Oxidation of organics in water by active chlorine performed in microfluidic electrochemical reactors: a new way to improve the performances of the process.

Wastewater polluted by organics can be treated by using electro-generated active chlorine, even if this promising route presents some important drawbacks such as the production of chlorinated by-products. Here, for the first time, this process was studied in a microfluidic electrochemical reactor with a very small inter-electrode distance (145 µm) using a water solution of NaCl and phenol and a BDD anode. The potential production of chloroacetic acids, chlorophenols, carboxylic acids, chlorate and perchlorate was carefully evaluated. It was shown, for the first time, up to our knowledge, that the use of the microfluidic device allows to perform the treatment under a continuous mode and to achieve higher current efficiencies and a lower generation of some important by-products such as chlorate and perchlorate. As an example, the use of the microfluidic apparatus equipped with an Ag cathode allowed to achieve a high removal of total organic carbon (about 76%) coupled with a current efficiency of 17% and the production of a small amount of chlorate (about 30 ppm) and no perchlorate. The effect of many parameters (namely, flow rate, current density and nature of cathode) was also investigated.

Economic Benefits of Waste Pickling Solution Valorization.

An integrated hybrid membrane process, composed of a diffusion dialysis (DD), a membrane distillation (MD) and a reactive precipitation unit (CSTR), is proposed as a promising solution for the valorization and onsite recycling of pickling waste streams. An economic analysis was performed aiming to demonstrate the feasibility of the developed process with a NPV of about EUR 40,000 and a DPBP of 4 years. The investment and operating costs, as well as the avoided costs and the benefits for the company operating the plant, were analyzed with an extensive cost tracking exercise and through face-to-face contact with manufacturers and sector leaders. A mathematical model was implemented using the gPROMS modelling platform. It is able to simulate steady state operations and run optimization analysis of the process performance. The impact of key operating and design parameters, such as the set-point bath concentration and the DD and MD membrane areas, respectively, was investigated and the optimal arrangement was identified. Finally, operating variables and design parameters were optimized simultaneously in a nonlinear framework as a tradeoff between profitability and environmental impact. We show how the integration of new technologies into the traditional pickling industry could provide a significant benefit for the issues of process sustainability, which are currently pressing.

Diffusion Dialysis for Separation of Hydrochloric Acid, Iron and Zinc Ions from Highly Concentrated Pickling Solutions.

Acid recovery from pickling waste solutions is an important step to enhance hot-dip-galvanizing industry process sustainability. Diffusion dialysis (DD) can be used to separate acids and heavy metals (e.g., iron and zinc) from pickling waters, promoting the circular use of such raw materials. In the present study, a laboratory scale unit operating in batch and a continuous large scale unit, both equipped with Fumasep anionic exchange membranes, were tested. Results obtained show that zinc and iron concentration affect the HCl recovery in opposite ways. Iron chlorides enhance acid recovery, while zinc chlorides considerably tend to diffuse through the membrane because of negatively charged chloro-complexes formation and slightly reduce the acid diffusion. A multi-components mathematical model, with a time-dependent and distributed-parameters architecture, was adopted enabling the prediction of operations with hydrochloric acid, zinc, and iron metals both in batch and in continuous dialyzers. As a result, a good comparison between model simulations and experiments was achieved in both configurations.

Comparative electrochemical treatments of two chlorinated aliphatic hydrocarbons. Time course of the main reaction by-products.

Acidic aqueous solutions of the chlorinated aliphatic hydrocarbons 1,2-dichloroethane (DCA) and 1,1,2,2-tetrachloroethane (TCA) have been treated by the electro-Fenton (EF) process. Bulk electrolyses were performed at constant current using a BDD anode and an air diffusion cathode able to generate H(2)O(2) in situ, which reacts with added Fe(2+) to yield OH from Fenton's reaction. At 300 mA, almost total mineralization was achieved at 420 min for solutions containing 4mM of either DCA or TCA. Comparative treatments without Fe(2+) (anodic oxidation) or with a Pt anode led to a poorer mineralization. The better performance of the EF process with BDD is explained by the synergistic action of the oxidizing radicals, BDD(OH) at the anode surface and OH in the bulk, and the minimization of diffusional limitations. The decay of the initial pollutant accomplished with pseudo first-order kinetics. Chloroacetic and dichloroacetic acids were the major by-products during the degradation of DCA and TCA, respectively. Acetic, oxalic and formic acids were also identified. The proposed reaction pathways include oxidative and reductive (cathodic) dechlorination steps. Chlorine was released as Cl(-), being further oxidized to ClO(3)(-) and, mostly, to ClO(4)(-), due to the action of the largely generated BDD(OH) and OH.

Electrochemical oxidation of organics in water: role of operative parameters in the absence and in the presence of NaCl.

The electrochemical oxidation of organics in water was investigated theoretically and experimentally to determine the role of several operative parameters on the performances of the process in the presence and in the absence of sodium chloride. Theoretical considerations were used to design the experimental investigation and were confirmed by the results of the electrochemical oxidation of oxalic acid (OA) at boron doped diamond (BDD) or IrO(2)-Ta(2)O(5) (DSA-O(2)) anodes in a continuous batch recirculation reaction system equipped with a parallel plate undivided electrochemical cell. Polarization curves and chronoamperometric measurements indicated that, in the presence of chlorides, the anodic oxidation of OA is partially replaced by an indirect oxidation process. This result was confirmed by electrolyses experiments that show that, in the presence of suitable amount of chlorides, oxidation of OA takes place mainly by a homogeneous process. Interestingly, a very different influence of the nature of the anodic material, the flow rate and the current density on the performances of the process arises in the absence and in the presence of chlorides so that optimization of the two processes requires very different operative conditions. In the absence of chlorides, high current efficiency (CE) is obtained at BDD when most part of the process is under charge transfer controlled kinetics, i.e. when low current densities and high flow rates are imposed. On the other hand, in the presence of NaCl, higher CE are generally obtained at DSA anode when high current densities and low flow rates are imposed, i.e. when a high concentration of chemical oxidants is obtained as a result of the chloride oxidation. The effect of other operative parameters such as the OA concentration and the pH were further investigated.