| Characterized by low pH and high concentrations of heavy metals as well as Fe and SO42-, acid mine drainage (AMD) is a major source of water and soil contamination in coal-and metal-mining districts, and thus it has raised widespread concern about how to avoid or minimize its impact on receiving streams, rivers, and the wider environment. The processes by which AMD is generated are currently quite well understood, that it is generated when sulfide minerals, such as pyrite and pyrrhotite, are exposed to water and oxygen. Although the oxidation of sulfide minerals can be abiotic, the oxidation rate can be enhanced by several orders of magnitude by sulfur-and iron-oxidizing bacteria and archaea. In particular Acidithiobacillus ferrooxidans (A.ferrooxidans) is a key contributor to pyrite oxidation. A.ferrooxidans is an acidophilic chemolithotrophic bacterium which has the ability to oxidize Fe2+ or reduced sulfur compounds in acidic sulfate solutions, coupling the energy thus derived to support carbon dioxide fixation and its growth. It is well known that abiotic oxidation of Fe2+ is kinetically inhibited below pH 4.5, but A.ferrooxidans can accelerate the oxidation rate by 105-106 times, and thus greatly enhance the oxidation of pyrite since the rate of pyrite oxidation by Fe3+ is much higher than by O2.During the biological oxidation of Fe2+, jarosite (basic ferric hydroxysulfate with the formula MFe3(OH)6(SO4)2, where M is usually K+, Na+, NH4+ or H3O+) tends to precipitate in FeS04 solutions containing suitable monovalent cations and an excess of SO42- at pH<3. The formation of jarosite has been widely adopted as a convenient means to efficiently remove iron impurities, especially in zinc hydrometallurgy. One of the advantages of the precipitate is easier to be filtrated, and thus the problems associated with settling and filtering neutralized iron solutions are avoided. Besides, jarosite is a high performance, rare and expensive yellow inorganic pigment.In the present work, the factors, such as cell density, pH, temperature, that influence the biooxidation of Fe2+, the hydrolysis of Fe3+ and the formation of jarosite were studied. The beneficial role of crystal seed (diatomite, quartz sand, already-formed jarosite) and jarosite-directing cation (K+) in biogenic jarosite formation was also reported. Besides, the effect of electrolytic reduction of Fe3+ on the precipitation of soluble iron was studied. Furthermore, due to the biological oxidation of Fe2+ is preferable to chemical oxidation for ecological and economic reasons, A.ferrooxidans was immobilized onto solid matrices, and then transferred to plexiglass columns to improve Fe2+ oxidation for the pretreatment of simulated Fe2+ -rich AMD. The packed-bed bioreactors were operated continuously, and the effluent was collected and neutralized with lime or limestone to reduce its acidity as well as metal contents. The main results were presented as follows:At pH 2.5, the chemical oxidation of Fe2+ by O2 was insignificant, but once A.ferrooxidans was introduced into the reaction solution, Fe2+ was oxidized quickly. Compared with the spent, iron-grown culture of A.ferrooxidans, concentrated cell suspension, the cell density of which is 50 times that of the former, was preferable in the reaction of Fe2+oxidation and jarosite formation, since not only was it devoid of Fe3+, NH4+ and K+, but also more convenient to control the inoculated bacteria concentration as well as to transport elsewhere in engineering applications. However, the concentrate produces an inferior effect on Fe2+ oxidation than that by spent, iron-grown culture of A.ferrooxidans with the same cell number. The capability of 25 mL (10%, v/v) of the spent, iron-grown culture to oxidize Fe2+ corresponded to 1 mL of concentrated cell suspension, of which the cell number was equal to 50 mL (20%, v/v) of the former.Biological oxidation rate of Fe2+ was proportional to the inoculated number of A.ferrooxidans cells. Thus, increasing the inoculum dose enhanced the formation rate of Fe3+ in the initial period of the reaction and hence the precipitation of jarosite. However, no matter how many A.ferrooxidans cells were added, the same degree of iron removal and the amount of jarosite would be expected if given enough retention time at the same concentration of Fe2+, since the bacteria density has no influence on the reaction equilibrium.To illustrate the effects of pH and temperature on the biooxidation of Fe2+ and the formation of jarosite, Fe2+, total iron (TFe), and mass of fresh jarosite were observed at the different initial pH values of 1.40,1.60,1.80,2.00,2.20,2.40 and 2.60, and different temperature of 10,18,28 and 38℃. The results showed that at 10℃, the rate of Fe2+ oxidation was so low that less that 40% of Fe2+ was oxidized after reaction of 72 h, as a result the formation of jarosite was seriously inhibited. In fact, only a little jarosite was formed at 10℃ with the pH at 2.40 or 2.60, whereas lower pH treatments presented no jarosite formation. At 18℃ with the pH at 1.80-2.60, the rate of Fe2+ oxidation was about 187 mg/(L-h), and all of the Fe2+ were oxidized within 48 h, the removal rate of total iron at 72 h increased from 3% to 20% when the pH ranging from 1.80 to 2.60.28℃ was the optimal temperature for A.ferrooxidans to oxidize Fe2+, under which the rate of Fe2+ oxidation reached 187 mg/(L-h) even at pH 1.40, and when the initial pH was set above 1.80, the rate of Fe2+ oxidation exceeded 300 mg/(L-h). The removal rate of total iron at 72 h was 38% at conditions of 28℃ and pH 2.60, which resulted in 2 g of jarosite. Temperature set at 38℃ showed the best effect for the formation of jarosite. Although the oxidation rate of Fe2+ decreased a lot when compared with that at 18℃, the removal rate of total iron at 72 h was much higher, which reached 32% at pH 2.60.pH between 1.80 and 2.60 showed no significant influence on the oxidative activity of A.ferrooxidans, while it would be decreased at pH 1.60 and strongly inhibited at pH 1.40; Increasing the pH and temperature is favorable to the hydrolysis of Fe3+ and hence the formation of jarosite.Both the rate and extent of the precipitation of Fe3+ as well as the formation of fresh jarosite increased with the increasing amount of diatomite, quartz sand, already-formed jarosite and K+ that added into the reaction system. Compared with the control, the treatments with 10 g of diatomite,10 g of quartz sand,10 g of already-formed jarosite, andr 80 mmol/L of K+ increased the removal efficiency of total iron to 47%,65%,52% and 56%, respectively, after 72 h of reaction in 160 mmol/L of initial FeSO4 biooxidation system. In this way, the former three acted as crystal seed that contributed to jarosite precipitation, while K+ was jarosite-directing cation. In the treatment with 10 g of already-formed jarosite and 80 mmol/L of K+, the removal efficiency of total iron was as high as 69%. The effect of the combined applications of crystal seed and jarosite-directing ion was a kind of synergistic effect.Increasing the concentration of K+ would facilitate the formation of jarosite, but most of them happened at 24-48 h, during which the concentration of Fe3+ is higher. However, this phenomenon was changed in the presence of jarosite seed, which could promote jarosite formation at low Fe3+ concentration. After the introduction of crystal seed, more than half of jarosite precipitated at the period of 0-24 h.To favor the precipitation of soluble iron in the form of jarosite, the effect of electrolytic reduction of Fe3+ was studied, and it was found that combining the electrolytic reduction of Fe3+ and the biooxidation of Fe2+ was benefit for the removal of soluble iron.In the treatment of solution with 1006 mg/L of Fe2+ and 489 mg/L of Fe3+, total iron removal efficiency reached 30.3% after reaction of 72 h in the presence of A.ferrooxidans and a continuous supply of electricity at a voltage of 2 V. Processing the biooxidation of Fe2+ after the electrolytic reduction of Fe3+ was more favorable to the conversion of Fe. The optimal reduction time and voltage were 5 h and 5 V, respectively. After the first reduction, the solution contained 4036 mg/L of Fe2+ and 2133 mg/L of Fe3+, then inoculating A.ferrooxidans into the system, total iron removal efficiency reached 42% after reaction of 120 h. By repeating the process of reduction/oxidation again, the total iron removal efficiency reached 71%. Introducing jarosite seed would accelerate the precipitation of iron. After adding 20 g/L of quartz sand or jarosite into the solutions with 3778 mg/L of Fe2+ and 2106 mg/L of Fe3+, total iron removal efficiency both reached 75% and 93% in the first and second process of reduction/oxidation, respectively. After three successive processes of reduction/oxidation, there was only 168 mg/L of TFe (Fe3+) left in the solution with 20 g/L of jarosite, which means that almost all of the soluble iron has precipitated in the form of jarosite, and the conversion of Fe reached as high as 97%.To investigate the feasibility of already-formed jarosite as a support for immobilization of A.ferrooxidans, the precipitates were intentionally taken as the biomass carriers to improve the oxidation rate of Fe2+ and the extent of iron precipitation. The initial pH of the media adopted was-2.50 which favors the precipitation of iron through the formation of jarosite. The number of viable cells retained on jarosite reached 3×108 cells/g after six successive cultures by inoculating previous iron precipitates to new media. The mean rate of Fe+oxidation by 5 g of cell-immobilized jarosite was about 265 mg/(L-h), which was equivalent to the oxidation capability of 25 mL of spent, iron-grown culture of A.ferrooxidans. Increasing the amount of cell-immobilized jarosite not only increased the oxidation rate of Fe2+, but also accelerated the initial iron precipitation rate, eliminated the induction period, and enhanced iron precipitation efficiency, which reached up to 53%in the culture inoculated with 10 g of cell-immobilized jarosite within 72 h. It is concluded that immobilization of Acidithiobacillus ferrooxidans on jarosite to improve the biooxidation and subsequent mineralization of Fe2+ is a potential approach to Fe+-rich AMD treatment.Another method that had been studied to treat Fe2+ -rich AMD in the present work was a combined biological-chemical process, which was based on two steps corresponding to enforced biooxidation of Fe2+ to Fe3+, and the following chemical neutralization. During the enforced biooxidation, A.ferrooxidans was immobilized onto ceramsite, lava, or active carbon, and then transferred to plexiglass columns to oxidize the simulated Fe2+-rich AMD continuously. The simulated AMD was fed at the bottom of the bioreactors, and the effluent was collected and neutralized with lime or limestone to reduce its acidity as well as soluble iron. The results showed that lava, ceramsite and active carbon have no negative influence on A.ferrooxidans growth. The highest oxidation rate of Fe2+ by cell-immobilized ceramsite, cell-immobilized lava and cell-immobilized active carbon was about 301 mg/(Lh),234 mg/(L-h) and 139 mg/(L/h), respectively. The highest value of pH that got in the neutralization processes with limestone was limited to 6, thus when treating the simulated AMD, the influent with high concentration of Fe2+ which is difficult to be precipitated unless pH is elevated to 8-9, the neutralizing agent could only use lime. The resulting sludge was colloidal precipitates, of which total solid content was 2.75%, Sludge velocity (SV30) was 24%, and capillary suction time (CST) was 13.7 s. After oxidized, all the soluble iron (Fe+) in the effluent could be precipitated at pH 4.06. Compared with lime, limestone is a more acceptable neutralizing agent, not only because it is cheaper, but also it could produce a kind of sludge with good dewaterability which is easier to be disposed. Total solid content, SV30 and CST of the sludge that got in the neutralization processed with limestone were 5.50%,4% and 8.9 s, respectively, while, that resulted from lime neutralization were 1.60%,28% and 21.1 s, respectively.Compared with the conventional treatment of AMD involving the addition of lime, this novel active treatment system consisting of enforced biooxidation and the following limestone neutralization could treat high Fe2+ AMD effectively. |
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