E.N. Peleka

Iron-modified hydrotalcite-like materials as highly efficient phosphate sorbents

Highly efficient sorbents for phosphate removal from aqueous solutions based on the calcined forms of Fe(III)-substituted Layered Double Hydroxides (LDH) materials have been developed in this study. Hydrotalcite-like materials with Mg/M(3+) approximately 3 (where M=Al(3+), Fe(3+) or combined) have been synthesized following simple co-precipitation method and were subsequently calcined in air at 450 degrees C. Both as-synthesized and calcined materials were characterized by means of X-ray Diffraction (XRD), Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES), elemental (C) analysis, N(2) porosimetry, Scanning Electron Microscopy (SEM). All the materials were evaluated for the sorption of phosphates by batch equilibrium sorption experiments and kinetic measurements (effect of contact time). It was shown that chlorides or nitrates, being the charge-balancing anions in the LDH structure, are more easily exchanged by phosphates compared to carbonates. In the Fe(III)-modified LDHs, an increase of the Fe loading led to the decrease of the sorption efficiency. The maximum uptake of phosphates for both the Mg-Al LDH and Mg-Fe LDH samples containing mainly carbonates as charge-balancing anions was relatively low (ca.<or= 25mgP/g sorbent) while it was higher for the LDH samples containing mainly chlorides (approximately 80mgP/g). On the other hand, the maximum sorption capacity for the calcined Mg-Al LDHs and the calcined Fe(III)-substituted sorbents were very high, ca. approximately 250 and approximately 350mgP/g, respectively. The sorption data of both the as-synthesized and calcined LDHs was best fitted by the Freundlich model. Both the Mg-Al and Fe-substituted LDH sorbents were regenerated with mixed aqueous solution of NaCl and NaOH and were reused with a small loss of removal efficiency.

Modeling the sorption of metal ions from aqueous solution by iron-based adsorbents

The possibility of using iron-based adsorbents (i.e. akaganéite or goethite) to remove heavy metal ions from aqueous solutions was the aim of the present review paper. Synthesized material was used in two forms, i.e. in fine powder of nanocrystals and in the form of grains (as granular). The main examined parameters were the quantity of sorbent, the presence of ionic strength, the pH value of solution and the metals speciation, including the presence of complexing agents. The removal efficiency of the packed-bed column was examined and compared. Typical adsorption models were discussed and the bed depth-service time equation has been applied to the sorption results in order to model the column operation.

Removal of Arsenic and Cadmium by Akaganeite Fixed‐Beds

The possibility of using a packed‐bed (in column configuration) of akaganeite to remove oxyanions like As(V) and cations like Cd from aqueous solutions was the aim of the present study. Synthesized akaganeite was used in two forms, i.e., in fine powder (of nanocrystals) and in the form of grains (as granular). The main examined parameters were: 1) the quantity of sorbent in the column and 2) the presence (or not) of ionic strength. The bed depth‐service time (BDST) model was applied to the sorption results to model the column operation. The removal efficiency of As(V) anions by the sorptive column was found to be higher than that of Cd cations. The presence of increased ionic strength caused a serious increase of removal efficiency for the case of arsenic anions. Granular akaganeite was less effective in all cases, but still, it remained a good adsorbent.

Alternative Flotation Techniques for Wastewater Treatment: Focus on Electroflotation

During the last decades (dissolved-air) flotation has found several applications in water and wastewater treatment. Flocculation is generally required in advance for a satisfactory separation and a membrane process is often applied downstream. Examples from the literature given in the present review include heavy metals, textile dyes, food, paper industry, oily effluents, laundry wastewaters, sludge etc. and are accompanied by typical laboratory results. Among the used techniques, the process known as electroflotation (or electrolytic flotation), which often incorporates electrocoagulation, was particularly focused in this paper due to certain advantages it offers; this process is certainly suitable for small-scale applications.

Effect of Cationic Surfactant on the Adsorption of Arsenites onto Akaganeite Nanocrystals

Abstract The current research focuses on removal of arsenite ions from aqueous solutions by a new adsorbent, surfactant modified akaganeite (Akm), prepared after the adsorption of the cationic surfactant, hexadecyl trimethyl ammonium bromide (N‐Cetyl‐N,N,N‐Trimethylammonium Bromide) onto akaganeite. The new adsorbent was investigated with Fourier transform infrared spectra and X‐ray photoelectron spectroscopy methods for a better understanding of the effects of surface properties on arsenite adsorption. Surfactant modified akaganeite was found to be an effective adsorbent for the removal of arsenite ions from aqueous systems. It presented a significantly higher arsenite adsorption capacity than the pure nanocrystalline akaganeite. Kinetics of adsorption obeys a second‐order rate equation. The maximum adsorption capacity was found to 328.3 mg g−1 over a wide pH range significantly higher than those of other adsorbents reported.

Application of Flotation for the Separation of Metal‐Loaded Resins#

Abstract The separation of two typical Lewatit‐type ion exchange resins (TP208 and TP260) was studied by the application of flotation, following their preliminary use for ion exchange of zinc, a common heavy metal found in many wastewater streams. The major examined parameters were the specific type of resin and the respective ion exchange capacity, the required resin concentration for the efficient removal of metal, the influence of wetting on both processes (i.e., ion exchange and flotation), the effect of pH value on the removal of metal and on the regeneration of resin, the type and concentration of used surfactant (primary amine or quaternary ammonium were examined), as well as the effect of frother (ethanol) on flotation efficiency. The regeneration of resins was studied, with respect to the desorption of metal and to the elution of surfactant. The efficiency of combined process, which includes the removal of Zn(II) by ion exchange and the application of flotation for the subsequent solid/liquid separation of metal‐loaded resin, showed that almost 100% of zinc was removed and more than 95% of used resin was recovered by flotation in a single stage. The operation was also examined during multiple cycles, i.e., following regeneration of resin; the high efficiency of both processes remained rather constant. Many thanks are due to the European Union – Energy, Environment and Sustainable Development (5th Framework programme) for funding the METASEP research project (for more information see reference 28) with contract no EVK1‐CT‐2000‐00083 and to the coordinator Dr. V. Mavrov (Inst. Envir. Compatible Process Tech., Univ. Saarland). Also, to Ms. S. Pataroudi, chemist, for experimental collaboration.

Fouling control in a lab-scale MBR system: Comparison of several commercially applied coagulants.

The Membrane bioreactors (MBRs) integrate the biological degradation of pollutants with membrane filtration-separation during wastewater treatment. Membrane fouling, which is considered as the main process drawback, stems from the interaction between the membrane material and the (organic or inorganic) foulants, leading to membranes efficiency deterioration. It is widely recognized that the mixed liquor colloidal and Soluble Microbial Products (SMP) are in principal responsible for this undesirable situation. As a result, the appropriate pretreatment of wastewater feed is often considered as necessary procedure and the coagulation/flocculation (C/F) process is regarded as a relevant viable option for wastewater treatment by MBRs in order to improve the effective removal of suspended solids (SS), of colloidal particles, of natural organic matter (NOM), as well as of other soluble materials. The objective of this study is the application of coagulation/flocculation for fouling control of MBR systems by using several commercially available chemical coagulant/flocculant agents. For this purpose, an appropriate lab-scale continuous-flow, fully automatic MBR system has been assembled and various (inorganic) coagulants (i.e. FeCl3∙6H2O, Fe2(SO4)3·5H2O, FeClSO4, PFS0.3, PAC A9-M, PAC-A16, Al2(SO4)3·18H2O, FO4350SSH, NaAlO2) have been examined. Filterability tests and SMP concentration measurements were also conducted in order to investigate the reversible, as well as the irreversible fouling, respectively. Based upon the obtained results and after selecting the most efficient coagulants (FeCl3·6H2O, Fe2(SO4)3·5H2O, FeClSO4, PAC-A9, PAC-A16), an attempt was subsequently performed to correlate the major fouling indices (i.e. TMP, TTF, SMP concentration) in order to improve the overall process operability by this fouling control method.

Bioremoval of Metal Ion and Water Treatment in a Hybrid Unit

Abstract Several biomass types, such as yeast (in the present Saccharomyces, a brewery waste), have been reported to remove heavy metals (i.e., zinc) from aqueous solution. The separation of metal-loaded biomass and hence, the production of a clean water stream using a hybrid flotation-microfiltration unit were investigated. The hybrid cell consisted of a microfiltration module submerged directly into a flotation cell. Air bubbling, constituting the transport medium during flotation, meanwhile has been used in order to limit the membranes fouling. The effects of air sparging, the solid particle content, and the type and concentration of flotation reagents on the performance of the hybrid process were the main examined parameters.

A New Hybrid Flotation-Microfiltration Cell

Abstract The investigated hybrid cell combines the advantages of both flotation and membrane separation, while overcoming their limitations and having as an outcome clean water from a industrial wastewater. Hence, metals recovery from dilute aqueous solutions was a promising application of this innovative process, further to solid/liquid separation. The specific objective was to apply the process for the efficient separation of effluents containing metals (here, zinc). The main examined parameters were the following: the metal initial concentration, flotation surfactant applied, and air flowrate. The successful contribution of precipitate flotation was highlighted, while the observed metal removals were of the order of ∼100%.

A hybrid flotation: microfiltration cell for effluent treatment

The investigated hybrid cell combines the advantages of both flotation and membrane filtration, while overcoming their limitations and having as outcome clean water from wastewater. The specific objective of the current paper was to apply the process for the efficient separation of effluents containing toxic metals (zinc cations and chromate anions). The main examined parameters were the sorbent material and flotation reagents concentration, the bubbles size and the backflushing required to reduce the fouling of membranes and to extend their useful operation time. Attention was also paid to the economics of this integrated process.