État du développement technologique en matière d'enlèvement des métaux des effluents industriels State of the art of technologies for metal removal from industrial effluents

Résumé

Cette étude trace un profil des diverses technologies utilisées et en développement pour la séparation et/ou la récupération des métaux dans les effluents industriels. Les principes de fonctionnement de ces technologies sont abordés, ainsi que leurs avantages et limites d'utilisation. Les procédés d'enlèvement et de récupération des métaux comprennent les techniques de précipitation (formation d'hydroxydes, de carbonates, de sulfures, etc.) et coprécipitation (sels de fer et d'aluminium, etc.), d'adsorption (sable, cellulose, charbon activé, pyrite, ciment, lignite, mousse de tourbe, sciure de bois, etc.) et de biosorption (bactéries, levures, moisissures, algues marines et d'eaux douces), d'électrodéposition et d'électrocoagulation, de cémentation, de séparation par membranes (osmose inverse et électrodialyse), d'extraction par solvant (acides carboxyliques, amines aliphatiques ou aromatiques, acides aminés, composés phénoliques, phosphates alkyl, etc.), et d'échange d'ions (résines naturelles et synthétiques). La précipitation ou la coprécipitation représentent les procédés les plus largement utilisés et étudiés pour l'enlèvement des métaux des effluents industriels, suivis des techniques d'adsorption. Les procédés plus sophistiqués tels que l'électrodéposition, l'extraction par solvant, la séparation par membranes et l'échange d'ions, bien que largement utilisés dans les procédés métallurgiques, sont relativement peu employés et examinés pour le traitement des effluents industriels. La biosorption a fait l'objet de plusieurs travaux de recherche au cours des dernières années et représente une option intéressante pour le traitement de divers types d'effluents contenant de faibles concentrations en métaux. Finalement, le recyclage et la gestion optimale des effluents constitue une avenue de plus en plus suivie par les industries soucieuses de satisfaire aux nouvelles réglementations et législations.

Mots-clés:

• Métal,
• enlèvement,
• précipitation,
• adsorption,
• électrodéposition,
• cémentation,
• membranes,
• extraction par solvant,
• échange d'ions,
• biosorption

Abstract

This study is dedicated to the review of the different technologies used and evaluated for the removal and/or recovery of metals from industrial effluents. The principles involved in these technologies are discussed, as well as the advantages and limits associated with these processes. The metal removal and recovery processes include the following techniques: precipitation, adsorption and biosorption, electrowinning and electrocoagulation, cementation, membrane separations, solvent extraction and ion exchange.

Precipitation and coprecipitation are the most used and studied methods for metal removal from industrial waste waters. The method of precipitation used most often to remove metals from waste water consists of precipitating them in the form of hydroxides. The usual procedure involves the addition of chemicals such as lime (CaO or Ca(OH)₂), Mg(OH)₂, NaHCO₃, Na₂ CO₃, (NH₄)₂ CO₃, NaOH or NH₄ OH. The precipitation of metals by carbonates or sulphides is an effective alternative to hydroxide precipitation. The use of carbonates allows the precipitation of metals to occur at pH values lower than those necessary with the hydroxides. Moreover, the precipitates thus formed are denser and have better characteristics of solid-liquid separation. Precipitation by sulphides is normally carried out with reagents such as: Na₂ S, NaHS, H₂ S or FeS. In acidic media, the lower solubility of metal sulphides (Cd, Co, Cu, Cr, Ni, Mn, Zn, etc.), makes it possible to reach concentrations lower than those obtained by precipitation as hydroxides. Coprecipitation with aluminum and iron salts is also an effective means for the removal of metals from effluents.

Adsorption methods are also widely applied and examined for this purpose. However, in most cases the use of adsorbents requires an effluent neutralization step. Indeed, the neutralization of acid effluents must take place to allow their disposal in sewerage systems. A wide variety of adsorbents can be employed, both organic and inorganic: aluminum or iron oxides, sand, activated carbon, mixtures of coal and pyrite, iron particles, gravel or crushed brick, cement, etc. Studies have demonstrated the possibility of eliminating metals by adsorption on vegetable matter: peat moss, sawdust and wood bark, etc. Chitin and chitosan, two natural polymers that are abundant in the cell walls of fungi and shellfish, also have excellent properties of metal fixation. The utilization of different agricultural by-products (peanut skins, coconuts, corn cobs, onions skins, tea leaves, coffee powder, canola meal, etc.) for metal adsorption has also been proposed.

Biosorption has been intensively studied in recent years as an economical treatment for metal recovery from dilute industrial effluents. Biosorption implies the use of live or dead biomass and/or their derivatives, which adsorb the metal ions with the ligands or functional groups located on the external surface of the microbial cells. Capacities for metal adsorption on various types of biomass (bacteria, yeasts, fungi, marine and freshwater algae) have been evaluated. The microorganisms used for the metal adsorption step must usually be immobilized in a matrix or in an easily recoverable support. The immobilizing agents or matrices most usually employed are alginate, polyacrylamine, polysulphone, silica gel, cellulose and glutaraldehyde.

Electrowinning is a well-established technology that is widely employed in the mining and metallurgical industries (heap leaching, acid mine drainage, etc.), in metal transformation industries (wastes from plating and metal finishing), and in the electronics and electrical industries for the removal and/or the recovery of metals in solution. Many metals (Ag, Au, Cd, Co, Cr, Cu, Ni, Pb, Sn and Zn) present in the effluents can be recovered by electrodeposition using insoluble anodes.

Electrocoagulation is another electrochemical approach, which uses an electrical current to remove several metals in solution. In fact, the electrocoagulation systems can be effective in removing suspended solids, dissolved metals, tannins and dyes. The contaminants present in waste water are maintained in solution by electrical charges. When these ions and the other charged particles are neutralized with ions of opposite electric charge, provided by a electrocoagulation system, they become destabilized and precipitate in a form that is usually very stable.

Cementation is a type of precipitation method implying an electrochemical mechanism. In this process, a metal having a higher oxidation potential passes into solution (e.g., oxidation of metallic iron, Fe(0), to ferrous iron, Fe(II)) to replace a metal having a lower oxidation potential. Copper is the metal most frequently separated by cementation. However, the noble metals (Ag, Au and Pd), as well as As, Cd, Ga, Pb, Sb and Sn, can also be recovered in this manner.

Reverse osmosis and electrodialysis are two processes using semipermeable membranes applicable to the recovery of metal ions. In electrodialysis, selective membranes (alternation of cation and anion membranes) fit between the electrodes in electrolytic cells. A continuous electrical current and the associated ion migrations, allow the recovery of metals. The techniques of membrane separation are very efficient for the treatment of dilute waste waters.

The metallurgical industry has used solvent extraction for many years for a broad range of separations. This technique is employed today for the removal of soluble metals (Cd, Cr, Co, Cu, Ni, Mo, U, V, Zn, etc.) from waste water. Separation is carried out in contact with an immiscible organic phase to form salts or complex compounds, which give a favorable solubility distribution between the aqueous and organic phases. Various types of reagents can be used for the extraction: carboxylic acids, aliphatic or aromatic amines, amino acids, alkyl phosphates, phenolic compounds. The non-selective removal of metal contaminants in aqueous solutions can be obtained with a whole range of organic reagents. Promising new reagents have been proposed recently for the selective extraction of metals, such as Cd, Co, Cr and Zn.

Ion exchangers are insoluble substances having in their molecular structure acidic or basic groups able to exchange, without modification of their physical structure, the positive or negative ions fixed at these groups. The first ion exchangers used were natural substances containing aluminosilicates (zeolites, clays, etc). Nowadays, the most-used ion exchangers are mainly organic in nature (resins). For the extraction of metals, the removal of cations in solution is usually done with the sulphonic acid group (-SO₃^- H⁺) of a polystyrene resin, or, with a chelating resin containing iminodiacetate functional groups. Ion exchange has recently received considerable attention for the separation and concentration of metals from waste water. These developments are especially applicable to the plating and metal transformation industries, for the removal of Cr, Co, Cu, Cd, Ni, Fe and Zn.

The more sophisticated processes, such as electrowinning, solvent extraction, membrane separations and ion exchange, although frequently used in metallurgical processes, are less popular for wastewater treatment than are precipitation methods. Finally, recycling and optimal management of effluents constitutes an approach more and more widely applied by industries to satisfy new environmental regulations and laws.

Keywords:

• Metal,
• removal,
• precipitation,
• adsorption,
• electrowinning,
• cementation,
• membranes,
• solvent extraction,
• ion exchange,
• biosorption