Phosphonate antiscalants are commonly used in nanofiltration and reverse osmosis water treatment to prevent membrane fouling by mineral scale. In many circumstances it is desirable to remove these phosphonate compounds before concentrate disposal or further treatment. This research investigated the removal of phosphonate compounds from simulated membrane concentrate solutions using ferric hydroxide adsorbents. Two phosphonate antiscalants were investigated, Permatreat 191® (PT191) and nitrilotrimethylphosphonic acid (NTMP). Batch adsorption isotherms and column breakthrough and regeneration experiments were performed on two commercial adsorbents and a ferric hydroxide loaded polyacrylonitrile fiber adsorbent prepared in our laboratory. The best performing adsorbent was Granular Ferric Hydroxide® (GFH) obtained from GEH Wasserchemie. Adsorption isotherms measured after 24-hour equilibration periods showed initial concentration effects, whereby the isotherms were dependent on the initial adsorbate concentration in solution. Significant differences in adsorption behavior were observed between the PT191 and the NTMP adsorbates. Differences in adsorption behavior between NTMP and PT191 are all consistent with the PT191 containing fewer phosphonate functional groups per molecule than NTMP. Desorption rates were bimodal, with 40-50% of the adsorbed phosphonate being released on a time scale of 10-24 hours, while the remaining fraction was released approximately one order of magnitude more slowly. The slow desorbing fraction primarily resulted from equilibrium effects resulting from significant phosphonate adsorption, even in 1.0 mol/L NaOH solutions. Complete regeneration could not be achieved, even after eluting the adsorbent columns with more than 300 bed volumes of 1.0 mol/L NaOH. However, the incomplete regeneration had only a minor effect on phosphonate uptake in subsequent column breakthrough experiments.

Resource recovery has become essential to compensate for costly and complex technical requirements to implement zero-liquid discharge (ZLD) policies. Antiscalant removal, especially phosphonate-based antiscalants, is a clear example of resource and potential subsequent valorization. The global market for phosphonate-based antiscalants is expected to grow in the coming years along with increasing membrane desalination projects. It implies the disposal in surface waters of tons of phosphonates and trace elements, present in wastewater and concentrates, every day. Removing the phosphonates and trace elements allows their subsequent recovery and valorization, minimizing squeezing produc-tion/extraction procedures and saving the environment from suffering any im-pact because of them. The first part of this thesis focuses on phosphonate removal with iron and aluminum-based adsorbents. Porous iron and aluminum (oxi)hydroxides can remove phosphonates from concentrates completely. The main limitation of this process is the diffusion of the phosphonates through adsorbent particles. As proved in this thesis, temperature significantly improves the adsorption kinetics of the phosphonates on both adsorbents as a result of the variation of the diffusion coefficient. The presence of calcium also plays an important role, since accelerates the adsorption at the first stages of the process, but limits and saturates the capacity of the adsorbent surface for further adsorption. More research on the role of calcium is needed in this regard to better understand how the adsorption/diffusion of the phosphonates is affected by this common element present in concentrates. Electrocoagulation was studied in the second part of this thesis as a potential approach for phosphonate removal. Using pure iron electrodes, the applied current density can be easily optimized. As a result, dissolved phosphonates are quickly removed from a large concentrate volume at a relatively low cost and minimal sludge production. The benefits of this technique lie in the possibility of producing the substrate (adsorbent) in situ for the phosphonate to be adsorbed. Furthermore, the time required to completely remove the phosphonates is remarkably shorter compared to adsorption, which, as pointed out above, is limited by diffusion phenomena. Alkaline washing was relatively successful at recovering the phosphorus from the sludge, depending on the dissolved phosphonate in the concentrate. Although experimental results may look promising, further research on finding the optimum working conditions has to be addressed. The process is open to improvement in terms of new electrode materials, reactor design, phosphorus recovery, or optimal working temperature. In the third part of the thesis, the adsorption potential of previously tested adsorbents for the removal of elements at trace levels. The iron-based adsorbent, commercialized for phosphate and arsenic removal, turned out to be excellent at removing transition metals (TM) and rare earth elements (REE). The study was carried out in parallel with the exploration of the capabilities of a high-resolution inductively coupled plasma mass spectrometry (HR ICP-MS) instrument. The potential of this analytical procedure allows the detection and quantification of all the isotopes at the ultra-trace level (in the range of a few ng·L-1) and in only one measurement round. Furthermore, interferences from polyatomic species, formed during the ionization in the plasma, are easily resolved due to the high-resolution mode. As a result, the detection of the targeted element is easily discriminated from the potential interferences. This feature makes a remarkable difference regarding ordinary ICP-MS, which requires different analytical procedures to properly resolve overlapping signals. This analytical procedure opens new possibilities to test the adsorbents in new conditions and develop analytical methods for water speciation.