Arsenic binding to dissolved humic acids in aqueous solution
Arsenic is a toxic metalloid which has gained growing attention during the last decades due to its potential to accumulate in the environment. Especially groundwaters with high arsenic concentrations are of concern, causing serious health problems in many countries of the world. One of the possible important factors influencing As mobility in groundwaters is the ubiquitously present natural organic matter.
The aim of this diploma thesis was the investigation of the binding behaviour of As(lll) and As(V) to humic acids dissolved in aqueous solution. The study was conducted by determining conditional distribution coefficients (Dom) for arsenate and arsenite under oxic laboratory conditions. For the determination of Dom values, dialysis experiments with three commercially available purified humic acids were conducted for varying pH values (4.6 - 8.4) and As concentrations (13 nmolL-1 - 12 μmoll-1). The DOC concentrations of the humic acids were varied between 30 and 100 mgL-1.
The results showed that As binds to dissolved humic acids. The Dom values of As(V) were quite high (≤ 18'000 Lkg-1) and up to 60% of total As(V) was bound to humic acids after equilibration under the used experimental conditions. The binding of As(lll) to humic acids seemed to be low and negligible. The Dom values obtained for As(lll) were ≤ 4'000 Lkg-1 and between 10% to 20% of total As(lll) was bound to humic acids after equilibration.
The Dom values for As(ll I) and As(V) increased with pH and decreased with [As]0/[DOC] ratio for intermediate to higher As concentrations of 67 nmolL-1 to 12 μmolL-1, implying a few strong and many weak binding sites. A proposed mechanism of the binding of As to humic acids is complexation by ligand exchange at the As centre. No evidence could be found for a complexation mechanism involving Al(lll) metal bridging, nor for a competition between As(lll) and As(V). Some competition was detected for phosphate and As(V). Experiments with concentrated NOM extracted from river water were also conducted. No increased binding of As to NOM was found compared to the experiments with commercially available humic acids.
This study gave many valuable hints for further investigations and the main factors involved in arsenic binding to humic acids. It is postulated that an evaluation of arsenic mobility in groundwaters has to consider the binding of arsenic to organic matter.