M.Sc Thesis

M.Sc StudentDabush Nitzan
SubjectRemoval of Inorganic Water Contaminates by Iron Oxide
DepartmentDepartment of Chemical Engineering
Supervisor PROFESSOR EMERITUS Raphael Semiat
Full Thesis textFull thesis text - English Version


Iron oxides/hydroxides are established adsorbents for the removal of inorganic contaminants from aqueous solutions. Silica scale is widely encountered problem in many industrial operations such as cooling towers, geothermal applications, saline waters desalination, and produced water reuse. The objective of the research was to develop a hybrid adsorption/filtration process for the removal of silica from aqueous solutions, focusing on adsorption by iron oxy/hydroxide agglomerates (IOAs) and ultrafiltration (UF). Batch experiments were conducted to characterize the adsorption kinetics and mechanisms (adsorption / co-precipitation) in distilled water and simulated brackish waters. The efficiency of the adsorption process was evaluated by the overall removal of the silica, its residual concentration in the treated water and by the cost of chemicals. Continuous fixed bed adsorption and stirred tank reactor experiments were carried out to examine the feasibility of applying continuous IOAs adsorption process.

Batch experiments revealed that silica removal by co-precipitation was found to be more effective than by adsorption to pre-formed IOAs. In both cases, adsorption is the dominant mechanism. Adsorption onto the IOAs was found to be pH-dependent with maximum removal obtained at pH around 9. The adsorption by the IOAs exhibited rapid initial uptake of silica followed by a continuous slower uptake. Silica adsorption onto the IOAs fitted two empirical kinetics models: pseudo second order and Elovich. The equilibrium isotherms of silica removal by IOAs was best fitted by the Freundlich model, which is based on heterogeneous surface adsorption. Co-existing ions did not affect the silica adsorption. These results enable to conduct the experiments with a simple distilled water solution. The cost of chemicals and a complex multi-step regeneration process led to the decision to abandon the regeneration/reuse approach.  The cost of silica removal by the IOAs was estimated at 0.22 $/m3 of brackish water.

IOAs retained by a fixed bed of granular activated carbon was found to be insufficient configuration for removal of silica from aqueous solution due to low IOAs capacity and iron leaching. Continuous stirred tank reactor (CSTR) system exhibited high adsorption capacity yet, up to 12% of iron was released to the product water during the experiment.  Hybridization of CSTR adsorption with separation of the spent IOAs by UF revealed high adsorption capacity as well as excellent separation of the adsorbent with the residual silica (unabsorbed) passing through to the product water. To summarize, combining adsorption and ultrafiltration shows potential to effectively remove silica from brackish water.