KU Leuven investigate the possibility of integrating phosphorus adsorbing materials in a circular process
Through extensive experimental work on a laboratory scale and through real field cases, the NuReDrain project partners have demonstrated that various iron and aluminium based adsorption materials are able to effectively treat phosphorous containing drainage water from tile drained fields and greenhouse effluent.
With the knowledge gained within the NuReDrain project, it is possible to reduce the phosphorus concentration below the standard limit for surface water at relatively high flow rates. The question can of course be asked what should happen when the filter material becomes saturated with phosphorus.
Alkaline desorption process
Within the NuReDrain project different options are being examined, of which the regeneration of the saturated sorbent and recovery of the adsorbed phosphorus, is one of them. Researchers at KU Leuven have developed a method to remove a large part of the adsorbed phosphorus from the adsorption material by means of an alkaline desorption process. The main objective here is not to damage the adsorption material so that it can retain its adsorption capacity as much as possible. In this manner, it is possible to integrate the P-adsorbing material in a circular process.
Aqua regia and ICP analysis
Continuous filter desorption experiments were carried out on Iron Coated Sand (ICS) to determine the most important operating conditions for a successful desorption process. The saturated ICS granules originated from a full-scale filter system set up at a florist. The composition of 1 g of saturated ICS granules was determined by a complete destruction of the granules by Aqua Regia and ICP analysis.
About 1.5% of the granules consists of phosphorus and almost 60% is iron. The influence of the NaOH concentration (0.1M, 0.5M and 1M) and the desorption time on the release of phosphorus is shown in Figure 1.
By increasing the contact time between the desorption solution and the saturated sorbent, more phosphorus was released from the sorbents. Figure 1 shows that only a concentration of 0.5 and 1M NaOH result in the desired desorption of phosphorus from the ICS granule.
At least 24 hours desorption time is necessary. During the first hour of the continuous desorption experiment only 0.4 mg P/g DS and 0.9 mg P/g DS can be leached by a NaOH concentration of 0.5 and 1M, respectively.
A concentration of 0.1M NaOH desorbed almost no phosphorus. Knowing the saturation capacity, i.e., 15.30 mg P/g sorbent, approximately 40% of the adsorbed P could be desorbed by a NaOH-solution of 0.5M after 24h.
Figure 1: (a) Continuous filter desorption experiment and the effect of the NaOH concentration on desorption capacity; (b) The progress of the desorption during the first hour of the continuous filter desorption experiment.
Analysis by Scanning Electron Microscope
Before using them in the desorption experiments, some of the saturated ICS granuleswere also analyzed by a Scanning Electron Microscope (SEM) equipped with EDX (Energy Dispersive X-Ray) Analyzer.
First, the granules were polished so that the inner structure of the granule is visible(Figure 2(a)). The EDX analysis allows us to visualize the locations at the granule cross section where the phosphorus is adsorbed, as can be seen in Figure 2(b).
The microscopic images of the granule structure and the analysis on the EDX give a better view on the mechanism of the phosphorusadsorption. From Figure 2(b) and (c) it can be clearly established that the phosphorus is accumulated at the sand core of the granule. This suggest that the phosphorus migrates towards the core of the granule when a sufficiently long contact time is maintained in the adsorption filter.
