FR2939426A1 - PROCESS FOR THE BIOLOGICAL TREATMENT OF ARSENATED WASTE FROM THE TREATMENT OF ACID EFFLUENTS - Google Patents

Abstract

The present invention relates in particular to a process for treating acid effluents containing arsenic waste. In particular, a process for oxidizing arsenites, included in an acidic effluent, to arsenates comprising: a step (i) of adding a basic reagent to said effluent in order to bring its pH to a value greater than or equal to 2, and a step (ii) of adding oxidizing bacteria to the effluent obtained after step (i).

Description

The present invention relates in particular to a process for treating acid effluents containing arsenic waste.

The bioleaching of refractory gold ores, in which gold is usually trapped in a pyrite and arsenopyrite matrix, generates acidic effluents (pH between 1 and 2) loaded with arsenic and iron. These effluents contain several g / l of arsenic in the form of a mixture of As (III) and As (V). The proportion of As (III) may be sufficiently high for the absolute concentration of As (III) to be greater than 1 g / l. The bioleaching effluents undergo a physico-chemical neutralization treatment (usually with lime) leading to the production of a solid waste, mainly composed of a mixture of iron oxy-hydroxides and gypsum. Arsenic is strongly associated with iron oxy-hydroxides in the form of bidentate-binuclear surface complexes (Foster 2003). The stability of iron-arsenic oxy-hydroxide complexes is maximal with As (V) and when the pH is between 4 and 6 (Wang et al., 2003). The solid residue obtained is all the more stable as the ratio As (V) / As (total) is high. This ratio depends significantly on the initial pyrite / arsenopyrite ratio in the starting sulphide concentrate (Morin et al., 1995): the higher the proportion of pyrite, the higher the redox potential of the medium and the proportion of As (V) are high. The As (III) present in the bioleaching effluent can be oxidized chemically, with hydrogen peroxide for example (Molnar et al., 1993, Grossin, 1994). The reaction is fast (30 minutes), however this reagent is expensive and should be used in excess (30 to 100%) to be effective. BRG002-EN-121 TEXT REMOVAL - 2 - Other processing methods are known to those skilled in the art. Thus, the patent FR2814751B1 protects the processes for treating arsenic-containing media to oxidize arsenites to arsenates, characterized by the inoculation of these media, in a bioreactor, with a bacterial population consisting essentially of bacteria, the sequence of which is 16S rDNA is identical to that of CAsO1 strain CNCM I-2518. Patent Application W00078402 protects a process for removing and immobilizing arsenic from arsenic-containing waste, by oxidizing arsenic in an aqueous medium, to contact As (V) with Fe (III) to form an insoluble Fe-As compound and to separate this compound from the aqueous medium. Arsenic is oxidized with oxidizing bacteria at a pH between 0.5 and 4 and at a temperature between 20 and 90 ° C in the presence of a mineral catalyst (often pyrite). In addition to the cost and handling of this catalyst, it generally contains impurities that increase the pollutant load of the effluent, as well as its acidity. The present invention consists in growing the As (V) / As (total) ratio in the solid waste by performing a biological oxidation of As (III) during the process of neutralization of the effluent with a basic reagent. This process may be completed by a liquid / solid separation step leading to the production of a solid waste of the oxy-hydroxide iron type. Arsenic is oxidized by bacteria using As (III) as a source of energy, at a pH between 2 and 7.

The present invention differs from the patent FR 2 814751 - Bl in that the treatment is not necessarily carried out in a bioreactor, but can advantageously be carried out in the tanks used for the conventional physicochemical treatment of the acid effluents BRG002-FR- 121 TEXT REMOVAL - 3 and loaded with metals. In addition, it brings novelties in the conditions of introduction of microorganisms into the environment. The present invention is also distinguished from the patent W00078402 - Al in that (1) the oxidation can be done by the bacteria in the absence of catalyst (As (III) is used as a source of energy by the bacteria ), (2) the initial product to be treated is not a solid waste, but an effluent, and (3) it is not necessary to add ferric iron to co-precipitate arsenic because it is already present in the effluent to be treated.

DETAILED DESCRIPTION OF THE INVENTION Thus, the present invention notably provides a process for oxidizing arsenites, included in an acidic effluent, to arsenates comprising: a step (i) of adding a basic reagent to said effluent in order to bring its pH to a value greater than or equal to 2, and a step (ii) of adding oxidizing bacteria to the effluent obtained after step (i). According to the invention, the term basic reagent refers to all chemical substances capable of capturing one or more protons. The basic reagents according to the invention are chosen from weak bases or strong bases. According to a preferred embodiment of the invention, the basic reagent is chosen from the group comprising lime (Ca (OH) 2), calcite (CaCO3), sodium hydroxide (NaOH), potassium hydroxide (KOH ). According to a particularly preferred embodiment, the basic reagent used in the process according to the invention is lime (Ca (OH) 2). According to the present invention, the term BRG002-FR-121 oxidative bacteria refers to bacteria capable of oxidizing arsenites. According to a preferred embodiment of the invention, the oxidizing bacteria are characterized in that they are autotrophic, aerobic bacteria capable of oxidizing As (III) to As (V) using CO2 as only source of carbon and As (III) as the sole source of energy. According to an even more preferred embodiment, the bacteria used have substantially identical 16S rDNA sequences SEQ ID No. 1 or SEQ ID No. 2. These sequences are obtained by analysis of the 16S rDNA region corresponding to positions 18-1492 according to E. coli (Brosius et al., 1981, Journal of Molecular Biology, 148: 107-127). In the context of the present invention, the term substantially identical means that the sequences have at least 80%, preferably 90% and even more preferably at least 95% identity between them. According to a most preferred embodiment, the oxidizing bacteria used in the process according to the invention belong to the CAsO1 strain (CNCM-I2518) or to the bacteria having a genome having more than 80% similarity with the bacteria of the strain. CNCM-I2518. According to another preferred embodiment, the bacteria that can be used in the context of the invention have the following characteristics: they support up to at least 1 g of As (III) without losing their ability to oxidize As (III), - the rate of oxidation of As (III) by these bacteria in bioreactor reaches at least 12 mg.171.h71, - their specific activity of oxidation of As (III) reaches at least 5x10-7 mg of As (III) per bacterium and per hour. With such a specific activity value, a bacterial suspension containing 107 bacteria per ml is capable of oxidizing 5 mg of As (III) per liter per hour. It follows that the oxidation of As (III) by bacteria requires only a very low production of bacterial organic matter. Therefore, the amounts of nutrients needed for the development of bacteria (nitrogen, phosphorus) are also very low, - they are able to reduce the residual concentration of As (III) to less than 50 pg.l-1. According to a preferred embodiment of the invention, the bacteria added in step (ii) are at a final concentration of between 10 l and 5 10 bacteria / ml. According to a preferred embodiment, the process according to the invention does not comprise a step consisting in adding to the effluent a mineral catalyst, in particular a noble or semi-noble metal or a metal or a pyritic metal complex such as pyrite. chalcopyrite or molybdenite. According to a preferred embodiment, the method according to the invention further comprises a step (i ') of mixing the basic reagent and the effluent. According to a still more preferred embodiment, this step is carried out by means of stirring means selected from the group comprising mechanical stirring, gas injection stirring (bubbling), and the use of a static mixer (by example circulation of the two fluids through baffles). According to a preferred embodiment, the basic reagent added during step (i) of the process according to the invention makes it possible to bring the pH of the effluent to a value greater than or equal to 2.0, even more preferentially to a value greater than or equal to 2.5. According to a preferred embodiment, the basic reagent added during step (i) of the process according to the invention makes it possible to bring the pH of the effluent to a value of less than 10%. 2.7 (pH of iron precipitation;) The use of oxidizing bacteria at such a low pH allows for better oxidation of arsenic waste, since at this pH a large part of the arsenic is still present in the phase. According to a most preferred embodiment, the reagent added during step (i) makes it possible to bring the said effluent to a value of between 2.5 and 2.7. in addition a step (ii ') of adding a basic reagent in the effluent from step (ii) in order to bring its pH to a value greater than or equal to 5 and ideally to a pH close to 5.5 This pH corresponds to the optimum pH of stabilization of the waste in the solid phase. it consists in adding said basic reagent in one or more times. According to a preferred embodiment, said basic reagent is added in several times. According to a most preferred embodiment, all or part of the said addition or additions of basic reagents during step (ii ') is followed by the addition of oxidizing bacteria in the effluent. According to a preferred embodiment of the invention, the effluent from step (i) or step (ii) is placed in a settling tank. According to another preferred embodiment, the method according to the invention further comprises a step (iii) of incubating the effluent obtained after step (ii) or (ii ') for 1 to 48 hours. According to another preferred embodiment, the method according to the invention further comprises a step (iv) of separating the liquid phase from the solid phase of the effluent obtained after step (iii). Figure 1 shows particularly advantageous embodiments of the method according to the invention. The process, illustrated in FIG. 1, consists of preparing a CASO1 inoculum with a simple mineral medium containing, for example, 100 mg / l of As (III). This culture is added in the tank (1) used to mix the effluent with the basic reagent (thanks to a mechanical stirrer (2)). The addition can be carried out continuously or discontinuously with a pump. The precipitation of Fe (III) in the form of oxy-hydroxides is mainly carried out when the pH is greater than 2.5. Bacteria oxidize As (III) during the precipitation process of Fe (= II). The bacterial oxidation treatment has an even greater effect on the proportion of As (V) in the solid phase that is performed early, that is to say at the lowest possible pH, before the As is trapped by iron oxy-hydroxides. The bacterial CAsO1 population, containing Thiomonas bacteria protected by the patent FR 2 814751-Bl, uses As (III) as a source of energy. The following reaction is carried out for a pH of between 2 and 7: H2AsO3 + O: -> HAsO4 + H +

These microorganisms do not need any particular nutrient input, and catalyze the oxidation of As (III) to As (V) in the presence of a very low oxygen concentration. The addition of bacterial suspension can be continued until the final pH of the precipitation stage is reached. The stability of the final residue, in terms of immobilization of arsenic, will be optimal for a pH of between 5 and 6. However, the final pH of the precipitation step will depend on the effluent to be treated, and in particular the metals present in this effluent. The oxidation process of As (III) continues during the settling phase in the decanter (3). BRG002-FR-121 TEXTE REMOVAL - 8 bacteria present in the aqueous phase continue to oxidize the residual As (III) during this step, the equilibrium [As liquid phase] / [As solid phase] promoting the release of As (III) in solution compared to that of As (V). The decanter supernatant can be used as an inoculum to be injected into the precipitation vessel. This inoculum could be reactivated, if necessary, by addition of As (III) in an intermediate tank (4).

The process of the invention is particularly useful for treating the bioleaching effluents of refractory gold ores. It is also applicable to other acid effluents rich in iron and arsenic, and to the acidic waters of mines (drainage, resurgence) also rich in iron and arsenic. Thus, the present invention also relates to the use of the process according to the invention for the treatment of effluents for the biolixivation of refractory gold ores. The following examples give by way of illustration, but without limitation, performance indicators obtained during the application of the method of the invention to actual samples of bioleaching effluents. FIG. 1 is a schematic representation of the process for increasing the stability of the waste resulting from the neutralization of acid effluents charged with iron and arsenic. FIG. 2 represents the evolution of the concentration of As (III) in the liquid phase during treatment with lime, in the absence and in the presence of As (III) oxidizing bacteria. FIG. 3 represents the evolution, as a function of pH, of the concentration of total arsenic leached in demineralised water or in mineral culture medium, from solid waste resulting from lime treatments BRG002-FR-121 TEXT REMOVAL - 9 - in the presence of bacteria. Figures 4 to 7 show the evolution of arsenic concentrations in the liquid phase after contacting the solid resulting from the lime neutralization of a bioleaching effluent with As (III) - oxidizing bacteria.

Example 1. Application of the biological oxidation treatment of As (III) during the lime treatment of a real effluent of bioleaching of a gold concentrate

A gold concentrate, obtained by flotation, containing approximately 40% arsenopyrite (FeAsS) and 30% pyrite (FeS2), was subjected to a batch bioleaching treatment in a stirred and aerated reactor. The biolixiviation conditions were as follows: 10% solid, OKm medium (Collinet-Latil, 1989), initial pH 1.75. The inoculum was a bacterial consortium preserved in BRGM and used for the bioleaching of sulphide ores. The final characteristics of the biolixiviation medium were as follows: pH 1.11, redox potential = 519 mV (ref Ag / AgCl), total arsenic concentration = 11 g / l, total iron concentration = 11 g / l. During the gold ore bioleaching process, the gold remains in the solid phase. The liquid phase is considered an effluent and must be treated. The biolixiviation pulp was filtered, and the filtrate was recovered to test the efficiency of the As (III) biological oxidation process. The filtrate was divided into two samples of equivalent volume (1 liter each). One of these samples has undergone a classic treatment with lime, without bacteria. The second sample was treated in the presence of BRG002-FR-727 TEXTE REMOVAL bacteria. In both cases, the filtrate was placed in a glass reactor equipped with a double jacket and a mechanical stirring system (motor, stirring axis and marine propeller 316L stainless steel). Lime pulp Ca (OH) 2 at 200 g / l was used to induce an increase in pH and to precipitate the iron and arsenic present in the bioleaching effluent. The pH was gradually increased from 1.11 to 4.00. This simple procedure was applied for the test in the absence of bacteria. In the case of treatment in the presence of bacteria, the following operations were performed. The pH was increased in a conventional manner from 1.11 to 2.70, by adding only lime. From pH 2.70, a bacterial suspension was added to the reactor at a rate of 100 ml of bacterial suspension at each of the following pH levels: 2.70; 3.02; 3.11; 3.30; 3.52; 3.71; 3.89; 4.00. The bacterial suspension was prepared with the CAS01 inoculum grown in the minimum autotrophic medium described in Battaglia-Brunet et al., 2002, in the presence of 100 mg / l of As (III). The evolution of the concentration of As (III) in the liquid phase of the reactors, in the absence and in the presence of bacteria, is given in FIG. 2. The concentrations of As (III) were obtained by voltammetric method. Additions of bacterial suspension induce a fall in the concentration of As (III) in the liquid phase of the lime treatment reactor. This effect is already sensitive between pH 2.7 and pH 2.9. The oxidation of As (III) is complete at pH 3.5, whereas at the same pH, the liquid phase still contains 800 mg / l of As (III) in the absence of bacteria. The pulps resulting from the treatment with lime, in absence or in the presence of As-oxidizing bacteria (III), were filtered. The solid phase recovered after filtration (without rinsing) was stored under nitrogen at 7 ° C. Its As (III) content was determined by microwave extraction in a mixture of phosphoric and ascorbic acids. The final concentration of AS (III) in the lime-treated solid conventionally amounts to 54 mg per g of dry solid (uncertainty on analysis 5 mg / g). In the lime-treated solid in the presence of As (III) -basing oxidizing bacteria, the final concentration of As (III) is 16 mg per g of dry solid (uncertainty on the assay 0.4 mg / g ). The addition of bacteria to the lime treatment reactor reduced the As (III) content in the solid residue of the treatment by 70%.

Solid samples from both treatments were placed in flasks in the presence of either deionized water or mineral medium for As (III) -oxidative bacteria, the solid / liquid ratio being 25 g / 100 ml. These flasks were placed under mutual agitation. Table 1 gives the arsenic values in solution after 24 h and 1 month of solid contact / water. The unadjusted pH of these mixtures equilibrates between 3.6 and 3.9. The pH was adjusted with 1M sodium hydroxide to study the influence of this parameter on the stability of the solids. The supernatant was filtered at 0.45 μm, and then a separation of As (III) and As (V) on anionic resin (after Kim, 2001) was performed. The concentrations of As (III) and As (V) were then determined by atomic atomic absorption spectrometry (AAS, Varian, Palo Alto, CA, USA, T = 2050 ° C, 193.7 nm). BRG002-EN-121 INSTALLATION TEXT - 12 - As 1 month without pH adjustment 1 month after adjustment of pH to 5 Time of 24 hours, contact (Solid treated As (III) 0.28 bio, water mg / 1 demineralized As (V) 0.29 mg / 1 As (III) 0.22 mg / 1 As (V) 0.89 mg / 1 As (II1) 0.01 mg / 1 As (V) 0.31 mg / 1 Solid treated as (III) 0.68 bio, medium mg / 1 mineral As (V) 1.27 mg / 1 As (III) 0.25 As (III) <mg / 1 0.01 mg / 1 As (V) 1.61 As (V) 0.6 mg / 1 mg / 1 Control without As (III) 151 As (III) 173 As (III) 38 bacteria, mg / 1 mg / 1 mg / 1 water As (V) 2 , 47 As (V) 0.33 As (V) 0.36 demineralized mg / 1 mg / 1 mg / 1 Control without As (III) 221 As (III) 252 As (III) 54 bacteria, mg / 1 mg / 1 mg / 1 medium As (V) 3.23 As (V) 0.62 As (V) 0.56 mineral mg / 1 mg / 1 mg / 1 Table 1. Concentrations of arsenic leached in demineralized water or in mineral culture medium, from solid waste resulting from treatments with conventional lime or in the presence of bacteria, Table 1 shows that the solid obtained by lime treatment in the presence of series is much more stable than the solid obtained by conventional treatment. According to Dixit and Hering (2003), the number of As binding sites on iron hydroxides is not greater than 0.3 moles of sites per mole of Fe. The Fe / As ratio should be greater than at 3.3 for retention BRG002-EN-121 TEXTE REMOVAL - 13 - effective of arsenic by the solid. In the present example, despite an initial molar ratio Fe / As unfavorable (1.33), the solid treated in the presence of bacteria releases little arsenic in solution. Precipitates consisting mainly of As (V) and ferrihydrite-type iron hydroxides having a Fe / As ratio of 1.45 have already been observed in an arsenic-rich geothermal source (Macur et al., 2004). Table 1 also shows that the arsenic released by the solid formed in the presence of bacteria is predominantly in As (V) form, whereas the solid obtained by conventional lime treatment generates a leachate mainly containing As (III) . The evolution of the concentration of total As as a function of the pH is given in FIG. 3. Between pH 3.5 and pH 7, the total concentration of As is always lower when the solid has been treated in the presence of bacteria than when the solid was obtained by conventional treatment. The concentration of As is minimized, for the solid obtained in the presence of bacteria, at pH 5.2.

EXAMPLE 2 Application of the biological oxidation treatment of As (III) to the solid resulting from the neutralization of a bioleaching effluent with lime In this example, the bioleaching effluent was conventionally treated with lime then was subjected to the action of As (III) -oxidative bacteria. The observed reactions correspond to those which would be carried out during the step following the precipitation phase, that is to say when the bacteria would be introduced into the settling tank. A gold ore bioleaching effluent, whose composition is given in Table 2, was subjected to two different treatments: a conventional treatment with BRG002-FR-121 TEXTE REMOVAL - 14 - lime up to pH or a conventional treatment with lime up to pH 7. As (III) 1.35 g / l Table 2. Composition of the treated bioleaching effluent in Example 2

One liter of bioleaching effluent was placed in a mechanically stirred reactor with a helix. The pH of the effluent was raised to 4 or 7 by adding a lime milk Ca (OH) 2. The amounts of lime required to increase the pH from 1.08 to 4 or 7 were 98 and 125 g, respectively. During this phase, iron and arsenic were almost completely precipitated. The final concentrations of arsenic in the liquid phase were 8.8 mg / l and 1.36 mg / l at pH 4 and 7, respectively. The solid phases were recovered by filtration and then stored under nitrogen at 7 ° C. . The tests for bringing into contact with the As (III) oxidizing bacteria were carried out in 250 ml flasks, containing 60 g of solid and 90 ml of mineral medium. The inoculum was prepared in the same manner as in Example 1. The flasks were inoculated with 10 ml of bacterial suspension. Control flasks without bacteria were prepared. The flasks were incubated at 25 ° C with reciprocal shaking. The arsenic analyzes in the supernatant and in the solids were carried out in the same way as in Example 1. The evolution of the arsenic concentrations in the BRG002-FR-121 TEXTE REMOVAL oH 1,08 Total Fe 30, 5 g / l Fe (II) 0.935 g / l As total 13.5 g / l - 15 - liquid phases is given in Figures 4 to A pH 4, the concentration of As (III) remains close to 10 mg / 1 during the 50 days of incubation, whereas in the presence of bacteria, the concentration of As (III) decreases rapidly and remains below 0.5 mg / l after 3 days of incubation. At pH 7, the concentration of As (III) decreases in both inoculated and uninoculated vials. However, this decrease is faster in inoculated flasks (3 days) than in noninoculated flasks (10 days). At pH 4, the As (V) concentration is lower in the inoculated vials than in the uninoculated vials. At pH 7, As (V) concentrations in inoculated and uninoculated flasks are not significantly different. The final results, in terms of arsenic concentrations in the liquid phases, are given in Table 3. The lowest concentration of total As is obtained at pH 4 in the presence of bacteria.

Condition As (III) mg / l As (V) mg / l As Total mg / l Inoculated pH4 0.07 0.02 0.09 Not inoculated 9.75 0.39 10.14 pH4 Inoculated pH7 <0.01 0 , 34 0.34 Not inoculated <0.01 0.48 0.48 pH7 Table 3. Contacting the solid resulting from the neutralization with lime of a bioleaching effluent with the As (III) -oxidative bacteria, concentrations in arsenic after 50 days of contact (Example 2)

References: BRG002-FR-121 TEXT REMOVAL - 16 - Battaglia-Brunet F., Dictor M.-C., Morin D., Baranger P. (2000) bacteria that can be used to oxidize arsenic, a process for their selection and their applications for treating media containing arsenic. Patent FR 2 814751 - B1.

Battaglia-Brunet F., Dictor MC, Garrido F., Crouzet C., Morin D., Deleyser K., Clarens M., Baranger P. (2002) An arsenic (III) -Oxidizing bacterial population: selection, characterization, and performance in reactors. J. Appl. Microbiol., 93, 656-667.

Dixit S. and Hering J.G. (2003) Comparison of Arsenic (V) and Arsenic (III) Sorption on iron oxide minerals: implications for arsenic mobility. About. Self. Technol. 37: 4182-4189.

Collinet-Latil M.-N. (1989) Bacterial leaching by Thiobacillus ferrooxidans and Thiobacillus thiooxidans from a gold-bearing arsenopyran flotation concentrate refractory to direct cyanidation. PhD thesis, Université de Provence Aix-Marseille I, specialty Cell biology and microbiology.

Foster A.L. (2003) Spectroscopic investigations of arsenic species in solid phases. In: Welch AH, Stollenwerk KG (eds) Arsenic in groundwater: geochemistry and occurrence, Kluwer Academic Publishers, Boston, pp. 27-65.

Grossin C. (1994) Study of the morphology and stability of synthetic iron arsenates. PhD thesis, University of Orléans, Chemistry-Physics option.

Kim, MJ. (2001) Separation of inorganic arsenic BRG002-EN-121 TEXTE REMOVAL - 17 Bull About Contam Toxicol 67: 46-51.

Macur R.E., Langner H.W., Kocar B.D. and Inskeep W.P. (2004) Linking geochemical processes with microbial community analysis: successional dynamics in an arsenicrich, acid sulphate-chloride geothermal spring. Geobiology, 2, 163-177.

Molnar L., Vircikova E. and Lech P. (1993) Experimental study of As (III) oxidation by hydrogen peroxide. Hydrometallurgy, 35, 1-9.

Morin D., Battaglia F., Hau J. M, Pooley F. D, Tidy N. P, Adam K. and Papassiopi N. (1995) Optimization of a bacterial leaching process for the treatment of auriferous arsenical pyrites. Contract No. MA2M - CT91 - 0055 Final Technical Report. Project funded by the European community under the Brite / Euram Program.

Ruitenberg R., Buisman, C.J.N. (2000) A method of immobilizing wastes containing arsenic. Patent W00078402 - Al.

Wang J. W., Bejan D., Bunce N.J. (2003) Removal of arsenic from synthetic acid mine drainage by electrochemical pH adjustment and coprecipitation with iron hydroxide. About. Sci. Technol., 37, 4500-4506. BRG002-PR-121 TEXT REMOVAL

Claims (17)

REVENDICATIONS1. A process for oxidizing arsenites, included in an acidic effluent, to arsenates comprising: a step (i) of adding a basic reagent to said effluent to bring its pH to a value greater than or equal to 2, and a step ( ii) adding oxidizing bacteria to the effluent obtained after step (i). 2. Method according to claim 1, characterized in that the reagent added during step (i) makes it possible to bring said effluent to a value greater than or equal to 2.5. 3. Method according to claim 2, characterized in that the reagent added during step (i) makes it possible to bring said effluent to a value less than or equal to 2.7. 4. Method according to one of claims 1 to 3, characterized in that it does not include a step of adding in the effluent a mineral catalyst. 5. Method according to claim 4, characterized in that the reagent added during step (i) makes it possible to bring said effluent to a value between 2.5 and 2.7. 6. Method according to claim 5, characterized in that it further comprises a step (ii ') of adding a basic reagent in the effluent from step (ii) to bring its pH to a value greater than or equal to 5. 7. Method according to one of claims 1 to 6, characterized in that the effluent from step (i) or step (ii) is placed in a settling tank. 8. Method according to one of claims 1 to 6 characterized in that it further comprises a step (iii) of incubating the effluent obtained after step (ii) BRG002-EN-121 TEXTE DEPOSE- 19 - for 1 hour to 48 hours. 9. Method according to one of claims 1 to 8, characterized in that it further comprises a step (i ') of mixing said basic reagent and said effluent. 10. Process according to claim 9, characterized in that step (i ') is carried out by means of stirring means selected from the group comprising mechanical stirring, agitation by gas injection (bubbling), and the use of static mixer. 11. Process according to one of reactions 1 to 10, characterized in that the basic reagent is lime (Ca (OH) 2). 12. Method according to one of claims 1 to 11, characterized in that said oxidizing bacteria are bacteria whose DNA sequence 16s is substantially identical to that of the CAsO1 strain (CNCM-I2518) or bacteria having a genome possessing more than 80 similarity with the bacteria of strain CNCM-I2518. 13. The method of claim 8, characterized in that it further comprises a step (iv) of separating the liquid phase from the solid phase of the effluent obtained after step (iii). 14. Method according to one of claims 1 to 13, characterized in that the bacteria added in step (ii) are at a final concentration of between 5 10j and 5 106 bacteria / ml. 15. Use of a process according to one of claims 1 to 14 for the treatment of bio-lixivation effluents of refractory gold ores. 16. Use of a process according to one of claims 1 to 14 for the treatment of acid effluents rich in iron and arsenic. 17. Use of a method according to one of the BRG002-FR-121 TEXTE DEPOSE- claims 1 to 14 for the treatment of acidic water mines (drainage, resurgences) rich in iron and arsenic. BRG002-EN-121 TEXT REMOVAL