M. Fresnedo San Román

Overview of the PCDD/Fs degradation potential and formation risk in the application of advanced oxidation processes (AOPs) to wastewater treatment

Polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) are a family of unintentionally produced persistent organic pollutants (POPs) that have received considerable public and scientific attention due to the toxicity of some of their congeners, more specifically those with chlorine substitution in the 2,3,7,8 positions. The environmental management and control of PCDD/Fs is addressed at a global level through the Stockholm Convention that establishes that POPs should be destroyed or irreversibly transformed in order to reduce or eliminate their release to the environment. Several technologies, including advanced oxidation processes (AOPs) such as photolysis, photocatalysis and Fenton oxidation, have been considered as effective methods for destroying PCDD/Fs in polluted waters. Nevertheless, during the remediation of wastewaters it is critical that the treatment technologies applied do not lead to the formation of by-products that are themselves POPs, especially if PCDD/Fs precursors or chlorine are present in the reaction medium. Despite the high effectiveness of AOPs in the oxidation of major contaminants, scarce references deal with the monitoring of PCDD/Fs in the course of the oxidation process, revealing that a detailed assessment of non-combustion technologies with respect to PCDD/Fs formation is still lacking. This study reports a review of the state of the art related to the potential remediation and/or formation of PCDD/Fs as a result of the application of AOPs for the treatment of polluted waters, warning on the correct selection of the operating conditions.

Selective Separation of Zinc and Iron from Spent Pickling Solutions by Membrane-Based Solvent Extraction: Process Viability

Abstract This work reports the viability of the selective separation of zinc from spent pickling solutions by means of membrane‐based solvent extraction to get a high concentrated zinc solution with a negligible content of iron and other metals that could be used in a electrolytic process to recover zinc in a metallic form. By working with waste effluents as feed solutions that contained these main components, Zn 78 g/L av., Fe 90 g/L av., and HCl 237 g/L av., Tributyl phosphate (TBP) and water were selected as the extractant and back‐extraction agent that allowed maximum zinc separation and recovery. With the kinetic results obtained in a bench‐scale set‐up containing two hollow fiber (HF) modules the selectivity of zinc over iron recovery, αZn/Fe, was calculated. The parameter αZn/Fe depended on the initial metallic concentration in the feed solutions and reached a maximum value of αZn/Fe = 146 (g Zn/g Fe).

Quantitative assessment of the formation of polychlorinated derivatives, PCDD/Fs, in the electrochemical oxidation of 2-chlorophenol as function of the electrolyte type.

The electrochemical degradation of 2-chlorophenol (2-CP) on boron-doped diamond (BDD) anodes was carried out using two electrolytes, NaCl and Na2SO4. Both electrolytes supported complete mineralization of 2-CP, but faster rates of degradation were observed in NaCl. After 4 h of oxidation, the total organic carbon (TOC) balance neared 100% with Na2SO4 for identifiable compounds, whereas 4 mM of TOC remained unidentified with NaCl. Since chlorophenols are known to be precursors of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs), a rigorous assessment of intermediate products was carried out. When near complete mineralization was achieved, the use of NaCl resulted in the concentration of total PCDD/Fs increasing 2.68 × 10(4) times compared to the untreated sample, and to toxicity values several times higher than the maximum level established by U.S. Environmental Protection Agency for water ingestion. When Na2SO4 was used, the increase in total PCDD/Fs concentration was 134 times lower than with NaCl and there was no significant 2,3,7,8-PCDD/Fs formation. Thus, we emphasize the importance of electrolyte selection in electro-oxidation processes, especially when PCDD/Fs precursors are initially present or may be formed in the treated water samples.

Modelling of the Extraction and Back‐Extraction Equilibria of Zinc from Spent Pickling Solutions

Abstract This work reports the study of the chemical equilibria of the extraction and back‐extraction of zinc from spent pickling solutions. Tributyl phosphate was used as a selective extractant and service water as a back extraction agent. Feed solutions had an average composition of 6.4 M Cl−, 1.2 M Zn+2, 1.7 M Fe+2 and traces of other heavy metals i.e., Pb, Ni, Cu, Mn. Under the experimental conditions the ion ZnCl4 2− was the predominant zinc species in feed. A careful experimental design was performed obtaining the equilibria mathematical models and parameters, KEX=0.137 (mol/L)−4, and KBEX=1.46 [−].

Optimal synthesis of an emulsion pertraction process for the removal of pollutant anions in industrial wastewater systems

Abstract This work reports a synthesis model for the optimal design of an emulsion pertraction process for the removal and recovery of pollutant anions from industrial wastewaters. The goal is to define the minimum membrane area needed in order to obtain an environmentally acceptable stream with minimum concentration of the pollutant and at the same time a concentrated solution for further processing. A superstructure is proposed which consists of a prespecified number of modules that are interconnected in all possible ways in order to account for all potential configurations. The selection of the optimal design from this superstructure is formulated as a non-linear programming (NLP) problem that is solved with CONOPT2 from GAMS. In order to reduce the membrane area, alternative configurations that provide flexibility to the emulsion phase inlet and higher residence time values were analyzed obtaining 56% reduction of the membrane area.

Assessment of PCDD/Fs formation in the Fenton oxidation of 2-chlorophenol: Influence of the iron dose applied

Toxic polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) may be formed during remediation of chlorinated phenols via Fenton oxidation. To highlight the need for monitoring the production of toxic byproducts in these reactions, this work assessed the influence of iron dose (0.09-0.36 mM) on the Fenton oxidation of 2-chlorophenol (2-CP, 15.56 mM), a potential precursor of PCDD/Fs, by quantifying 2-CP removal and mineralization rates as well as byproducts yields, including PCDD/Fs. Although the increase in the iron dose showed positive contribution to 2-CP oxidation, under the operating conditions of the current study (H2O2 at 20% of the stoichiometric dose and 20 °C), there was no effect on the mineralization rate, and TOC and chlorine balances were far to be closed, depicting the presence of chlorinated organic byproducts in the reaction medium. After 4 h of treatment, the total PCDD/Fs concentrations increased by 14.5-39 times related to the untreated sample when the iron doses tested decreased from 0.36 to 0.09 mM, with preferential formation of PCDFs over PCDDs and dominance of lower chlorinated congeners such as tetra and penta-PCDD/Fs. The treatment with the highest iron dose (0.36 mM) exhibited the lowest PCDD/Fs yields and was thus most successful at mitigating toxic byproducts of the Fenton oxidation, leading to lower sample toxic equivalence (TEQ) value.

Minimum membrane area of an emulsion pertraction process for Cr(VI) removal and recovery

The design of an emulsion pertraction membrane (EPM) process for the treatment of wastewater containing hexavalent chromium and its recovery for industrial reuse is presented. EPM is a new technology that combines the efficiency of emulsion liquid membranes with the advantages of using hollow fibber membrane modules. It can selectively remove and concentrate metals from diluted process streams. The total membrane required is minimised, complying with effluent disposal regulations and concentrations levels for Cr(VI) reuse. The algebraic and differential equation system modelling the mass transfer along the membrane modules is presented. The differential equations are discretised using orthogonal collocation on finite elements to solve a non-linear programming problem. An improved process configuration recycling the treated wastewater and eliminating fresh water consumption is presented. The sensitivity of the objective function to emulsion tank volumes and aqueous to organic phases ratio in the emulsion are quantified suggesting further improvements for the industrial application of this cleaner technology.

The critical role of the operating conditions on the Fenton oxidation of 2-chlorophenol: assessment of PCDD/Fs formation.

This work assesses the influence of the operating conditions H2O2 dose (20 or 100% of the stoichiometric amount), temperature (20 or 70°C), and the presence of chloride in the oxidation medium in the formation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) during Fenton treatment of aqueous samples of 2-chlorophenol, 2-CP, one of the strongest precursor of PCDD/Fs. After 4h of oxidation in the experiments carried out with 20% H2O2 chlorinated phenoxyphenols and biphenyls, which are intermediates in PCDD/Fs formation, as well as PCDD/Fs were observed, resulting in concentrations 11 times higher than in the untreated sample. Additionally, when NaCl was also present in the reaction medium, PCDD/Fs were formed at higher extent, with a total concentration 74.4 times higher than in the untreated 2-CP solution. Results depicted a preferential formation of PCDFs over PCDDs, with dominance of lower chlorinated PCDD/Fs (tetra and penta-PCDD/Fs). Besides, the formation of the most toxic PCDD/Fs congeners (2,3,7,8-PCDD/Fs) was not favored under the operating conditions used in this work.

Analysis of the back-extraction of cadmium–nickel–D2EHPA organic phases

In this work the back-extraction reaction and equilibrium parameter of organic phases containing Cd, Ni, and di(2-ethylhexyl)phosphoric acid (D2EHPA) are presented. Using sulphuric acid solution as stripping agent, this study is part of the analysis of the integral separation process of Cd and Ni acidic mixtures by means of non-dispersive solvent extraction. Discrimination of different back-extraction mechanisms was achieved from the comparison of kinetic results to simulated data with a previously-reported mathematical model. Finally, it was concluded that the formation of the organic complex species is responsible for the back-extraction of both metals with the equilibrium parameter

Membrane contactors (NDSX and EPT): an innovative alternative for the treatment of effluents containing metallic pollutants

This work presents an overview of membrane-based solvent extraction technologies using membrane contactors as an innovative alternative for the remediation of effluents containing metallic pollutants. The discussion is focused on the description of Non-Dispersive Solvent Extraction (NDSX) and Emulsion Pertraction Technologies (EPTs). Three case studies are reported to demonstrate the viability of NDSX and EPT for the removal and recovery of metallic pollutants present in aqueous streams: