membranes soaked previously in porewater solution. The retention effects of HS can be attributed to the enhanced adsorption of metal ions on the surfaces of the membranes. Further study in saturated sediments verified that the membrane dialysis technique could underestimate the concentrations of metal ions by 13.3-40.2 % at the presences of HS. These findings suggest that collected porewater data using the membrane dialysis technique should be interpreted with caution with the consideration of in situ geochemical conditions such as HS or pH in water-saturated environments.