Metal Recovery

The present invention is directed to the treatment of aqueous solutions containing cations of heavy metals with a biomass reaction product derived from a microorganism, such as Bacillus subtilis, selective to the removal of heavy metals from solution. The invention is also directed to a process of enhancing metal uptake properties of microorganisms and to relatively stable biomass products produced by said process having enhanced metal uptake properties.

Waste waters from many industrial processes, such as metal finishing, which contain a variety of metal ions some of which are toxic and some of which are valuable, are generated in large quantities. These liquids cannot be released into streams or sewers without causing environmental damage. Furthermore, such release would, in many cases, represent a violation of stringent environmental regulations. Treatment of such waters to remove the metal ions is required. Existing processes for treating such waste waters suffer from many disadvantages, among which are included high cost, production of metal-containing sludges which are difficult to treat for metal recovery and hence are dumped in landfills, complex technology, etc. The use of caustic preciptation, sulfide precipitation, electrolysis, evaporation, reverse osmosis, ion exchange, etc., are all known and suffer from one or more difficulty in terms of energy requirement, limited applicability, low absortion capacity, requirement for precise control, etc.

Attempts to use biomass of living microbes for metal recovery are reported in U.S. Pat. Nos. 4,407,954 and 4,293,333. T. J. Beveridge, et al. have reported that cell walls of B. subtilis will take up metals from solution (J. Bacteriol, 1976, 127(3), 1502-18). However, living microbes must be cultured, an expensive, capital-intensive process, and are subject to contamination by other microbes and inhibition by high metal concentrations or extremes of pH and other toxic components of waste streams. They are also subject to putrefaction. U.S. Pat. Nos. 4,293,334 and 4,320,093 disclose use of nonliving biomass derived from microbes but these processes display limited metal sorption or uptake capacity.

One embodiment of the invention contemplates the recovery of the metal present in cationic form in aqueous streams by using a relatively insoluble solid biomass product in particulate form derived chemically from microorganisms characterized by cell wall structures, a preferred microorganism being the bacterium, Bacillus subtilis. The sorption or metal uptake capacity for metal ions is dramatically improved by treating the microorganism with a caustic solution which converts it to a substantially solid biomass reaction product. The term "solid" also includes a reaction product having a porous structure.

The term "caustic" is meant to cover alkaline solutions having a relatively high intrinsic pH, e.g., in excess of 9, preferably in excess of 10 or 11, prior to reaction with the microorganism, the alkaline solution being one which when reacted with a selected microorganism produces a relatively solid biomass reaction product having enhanced metal uptake capacity compared to the metal uptake properties of the microorganism prior to the caustic treatment. The term caustic is defined in Webster's Collegiate Dictionary as capable of destroying or eating away by chemical reaction, such as occurs during caustic treatment of microorganisms in accordance with the invention.

The term "sorption" is used in the broad sense to include all forms of metal uptake, whether by adsorption, absorption, ionic bonding, among other forms of metal uptake.

Another embodiment of the invention resides in a process for enhancing the metal uptake properties of microorganisms and the biomass reaction produced by the process. The microorganisms selected are those which have cell wall structures (e.g., Bacillus subtilis) which when reacted with a caustic solution form a substantially solid biomass reaction product having enhanced metal uptake properties. The "causticized" biomass or reaction product having the enhanced uptake capacity is slightly alkaline when dispersed in water. Thus, the invention provides a causticized microorganism in the form of a substantially solid biomass reaction product, the product being preferably in the particulate form.

In producing the solid biomass reaction product, solutions of NaOH and KOH are preferred having concentrations ranging from about 0.25 molar to 1.25 molar. The microorganism, e.g., Bacillus subtilis, is treated with the caustic solution at temperatures ranging up to boiling, preferably above ambient temperature, e.g., from about 50.degree. C. to 100.degree. C., to form the biomass reaction product which is then washed to remove excess alkaline solution and then dried. The treated dried biomass may be in the form of hard, grindable bodies, such as plates. The biomass reaction product in the particulate form is slightly alkaline and can be used in a column to contact the metal-containing aqueous stream.

When the metal-containing aqueous stream is contacted with the treated biomass, rapid metal uptake occurs. For example, when the solid biomass reaction product in the particulate state is suspended in the aqueous stream and the biomass particles become loaded with metal, the loaded particles, because of the increase in weight, settle readily and can be separated from the aqueous stream by conventional means and the metal values thereof easily recovered.

Process waste waters treated in accordance with the invention may contain widely varying amounts of a heavy metal e.g., about 20 milligrams or less to about 2,000 milligrams (i.e., 2 grams) or more per liter. To be treatable in accordance with the invention, the metal must be in cationic form. Anionic complex ions such as cyanide-metal complexes, may be decomposed by known techniques prior to treatment. Common anions such as sulfates, chlorides, nitrates, phosphates, carbonates, etc., may be present without harm and solutions can successfully be treated over the pH range of about 2 to 11, for example, about 3 to about 11, preferably about 4 to 8, e.g., about 4 to 6. The heavy metal usually will have an atomic number greater than 20, although aluminum, atomic number 13, can be recovered from aqueous solutions.

The solid biomass product derived from B. subtilis cells is employed in the caustic treated condition, since greater uptake capacity for metal cations thereby results. As previously stated, the treatment preferably is effected by heating the cell biomass at 50.degree. to 100.degree. C. or boiling, for 1 to 15 minutes with a solution preferably containing about 0.25 molar to about 1.25 molar NaOH concentration. KOH in equivalent amounts may be used. NH.sub.4 OH may be employed but is not as effective. Any excess sodium or potassium hydroxide is removed by water washing. The cell biomass may be hydroxylated and/or "cleaned" by removing lipids and other organic matter which masks active sites by the caustic treatment. In any event, metal uptake by the causticized biomass is rapid, the treated biomass exhibiting high metal uptake capacity. Treatment with caustic at elevated temperatures, e.g., boiling temperatures, destroys autolytic enzymes (enzymes that the organisms possess that cause putrefaction). Biomass treated with hot or boiling sodium hydroxide solution to kill the cells and inactivate potentially putrefiable matter may be dewatered and dried prior to use. Centrifugation, filtration, etc. may be employed for dewatering. When drying is accomplished in shallow pans, a solid, hard, rock-like plate resembling slate is produced. This hard material can be crushed, ground and sized for use in a metal extraction column. When drying is accomplished in such a manner as to produce the biomass reaction product in powder form, the powder can be agglomerated for use in a metal extraction column.

Standard water treatment units may be used. For example, processing equipment comprising a mixing unit for contact between cell biomass product and metal-containing liquid may be followed by a settling unit for removing metal-containing particles from suspension with optional final filtration. This characteristic of the particulate biomass facilitates separation from the treated liquid. A continuous or batch-type ultra-filtration polymer membrane unit may be employed for mixing, sorption and filtration. Other modes for contacting metal-containing liquids with solid caustic-treated biomass include, for example, contact of the solution with the treated biomass immobilized in a matrix, such as a gel, or with a granular biomass contained in a column configuraton operated as a packed or fluidized bed. During metal accumulation in caustic-treated biomass, the pH increases even though residual caustic is washed from the treated biomass. It is believed that, at equilibrium, powdered, caustic-treated biomass has greater ultimate metal-loading capactity than coarse granulated, caustic-treated biomass, this being due to the increased surface area of the powdered product. However, it is found that the caustic-treated biomass can be metal saturated more efficiently than powdered biomass reaction product if it is dried, ground, sized, for example to -35+60 mesh and then contacted with a continuous flow of metal solution at a given concentration in a packed or fluidized bed contactor. Thus, packed or fluid bed systems containing a microorganism-derived granular biomass reaction product will exhibit enhanced metal-loading capacity when compared to coventional mixing/settling systems using powdered biomass.

The metal may be recovered from the loaded biomass in a number of ways. For example, copper, silver and gold can be separated from dewatered, loaded biomass by contact with iron. The initial contact can be very small, for example, a scratch in a Teflon-coated steel pan, to initiate separation of metal from the biomass and once separation is initiated, it continues to form fan-like metal figures radiating from the initial point of contact to produce dendritic-appearing crystals of almost pure metal. Traces of iron are found near the initial contact point. High recovery of loaded metal is obtained. Precipitation of loaded metal appears to proceed on an atom-by-atom basis, at low temprature, e.g., 80.degree. C. This observation indicates that the observed separation proceeds by galvanic action and thus indicates the invention is of particular value in extracting from solution metals which are below iron in the electromotive series; namely, cadmium, cobalt, nickel, tin, lead, antimony, arsenic, bismuth, copper, mercury, silver, palladium, platinum and gold. Other heavy metal ions can be extracted.

Metal can also be recovered pyrolytically from the loaded, dewatered biomass reaction product by fluid-bed combustion or other incineration means using the biomass as fuel.

An example will now be given:

This example demonstrates that metal accumulation in caustic-treated biomass obtained as a biomass reaction product from B. subtilis is greatly enhanced as compared to that obtained with untreated B. subtilis biomass. Metal accumulation is also enhanced when the treated biomass is dried to a hard, solid body, ground and sized, and the biomass product in particulate form contacted countercurrently with a continuous flow of metal solution in a packed or fluidized sorption column. Results obtained in treating dilute silver and copper solutions with caustic-treated biomass as produced after boiling with 0.75M NaOH and which was then dried and ground to -60 mesh are shown in the following Table 1:

A number of other microorganisms, such as Aspergillus flavus and Saccharomyces uvarum, were given substantially the same caustic treatment as described for Bacillus subtilis in Table 1. The NaOH -treated microorganisms tested were examined for metal uptake and were found with respect to certain metal ions to provide improved effectiveness in recovering metal ions from solution, but in some instances, not as effective as the NaOH-treated Bacillus subtilis. Examples of such microorganisms are listed in Table 2 below.

Additional tests with microorganisms have indicated that some microorganisms tended to be more selective to the recovery of certain metal ions than others. For example, a microorganism selected might be superior in metal uptake capacity as to a particular metal ion when compared to B. subtilis, while being less effective compared to B. subtilis as to another metal ion.

Thus, in carrying out the invention, the microorganism selected is that microorganism which provides enhanced metal uptake capacity as to the particulate metal ion of interest, following treatment of the microorganism with caustic. For example, in the treatment of a solution containing metal ions A and B and in which the metal ion of interest is B, the microorganism selected would be one which is particularly selective to the uptake of B. The remaining metal A may then be removed by a biomass reaction product produced from another microorganism more selective to A, although some A may have been taken up by the first biomass reaction product.

Comparative tests were conducted on a series of microorganisms selected from the aforementioned group consisting of bacteria, yeasts, filamentous fungi and algae. In making the comparison, Bacillus subtilis was used as a standard. The results indicated that certain of the microorganisms tended to be more selective to the recovery of a particular metal ion than others. As illustrative of this selectivity, the following example is given.

Different microorganisms were grown and prepared for comparison of their respective capacity for metal accumulation from solutions containing lead as Pb(NO.sub.3).sub.2 and silver as AgNO.sub.3. The microorganisms were tested in the untreated and the caustic-treated state.

Biomass, which was not caustically treated, was harvested from growth medium using centrifugation. The cell paste was washed with water to remove residual medium components and the washed cell paste was oven dried and ground to yield -60 mesh particles.

Caustic-treated biomass was prepared by mixing cell paste with 3% NaOH, removing residual caustic, washing the paste, oven-drying the paste to yield a hard slate-like material, grinding and sizing the material to -60 mesh.

The oven temperature was in the neighborhood of about 80.degree. C. to 100.degree. C.

One-half gram of each prepared biomass granule (-60 mesh size) was added to 990 ml of an approximate 100 mg metal/liter solution shaken for 24 h at 150 rpm and 30.degree. C. The metal loading of the granules was calculated by determining the initial and final metal concentrations in the test solution. Various conditions were used in carrying out the experiment. The biomass was either not treated with NaOH or treated using 3% NaOH. In determining the effect of pH on metal accumulation, one test run for lead accumulation using a solution pH adjusted so that the NaOH-treated granules had a test pH near the value of that for the non-treated granules. The results are summarized in Table 3.

The NaOH treatment enhanced accumulation of silver and lead for every microbe tested except for the yeast, Saccharomyces uvarum, which was selective to lead.

The enhanced lead accumulation was not solely an effect of high pH; the pH-adjusted tests using the NaOH granules indicated similar uptake regardless of pH (about 9 compared to about 6).

Comparison of the microbes tested with B. subtilis showed that most of the microbes exhibited improved metal uptake capacity when treated with caustic. Many of the microbes tested showed higher metal uptake capacity than B. subtilis. Other microorganisms, such as Ps. aeruginosa, S. uvarum and A. flavus, were not as effective, but nevertheless did show improvement with respect to certain metal ions when treated with caustic. However, B. subtilis is preferred in that the biomass reaction product produced therefrom is easier to handle when used to recover metal ions from aqueous solutions.

The granules produced from the B. subtilis, E. coli and S. uvarum are relatively solid but not brittle hard. In a subjective sense, the biomass reaction product might be considered soft, although they are quite stable and substantially insoluble in aqueous solutions. Biomass reaction products produced from Ps. aeruginosa, N. crassa, R. arrhizus, A. flavus, C. vulgaris and C. pyrenoidosa were subjectively softer than the biomass reaction products of the aforementioned microorganisms. However, they gave very effective results with lead and/or silver.

Not all caustic materials have the same properties of NaOH and KOH in producing granules of biomass reaction product. For example, calcium hydroxide is effective, but not as effective as NaOH. Caustic materials included within the invention are those alkaline materials which convert the microorganism into a biomass reaction product which is substantially solid and stable, capable of forming granules and which granules are substantially insoluble in the aqueous solutions being treated. The term "granules" covers particulate biomass reaction products of any particle size capable of being easily handled in a system for carrying out the process.

Granules of the substantially solid biomass reaction product are advantageous in recovering metal ions from solution in that the granules, because of their low density, can be easily suspended like a fluid bed in a column of solution in which the solution is caused to flow upwardly through a supporting column at a residence time sufficient to effect extraction of the metal ions of interest from the solution.

As the granules or particles of the biomass reaction product become loaded with the metal ions of interest, the granules settle to the bottom of the column from which they are subsequently removed.

A glass column is employed which contains the biomass granules at a specified depth. The dry weight of the granules is recorded. A solution at a specified metal concentration is pumped in the upflow direction through the column. Using this process, tests were conducted on a lead nitrate solution using a biomass reaction product produced from causticized B. subtilis. A plurality of one liter solutions containing lead nitrate were passed separately through the column containing 4 grams of the biomass reaction product until a total of 94 one-liter influent volumes had passed through the column. After passage of each one-liter volume, the amount of lead extracted was determined by analyzing the one-volume effluent for residual lead. When the percent recovery reaches less than about 90%, the experiment is terminated.

Following termination, the depth of the settled granules and the final weight (dry weight) are determined. The bottom one inch of granules in the column is assayed for metal content; the bottom one inch is assumed to be near saturation loading. The remaining granules in the column are blended and assayed for metal content. The results are given in Table 4. As will be noted, 99% of the metal is removed from the influent solution. This is determined by analyzing the metal content of the effluent solution.

The solutions tested were dilute, except for two solutions (volumes 20 and 25) which contained shock loadings of 508 and 494 mg/l of lead, respectively. Each pass-through of solution from volumes 1 to 87 showed substantial removal of lead of about 99% for each volume. This test illustrates the very high metal uptake capacity of the 4 grams of biomass reaction product, despite the shock loading of volumes 20 and 25. Substantially all of the lead was removed from the solution by the granules.

Additional tests were conducted on copper sulfate solution. These tests are reported in Tables 5 and 6. The solutions treated in Table 5 had a fairly high influent loading of copper of about 100 ml/l; whereas, in Table 6, the influent loading of copper was much lower and ranged from about 8.5 to 9.5 mg/l of copper.

Both tests showed a fairly high metal uptake capacity of the biomass reaction product.

As will be apparent from the tests summarized in Tables 4 to 6, a biomass reaction product may exhibit different metal uptake properties for different metal ions. Likewise, as will be apparent from Table 3, different biomass reaction products exhibit different selectivity for different metal ions.

Despite such differences, the biomass reaction products described herein have great utility. For example, where it is desired to cleanup wastewater containing toxic metal ions, at least two of which are selective to different biomass products, a mixture of two or more biomass products may be employed to remove the toxic elements.

With regard to the caustic reagent used to carry out the invention, it has been found that a strong detergent material sold under the trademark "ALCONOX" appears to act similarly to sodium hydroxide in treating B. subtilis. This detergent is a long chain alkyl sulfonate which is a wetting agent. It is biodegradable. It contains about 7.3% phosphorus by weight as trisodium phosphate. The pH of the detergent is 9 to 9.5.

Attempts have been made to explain the reason for the selective behavior of caustic-treated microorganisms. While we do not wish to be held to any particular theory, it has been stated that the cell walls of Bacillus subtilis (a Gram-positive bacterium) are considered to be polymeric in nature including strands of mucopeptides (peptidoglycans), teichoic acids or teichuronic acids distributed and attached along their length. The long polymers comprising the mucopeptides are composed of three kinds of building blocks; namely, N-acetylglucosamine (AGA), N-acetylmuramic acid (AMA), and a peptide consisting of four or five amino acids; i.e., L-alanine, D-alanine, D-glutamic acid and either lysine or diaminopimelic acid. A substantial proportion, e.g., about 35% of the mucopeptide strands are cross-linked together through transpeptide (D-alanyl-(L)-meso-diaminopimelyl) bonds. The polymeric structure possesses chemically reactive groups including oxygen, nitrogen, sulfur and phosphorus which carry lone electron pairs that bind metals. Such atoms are effective as electron donors for metal ions. The effects of sodium hydroxide upon such materials are not readily evident although, inter alia, it is believed sodium hydroxide removes lipids and other organic impurities that mask reactive sites. Caustic-treated cell wall material from other Gram-positive bacilli having mucopeptide strands cross-linked with peptide bonds and reacting similarly to caustic treatment may also be employed in accordance with the invention.

The Gram-negative bacterium, E. coli, is also particularly useful as a starting material, in carying out the invention.

Conversion of the caustic-treated biomass derived from B. subtilis cells into particles by drying to a hard or solid body and grinding facilitates handling and promotes treatment of metal-containing aqueous streams to recover the metal content thereof by continuous counter-current decantation, by pulsed bed, fluid bed and other technologies in which a column of liquid undergoing treatment can be employed. A bed of ground particles contacted with upwardly-flowing metal-containing stream can become classified with coarser particles in the lower portion of the bed and finer, more active particles in the upper portion of the bed. In this way, the coarser particles contact fresh feed while the finer particles contact a partially metal depleted stream. This action promotes high loading of the particles and high recovery of metal from the incoming stream.

In summary, the invention comprises various embodiments.

One embodiment is directed to a process for enhancing the metal uptake properties of Bacillus subtilis from aqueous solutions containing metal cations in which cells of the bacterium, Bacillus subtilis, are treated with a caustic solution to form a causticized material consisting essentially of a biomass reaction product. In another embodiment, the biomass reaction product is recovered and dried to form a relatively solid, stable product. The biomass reaction product in the particulate state exhibits substantially enhanced metal uptake capacity as compared to the metal uptake capacity of the Bacillus subtilis in the untreated state. A preferred caustic solution is one selected from the group consisting of NaOH and KOH.

As stated herein, the caustic treatment may be carried out at temperatures ranging up to boiling and preferably over a temperature range of about 50.degree. C. to about 100.degree. C.

The process is applicable to a relatively broad range of microorganisms provided they are characterized by cell wall structures. Thus, the process is applicable to cells of bacilli, such as Gram-positive bacteria (e.g., B. subtilis) and Gram-negative bacteria (e.g., E. coli). Examples of microorganisms are bacteria, yeast, fungi and algae, so long as the microorganism is characterized by a cell wall structure and is selective to uptake of metal ions, and which when treated with a caustic solution produces a relatively solid, stable biomass rection product having substantially enhanced metal uptake capcity for metal ions compared to the uptake capacity before treatment.

The biomass reaction product of the invention consists essentially of a causticized non-living microorganism.

A particulr advantage of the invention is that a microorganism-derived biomass reaction product, in the form of a solid, stable particulate material, can be used as a fluid bed in an aqueous solution containing metal cations with the solution flowing countercurrently through the bed. As the particles of the bed become loaded, they segregate to a lower part of the bed from which they can be removed for the subsequent recovery of metal therefrom.

The particulate or granular biomass may be configured in a packed column suitable for either upflow or downflow operation.

The granular biomass may be employed in an unrestrained, upflow column to allow fluidization of said granular biomass.

Where the treated biomass has been formed into a hard, grindable body, the biomass is ground to provide a variety of particle sizes which may be employed in an upflow column. The aqueous solution is passed at a rate causing stratification of the treated biomass with larger biomass particles in the lower portion of the column and finer biomass particles in the upper portions of the column, such that the smaller and more active biomass particles or granules contact the solution after the bulk of the cations have been removed therefrom by the larger particles. The biomass in the column may be operated as a pulsed bed, with the solution moving upwardly in the column, while freshly treated biomass is introduced position-wise at a point toward the top of the column while metal-laden biomass is removed from a point toward the bottom of the column. The respective volumes of the fresh biomass and the metal-laden biomass may be substantially equal, the metal-laden biomass being removed at substantially the same time that the fresh biomass portions are added.

The treated biomass may comprise polymeric cell wall material derived from a Gram-positive bacterium having mucopeptide strands cross-linked with transpeptide bonds.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.