Environmental Electrochemistry: Separation Processes PDF

Environmental Electrochemistry Lesson11: Separation Processes Il presente documento è fornito ad uso esclusivo dell’ Insegnamento di “Environmental Electrochemistry” e non può essere altrimenti utilizzato, riprodotto o distribuito sotto qualunque forma. This document is intended for the students o...

Environmental Electrochemistry Lesson11: Separation Processes Il presente documento è fornito ad uso esclusivo dell’ Insegnamento di “Environmental Electrochemistry” e non può essere altrimenti utilizzato, riprodotto o distribuito sotto qualunque forma. This document is intended for the students of the course of “Environmental Electrochemistry”. Any copying, dissemination or disclosure, either whole or partial is prohibited.  Electrochemical separation processes By electroflocculation By electrolyses By electrodialysis By electro‐electrodialysis  Coagulation/Flocculation A Coagulant is typically a trivalent ion that shields (neutralizes) the charge of suspended particles. Al3+ and Fe3+ are the most commonly used A Flocculant is a polymer that «joins» the particles into a floc that can easily precipitate. Flocculants are soluble polymers*, although trivalent ion‐hydroxides can also initially play the role *inorganic, like bentonite or silica, or organic, https://kenkidryer.com/2020/05/10/flocculation‐treatment/ like polielectrolytes or synthetic polimers  …by electrocoagulation Suspended particles’ charge is “shielded” by trivalent ions, produced at a sacrificial anode. Some of these float to the surface (flotation‐also thanks to gases produced at the cathode), while the heaviest sinking to the bottom of the reaction binding of the pollutant to insoluble oxides effectively makes most captured From patent: EP 2050723 A1 pollutants non‐leachable and suitable for landfill.  …by electroflocculation http://soneerawater.com  …by electrolyses (metal electrodeposition) Metal recovery from – Surface treatments processes E.g. metal pickling (decapaggio) – Exhausted electrodeposition baths Galvanic processes – Water used for washing galvanic plants – Recovery of batteries E.g., Pb, Cd – Other industrial waste streams E.g. tan industry (conciaria)  Electrodialysis A useful and often used technology for – Dissalation/concentration – Producing high purity water – Acids/bases regeneration – Metal recovery – Wastewaters treatment – Separation of ionic species (or species that can be ionized for example. Aminoacids, organic acids) – Production of chemical intermediates MEMBRANES In the presence of water, the terminal groups follows dissociative equilibria: R-SO3H + SH = R-SO3 + SH2+ R- R3NOH + SH = R- R3N+ + S + H2O Or, for an anionic membrane: R- R2N + HX = R- R2NH+ + X This leads to the formation of fixed charges (polymer exchange sites) and mobile charge.  C X M+ The presence of charged groups within the membrane allows the repulsion of ions with the same sign; the ions with the opposite charge are the only able to transport the charge across the membrane The franction of the total charge transoprted by the ion i is the transport number, ti An ideal cationic membrane has anionic transport =0 An ideal anionic membrane has cationic transport number =0 The solvent can also move within the membrame: ‐by osmosic ‐for the solvation of moving ions The second contribution usually prevails Equivalent weight , EW, expresse as (g of dry membrane)/(moles of sites), The reciprocal: Ion exchange capacity, IEC (IEC=1/EW), in (moles of sites)/(g of dry membrane); The nature of functional groups either cationic (ANIONIC membranes) of anionic (CATIONIC membranes). The ions allowed to pass through the membrane are called counter‐ions. Permselectivity: according to IUPAC ”A term used to define the preferential permeation of certain ionic species through ion‐exchange membranes.” It can be quantified as the ratio between the charge transported by the counter‐ions over the total passed charge. Solvent Transport, as moles of solvents transported across the membrane per mole of electrons Other important properties: ‐Chemical stability ‐Thermal and mechanical stability ‐Resistivity: typically espressed as R S=ρ L in Ω cm2 ‐Quantity of absorbed solvent: membrane swelling ‐Electrolyte uptake  Electrodialysis cout, concentrate cout, diluate cout, concentrate cout, diluate O2 H2 C A C A C Na+ Cl anode Cl Cl cathode Na+ Na+ + H2O H2O H2O Na H2O H2O cin, concentrate< cin, diluate cin, concentrate cin, diluate  Water Splitting A M+ X c’s c’b cb cs H2 O OH H cs = 0  Boundary conditions Electrolyte pre‐treatment filtration Coagulation Water softening Control of concentration gradient across the membranes in order to: Avoid retrodiffusion Limit water osmotic flow Avoid membrane obstruction …extend membrane life by Complexing agents Periodic acid treatments of the cell Periodic reverse polarity  Other electrodialysis processes cout, concentrate N cout, concentrate N O2 H2 Recovery of a non- C A C A C electrolyte (neutral or M+ X anode X X cathode zwitterion) M+ M+ M+ cin, concentrate< N + salts cin, concentrate N + salts low c MX high c MY low c NY high c NX O2 H2 A C A C A M+ X anode Y cathode X N+ Double exchange reaction: M+ from MX + NY to MY + NX high c MX low c MY high c NY low c NX  EED ‐ ELECTRO‐ELECTRODIALYSIS Separation using ion exchange membranes + faradaic (electron transfer) process The typical example is the recovery of a salt in the corresponding acid or base 3 compartments cell HX out MX out MOH out Note that the transport of H+ e OH- decreases the current efficiency and O2 A C H2 increases the energy consumption + OH  H anode cathode M+   X   ½H 2 O = ¼O 2 +H + e H 2 O + e = ½H 2 + OH H 2O H2O HX in MX in MOH in  2 compartment EED HX + MX out MOH out Cationic membrane: recovery of O2 H2 the base C H+  anode OH cathode M+     ½H 2O = ¼O 2 + H + e H 2O H2O + e = ½H 2 + OH H2O MX in MOH in HX out MOH + MX out O2 H2 Anionic membrane: recovery of A  OH the acid anode H+ cathode  ½H 2O = ¼O 2 + H + e  X  H2O + e = ½H 2 + OH  H2 O H 2O HX in MX in  Multicell EED Using bipolar electrodes HX + MX out MOH out HX + MX out MOH out HX + MX out MOH out O2 H2 O2 H2 O2 H2 C C C + + H H H +    OH OH OH ANODE CATHODE M+ M+ M+ H 2O H 2O H 2O H2 O H2 O H2 O MX in MOH in MX in MOH in MX in MOH in  Multicell EED Using H2 evolving cathodes HX + MX out MOH out HX + MX out MOH out HX + MX out MOH out H2 H2 C H2 H2 C C + + H H H +    OH OH OH ANODE CATHODE M+ M+ M+ H 2O H 2O H 2O H2 O H2 O H2 O MX in MOH in MX in MOH in MX in MOH in  Multicell EED Using O2 reduction cathodes HX + MX out MOH out HX + MX out MOH out HX + MX out MOH out O2 C O2 O2 O2 C C H+ H+ H+    OH OH OH ANODE CATHODE M+ M+ M+ H 2O H 2O H 2O H2 O H2 O H2 O MX in MOH in MX in MOH in MX in MOH in Electrodialysis with Bipolar Membranes: EDBM HX + MX out HX + MX out HX + MX out MOH out MOH out MOH out O2 BM BM H2 C C C + + + H  H  H  OH OH OH M+ M+ H2O H2O M+ MX in MOH in MX in MOH in MX in MOH in  Example: HCl electrolysis Cl2+HCl dil O2+HCl Cl2+HCl dil O2+HCl Cl2+HCl dil O2+HCl Cl2 O2 Cl2 O2 Cl2 O2 C C C H2 O H2 O H2 O ANODO H+ H+ H+ CATODO Cl- Cl- Cl- H+ H+ H+ HCl in HCl in HCl in HCl in HCl in HCl in  Possible future uses Catalysts recovery Tratment of heavy metals (e.g. Cr(VI) Natural products estractions Enantioselective separations  Reversed Electrodyalisis Use of a salinity gradient for generating electricity. Therefore, osmotic pressure is transformed into a potential difference across the membranes that generates an electric current. The design is similar to those of an ED. Proposed demonstrative projects mostly useseawater and freshwater, for example in river deltas The main problem is the low conductivity of the freshwater compartment http://www.reapower.eu/