Iron oxide incrustations in wells. Part 2: chemical dissolution and modeling

The formation of Fe oxide and Mn incrustations seriously affects the performance of wells, piezometers and drains. Chemical dissolution can be a valuable tool for their removal. Standardized dissolution experiments were performed to study the efficiency of different chemicals on synthetic Fe oxides. They showed that buffered Na-dithionite and oxalic acid are the most effective agents, followed by ascorbic, malonic and sulfamic acid. Citric acid and especially NaOH proved to be ineffective. As expected, a strong dependency of the dissolution rates on the surface area of the individual Fe oxides was observed, goethite being far less soluble than ferrihydrite. In many cases linear, zeroeth order rate laws were sufficient to model the measured dissolution curves. In other cases more advanced model approaches had to be applied. Sometimes different models were equally well suited to describe a dissolution process.