Patent Publication Number: US-2012034286-A1

Title: Stabilization of biosolids using iron nanoparticles

Description:
FIELD OF THE INVENTION 
     The present invention relates to the stabilization and processing of biosolids. In particular, this invention is directed to stabilization of biosolids compositions using iron nanoparticles. 
     BACKGROUND OF THE INVENTION 
     The term “biosolids” refers to organic sludge generated in municipal wastewater treatment. Of the many approaches utilized, land application is the most commonly used disposal or remediation method. However, nuisance odors and the potential of pathogen and toxic chemical transmission have severely limited the practice of land application. Biosolids include, but are not limited to, substantial amounts of organic substances. For example, 59% to 88% by weight of biosolids from activated sludge treatment of municipal wastewater are biological materials and an assortment of organic compounds. 
     Biosolids are often disposed of, dispersed or distributed on agricultural land, in forests, rangelands, or on disturbed or environmentally impacted land in need of reclamation or remediation. These applications are referred to as land applications. Disposal, dispersal or distribution of biosolids through land application serves many beneficial purposes. It provides an economic method for the disposal of large volumes of biosolids generated everyday. Land application minimizes soil erosion, improves soil properties, including but not limited to texture and water holding capacity, making conditions on the treated land more favorable for root growth and increasing drought tolerance of vegetation. Biosolids also include nutrients and elements essential for plant growth, including but not limited to nitrogen and phosphorous (Table 1), as well as other essential nutrients including but not limited to iron and zinc. The nutrients in biosolids compositions serve as alternatives to, or substitutes for, expensive chemical fertilizers. Furthermore, the nutrients in biosolids offer certain advantages over those nutrients available in inorganic fertilizers because they are biological and organic in character and are released slowly to growing plants. Biological and organic forms of nutrients are less water soluble and, therefore, less likely to leach into groundwater or runoff into surface water. EPA estimates that more than 7 million dry tons of biosolids are generated annually for use, disposal, dispersal or distribution by the over 16,000 wastewater treatment facilities in the U.S., of which approximately 60% are land applied, composted, or used as landfill cover. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Comparison of nutrient levels in commercial fertilizers and biosolids 
               
            
           
           
               
               
               
               
            
               
                   
                 Nitrogen 
                 Phosphorus 
                 Potassium 
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 Fertilizers for 
                 5% 
                 10% 
                 10% 
               
               
                   
                 typical agricultural 
                   
                   
                   
               
               
                   
                 use 
                   
                   
                   
               
               
                   
                 Typical biosolids 
                 1.6-3.0% 
                 1.5-4.0% 
                 0-3% 
               
               
                   
               
            
           
         
       
     
     International Patent Publication WO 2003014031 A1 discloses a method of disinfecting and stabilizing organic wastes where organic waste is intimately mixed with one or more mineral by-products to produce a mixture having a pH of less than about 9. The mixture is heated and dried to produce a stable, granular bio-mineral product that may be used for example, as a fertilizer, soil amendment or as a soil substitute. 
     SUMMARY OF THE INVENTION 
     The present invention provides stabilized biosolids compositions by combining biosolids with nanometer sized metal particles having high surface areas. A surface promoted reaction of biological materials and organic compounds with the ultrafine metal particles (ultrafine defined herein as particles having particle sizes &lt;1 μm), including but not limited to iron particles, provides stabilized biosolids having reduced or minimal amounts of odorous and halogen containing biological materials and organic compounds in addition to inhibiting the growth of one or more pathogens. The iron particles are capable of degrading odorous inorganic sulfur and organosulfur compounds, chemically transforming (thus minimizing or eliminating) persistent and toxic organic pollutants such as polychlorinatedbiphenyls (PCBs) and chlorinated pesticides, immobilizing toxic metal ions such as mercury ions and lead ions, and improving the overall quality of the treated biosolids for land application and plant growth. Iron is an environment-friendly element and its compounds have a low adverse environmental impact. In fact, iron is an essential element for human, animal and plant growth. This invention offers significant advantages over prior art methods in the speed of treatment and processing of biosolids, in the efficiency of controlling and minimizing odors associated with biosolids, in the simplicity of implementing biosolids processing, in reducing overall operational costs associated with the processing of biosolids. 
     Accordingly, the present invention provides a stabilized biosolids composition comprising oxidizable iron particles having diameters between 1 to 200 nm and having specific surface areas from 1000 to 763,358 m 2 /kg. 
     The present invention also provides a method for stabilizing and reducing or eliminating one or more pathogens in a biosolids composition including the step of combining a biosolids composition with iron particles having diameters between 1 to 200 nm to form a stabilized biosolids composition, as compared to an untreated biosolids composition. 
     The present invention also provides a method for removing or eliminating odors associated with one or more pathogens in a biosolids composition including the step of treating a biosolids composition with iron particles having diameters between 1 to 200 nm, reducing or eliminating odorous biological and organic compounds, as compared to an untreated biosolids composition. 
     The present invention also provides a method for dehalogenating one or more halogen containing compounds comprising the steps of combining the biosolids composition with iron particles having diameters between 1 to 200 nm, and lowering or eliminating the halogen content, as compared to an untreated biosolids composition. 
     The present invention also provides a method for increasing, controlling or maintaining pH of a biosolids composition including the step of combining the biosolids composition with iron particles having diameters between 1 to 200 nm, as compared to an untreated biosolids composition. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  summarizes how iron nanoparticles serve as a versatile reagent for the treatment of biosolids. 
         FIG. 2  is a transmission electron micrograph of a single iron nanoparticle (˜66 nm in diameter). 
         FIG. 3  is a transmission electron micrograph of iron nanoparticles synthesized by the reduction of ferric iron (Fe 3+ ) using sodium borohydride 
         FIG. 4  summarizes changes of solution pH as a function of time and concentration of iron nanoparticles. 
         FIG. 5  summarizes changes of solution E h  as a function of time and concentration of iron microparticles. 
         FIG. 6  summarizes a E h -pH diagram of Fe(0) in water. 
         FIG. 7  summarizes immobilization of metal ions (“M n+ ) with nanoscale iron particles. 
         FIG. 8  summarizes structures of the environmentally significant HCH isomers, including the two HCH enantiomers. 
         FIG. 9  summarizes the effectiveness of nanoscale iron (“nFe”) particles in removing the HCHs from solution as a function of dosage. 
         FIG. 10  is a schematic diagram of an iron particle feeding and mixing system in combination with conventional lime stabilization. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Biosolids refer to sludge produced in various wastewater treatment processes. Biosolids consist primarily of organic substances. For example, 59%-88% by weight of biosolids from activated sludge treatment of municipal wastewater are organic materials. 
     Biosolids are often disposed on agricultural land, forests, rangelands, or disturbed land in need of reclamation. This is known as land application. Disposal of biosolids through land application serves many beneficial purposes. It provides an economic method for the disposal of large volumes of biosolids generated everyday. Land applications reduce the erosion of soil, improve soil properties, such as texture, and water holding capacity. This makes conditions more favorable for root growth and increases the drought tolerance of vegetation. Biosolids also contain nutrients essential for plant growth, including nitrogen and phosphorous (Table 1), as well as other essential nutrients such as iron and zinc. The nutrients in the biosolids serve as alternatives or substitutes for expensive chemical fertilizers. Furthermore, the nutrients in the biosolids offer certain advantages over those in inorganic fertilizers because they are organic and are released slowly to growing plants. Organic forms of nutrients are less water soluble and, therefore, less likely to leach into groundwater or runoff into surface water. 
     EPA estimates that more than 7 million dry tons of biosolids are generated annually for use or disposal by the 16,000 wastewater treatment facilities nationwide. Of this, approximately 60% are land applied, composted, or used as landfill cover. 
     Federal regulations require that biosolids be processed before they are applied to land. The treatment processes are often termed as “stabilization” as they help minimize odor generation, destroy pathogens (disease causing organisms), and reduce vector attraction potential. Details of the federal regulations can be found in “Standard for the Use and Disposal of Sewage Sludge, the Part 503 Rule” by U.S. Environmental Protection Agency. 
     Nuisance odor is often the number one complaint/objection associated with land applications. It is encountered frequently when the beneficial use sites are close to residential areas. Odors are mostly generated by biological activities in the biosolids with various biodegradable organic substances. For example, biosolids from activated sludge treatment contains a variety of proteins. Naturally, biosolids are a rich source of food for microorganisms. Due to the lack of oxygen, the biological reactions in biosolids are mostly anaerobic producing various odorous compounds. Organic and inorganic compounds of sulfur and nitrogen (Table 2) produce the most offensive odor causing compounds in biosolids, 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Examples of odorous compounds commonly found in biosolids 
               
            
           
           
               
               
               
               
            
               
                   
                 Compound 
                 Formula 
                 Threshold (ppm) 
               
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Ammonia 
                 NH 3   
                 46.8 
               
               
                   
                 Hydrogen sulfide 
                 H 2 S 
                 0.00047 
               
               
                   
                 Meth I merca ton 
                 CH 3 SH 
                 0.0021 
               
               
                   
                 Dimeth I sulfide 
                 CH 3 CH 3   
                 0.0001 
               
               
                   
                 Indole 
                 C 8 H 6 NH 
                 0.0001 
               
               
                   
               
            
           
         
       
     
     Biosolids also contain various metals in the form of water soluble metal cations. Those metals can be detrimental to plants and animals. The term “heavy metal” has been often used to denote several of trace metals in biosolids. Concentrations of heavy metals vary widely as indicated in Table 3. For land applications, the presence of heavy metals limits both the application rate and useful life span of the disposal site. 
     Biosolids also have a large variety of microorganisms and viruses. Some are disease-causing pathogens such as bacteria ( salmonella  sp.,  vibrio cholerae ), protozoa ( Giardia lamblia ), viruses (hepatitis and Norwalk). Table 3 gives examples of common pathogens in biosolids. 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Examples of pathogens potentially present in biosolids. 
               
            
           
           
               
               
               
            
               
                 Organism 
                 Disease 
                 Symptoms 
               
               
                   
               
               
                 Bacteria 
                   
                   
               
               
                 
                   E. Coli 
                 
                 Gastroenteritis 
                 Diarrhea 
               
               
                 
                   Legionella pneumophila 
                 
                 Legionnaires&#39; 
                 Malaise, fever, 
               
               
                   
                 disease 
                 myalgia, etc. 
               
               
                 
                   Salmonella 
                 
                 Salmonellosis 
                 Food poisoning 
               
               
                 Protozoa 
                   
                   
               
               
                 
                   Cryptosporidium parvum 
                 
                 Cryptosporidiosis 
                 Diarrhea 
               
               
                 
                   Giardia lambila 
                 
                 Giardiasis 
                 Diarrhea, nausea 
               
               
                 
                   Balantidium coli 
                 
                 Balantidiasis 
                 Diarrhea sent 
               
               
                 Helminthes 
                   
                   
               
               
                 
                   Ascaris lumbricoides 
                 
                 Ascariasis 
                 Roundworm infestation 
               
               
                 
                   Enterobius vermicularis 
                 
                 Enterobiasis 
                 Pineworm 
               
               
                 
                   T. solium 
                 
                 Taeniasis 
                 Park tapeworm 
               
               
                 Viruses 
                   
                   
               
               
                 Adenovirus 
                 Respiratory disease 
                   
               
               
                 Enteroviruses 
                 Gastroenteritis etc. 
                   
               
               
                 Hepatitis A virus 
                 Infectious hepatitis 
               
               
                   
               
            
           
         
       
     
     Conventional methods used in the treatment of biosolids include biological digestion, alkaline treatment, composting, heat drying and pelletizing (Table 4). These methods are often expensive, time consuming, and ineffective in terms of abating odors and immobilizing heavy metals associated with biosolids. 
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                 Common Methods for Biosolids Stabilization 
               
            
           
           
               
               
            
               
                 Treatment Processes 
                 Use or Disposal Methods 
               
               
                   
               
               
                 Aerobic or Anaerobic 
                 Produces biosolids used as s soil 
               
               
                 Digestion 
                 amendment and organic fertilizer on 
               
               
                   
                 pasture and row crops, forests, and 
               
               
                   
                 reclamation sites. 
               
               
                 Alkaline Treatment 
                 Produce biosolids useful for land 
               
               
                   
                 application and for use as daily landfill 
               
               
                   
                 cover. 
               
               
                 Composting 
                 Produces highly organic, soil-like biosolids 
               
               
                   
                 with conditioning properties for 
               
               
                   
                 horticultural, nursery, and landscape uses. 
               
               
                 Heat-Drying/Pelletizing 
                 Produces biosolids for fertilizers generally 
               
               
                   
                 used at a low rate because of higher cost 
               
               
                   
                 and higher nitrogen content 
               
               
                   
               
            
           
         
       
     
     The present invention provides a new procedure for biosolids stabilization. Specifically, reactive metal particles, especially ultra fine, nanometer sized iron nanoparticles are useful for degrading and stabilizing the above-described pollutants ( FIG. 1 ). The oxidizable iron nanoparticles are useful, according to an exemplary embodiment, for treating biosolids. 
     Nanometer sized or nanoscale iron particles in general refer to iron particles having diameters in the range of from 1 to 1000 nm, including from 1 to 500 nm, also including from 1 to 200 nm, including from 1 to 100 nm. The metal nanoparticles are characterized by transmission electron microscopy, for example, to determine particle size ( FIG. 2 ). 
     According to one exemplary embodiment, the iron particles are combined with other metal particles having diameters from 1 to 200 nm. According to a separate embodiment, the iron particles are combined with other metal particles having diameters from 200 nm to 1,000,000 nm (1,000 μm). For many environmental applications, the reactive component is the metallic or zero-valent iron (Fe 0 ). 
     Zero-valent iron (Fe 0 ) is a reactive material, which readily reacts with dissolved oxygen: 
       2Fe 0   (s) +4H +   (aq) +0 2(aq) →2Fe +2   (aq) +2H 2 O  (1)
 
     Equation 1 summarizes a classical corrosion reaction by which iron is oxidized from exposure to oxygen and water. In accordance with the invention, iron functions as an efficient and relatively inexpensive electron donor, as shown in equation 2: 
       Fe→Fe 2+ +2 e   −   (2)
 
     Oxidized ferrous iron (Fe(II)) can be further oxidized to ferric iron (Fe(III)). Although oxygen and water are common electron acceptors in the environment, other substances including many environmental contaminants can serve as electron acceptors. For example, trichloroethene (TCE), a common contaminant, can receive the electrons from iron and be reduced to ethene as summarized in equation (3): 
       C 2 HCl 3 +3H + +6 e   − →C 2 H 4 +3Cl −   (3)
 
     In accordance with the present invention, electrons released from iron oxidation are utilized for the stabilization of biosolids. Numerous studies on the hydrodechlorination of chlorinated organic contaminants using iron nanoparticles have been reported. Field tests have demonstrated the potential of iron nanoparticles for in-situ remediation. Recent work has expanded the applications to the remediation of polychlorinated biphenyls (PCBs), perchlorate, nitrate, heavy metal ions such as Cr(VI) and Cd(II), and organochlorine pesticides such as DDT and hexachlorocyclohexane. 
     Nanoparticles provide novel materials with unique reactivity toward targeted contaminants, enhanced mobility in environmental media, and ease of use. The foremost imperative advantage of iron nanoparticles is the large surface area. Iron oxidation is surface mediated. That is, the larger the iron surface, the higher the reaction rate. Stated in another way, the smaller the particle size, the higher the potential reaction rate. For a spherical particle with a diameter of d, surface area per unit of mass, or specific surface area (SSA) can be calculated by the following equation: 
     
       
         
           
             
               
                 
                   SSA 
                   = 
                   
                     
                       
                         Surface 
                          
                         
                             
                         
                          
                         Area 
                       
                       Mass 
                     
                     = 
                     
                       
                         
                           π 
                            
                           
                               
                           
                            
                           
                             d 
                             2 
                           
                         
                         
                           ρ 
                            
                           
                             π 
                             6 
                           
                            
                           
                             d 
                             3 
                           
                         
                       
                       = 
                       
                         6 
                         
                           ρ 
                            
                           
                               
                           
                            
                           d 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     Where ρ is the density (kg/m 3 ) of iron particles. 
     For the procedure described in this invention, particles both large and small can be used. However, smaller particles offer the advantage of large reactive surface area per unit of iron mass. Smaller amounts of iron are thus needed to achieve biosolid stabilization. This is particularly true for ultrafine particles with diameters less than 100 nanometers. For example, iron particles having particle sizes of 50 nm have SSA of 15,000 m 2 /kg (Table 3). In comparison, iron powders having a diameter of 1 mm have a theoretical SSA of only 0.77 m 2 /kg. 
     
       
         
           
               
             
               
                 TABLE 5 
               
             
            
               
                   
               
               
                 Theoretical specific surface areas (SSA) of spherical iron particles 
               
            
           
           
               
               
               
            
               
                   
                 Diameter (d) 
                 SSA (m 2 /kg) 
               
               
                   
               
            
           
           
               
               
               
            
               
                   
                 1 nm 
                 763,358 
               
               
                   
                 1 μm 
                 763 
               
               
                   
                 1 mm 
                 0.763 
               
               
                   
               
               
                 * Calculated using equation (4) with     at 7860 kg/m 3  for iron. 
               
            
           
         
       
     
     Iron Nanoparticles for pH Adjustment of Biosolids 
     Iron also reacts with water as summarized in equation (5): 
       Fe 0   (s) +2H 2  0 (aq) →Fe +2   (aq) +H 2(g) +20H −   (5)
 
     According to the above reaction, the iron-mediated reactions generate hydroxyl ions (OH − ) and result in a characteristic increase in pH. 
     Experimentally measured pH trends in water are illustrated in  FIG. 4 .  FIG. 4  summarizes changes of said solution pH as a function of time and concentration of iron nanoparticles. Typically, the addition of even a small amount (e.g., &lt;1 g/L) of iron particles can raise and maintain the water pH in the range of 8-10. 
     The pH of the environment is a key factor in the growth of organisms. Most bacteria cannot tolerate pH levels above 9.5 or below 4.0. Generally, the optimum pH for bacterial growth lies between 6.5 and 7.5. Alkaline treatment in which a large amount of lime is added to biosolids has been frequently used in biosolid stabilization. In this context, the addition of iron nanoparticles function to increase pH, inhibiting the growth of pathogens in biosolids. 
     As described above, iron rapidly reacts and consumes oxygen in water. At adequate doses, iron nanoparticles can deplete all dissolved oxygen in biosolids.  FIG. 5  summarizes changes of solution oxidation-reduction potentials, E h , as a function of time and concentration of iron microparticles. As shown in  FIG. 5 , solution oxidation-reduction potential decreases rapidly, indicating a highly reducing environment as a result of the addition of iron nanoparticles. As a result, microorganisms (including pathogens) grow much more slowly in the presence of iron nanoparticles. 
     Iron Nanoparticles for Controlling Odors in Biosolids 
     Biosolids often have their own distinctive odor depending on the type of treatment it has been subjected to. Nuisance odors can have detrimental effects on aesthetics, property values and the quality of life in communities subjected to them. Odor complaints often lead to long-term problems. For example, local public opposition can delay or prevent the beneficial reuse of biosolids. Nuisance odors are often the number one complaint associated with land disposal of biosolids. 
     Compounds that contain sulfur cause most odors. For example, hydrogen sulfide (H 2 S) is one compound that contributes to the odor associated with rotten eggs and gives off the characteristic pungent odor. Common odor-causing compounds in biosolids are listed in Table 2. Organic sulfur compounds such as mercaptans and methyl sulfide are identified as the most offensive odor causing compounds associated with biosolids handling and application. These compounds typically are released from biosolids by heat, aeration and digestion. 
     Sulfur, which is directly below oxygen in the periodic table of the elements, has many properties similar to those of oxygen. One of them is the reaction with metallic iron. For example, sulfate, like oxygen, can accept electrons from iron oxidation and be reduced to elemental sulfur: 
       3Fe+SO 4   2− +8H→3Fe 2+ +S⇓+4H 2 O⇑  (6)
 
     Sulfides (S 2− ) on the other hand precipitate with ferrous iron as both iron sulfide (FeS) and pyrite (FeS 2 ), and have a relatively low solubility in water: 
       S 2− +Fe 2+ →FeS⇓  (7)
 
       FIG. 6  summarizes oxidation-reduction potentials—pH (E h -pH) diagram of zero valent iron (Fe 0 ) in water. Thermodynamically, the most stable forms of sulfur are FeS and FeS 2  ( FIG. 6 ) in the presence of iron nanoparticles. 
     Iron also reacts with sulfur-containing organic compounds such as dimethyl sulfide, and dimethyl disulfide as summarized in equations (8) and (9): 
       CH 3 —S—CH 3 +Fe+2H + →Fe 2+ +2S⇓+2CH 4 ⇑  (8)
 
       CH 3 —S—S—CH 3 +Fe+2H + →Fe 2+ +2S⇓+2CH 4 ⇑  (9)
 
     The above reactions point to probable mechanisms for odor reduction where sulfur containing compounds can react and bind to reactive iron particles. 
     It should be noted that the formation of volatile H 2 S is not likely in the presence of zerovalent iron particles. The reactions of iron with water typically increase the solution pH to 8-10 as shown in  FIG. 4 . Sulfide (S 2− ) likely exists as HS −  in this pH range. 
     Iron for Metal Ion Reduction and Immobilization 
     As noted above, biosolids contain a large number of metals (Table 6). Among them, federal regulations (Part 503 Rule) have numerical limits on 10 metals. The ten metals are: arsenic, cadmium, chromium, copper, lead, mercury, molybdenum, nickel, selenium, and zinc. As shown in this invention, reactive iron can react and immobilize most regulated metals in biosolids. 
     
       
         
           
               
             
               
                 TABLE 6 
               
             
            
               
                   
               
               
                 Typical metal contents in biosolids (U.S. EPA 1984) 
               
            
           
           
               
               
               
               
            
               
                   
                   
                 Concentration range 
                 Median concentration 
               
               
                   
                 Metal 
                 (mg/kg) 
                 (mg/kg) 
               
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Arsenic 
                  1.1-230 
                 10 
               
               
                   
                 Cadmium 
                    1-3,410 
                 10 
               
               
                   
                 Chromium 
                    10-99,000 
                 500 
               
               
                   
                 Cobalt 
                  11.3-2,490 
                 30 
               
               
                   
                 Copper 
                    84-17,000 
                 800 
               
               
                   
                 Iron 
                  1,000-154,000 
                 17,000 
               
               
                   
                 Lead 
                    13-26,000 
                 500 
               
               
                   
                 Manganese 
                   32-9,870 
                 260 
               
               
                   
                 Mercury 
                 0.6-56  
                 6 
               
               
                   
                 Molybdenum 
                  0.1-214 
                 4 
               
               
                   
                 Nickel 
                    2-5,300 
                 80 
               
               
                   
                 Selenium 
                  1.7-17.2 
                 5 
               
               
                   
                 Tin 
                  2.6-329 
                 14 
               
               
                   
                 Zinc 
                   101-49,000 
                 1,700 
               
               
                   
               
            
           
         
       
     
     From classical corrosion chemistry, it is known that reactive metals such as iron have relatively low standard potentials and can serve as the electron donors or reductants for the reduction and precipitation of less reactive metal ions. As shown in Table 7, iron is more reactive and can reduce metal ions such as Ni, Pb, Cu, Ag, Cd, and Hg. The reduced metals could precipitate on solid surfaces such as iron and soil particles ( FIG. 7 ).  FIG. 7  summarizes immobilization of metal ions M n+  with iron nanoparticles. 
                     TABLE 7                  Standard electrode potentials at 25° C. Zero-valent iron can reduce metal       ions with standard potential higher than that of iron (−0.41 V).                         EO (volts)                                 Zinc (Zn)   Zn 2+  + 2 e− &lt;=&gt; Zn   −0.76       Iron (Fe)   Fe 2+  + 2 e− &lt;=&gt; Fe   −0.41       Cadmium (Cd)   Cd 2+  + 2 e− &lt;=&gt; Cd   −0.40       Cobalt (Co)   Co 2+  + 2 e− &lt;=&gt; Co   −0.28       Nickel (Ni)   Ni 2+  + 2 e− &lt;=&gt; Ni   −0.24       Tin (Sn)   Sn 2+  + 2 e− &lt;=&gt; Sn   −0.13       Lead (Pb)   Pb 2+  + 2 e− &lt;=&gt; Pb   −0.13       Copper (Co)   Cu 2+  + 2 e− &lt;=&gt; Cu   0.34       Silver (Ag)   Ag 2+  + e− &lt;=&gt; Ag   0.80       Mercury (Hg)   Hg 2+  + 2e− &lt;=&gt; Hg   0.86       Chromium (Cr)   Cr 2 0 7   2−  + I4H +  + 6e −  &lt;=&gt;2Cr 3+  + 7H 2 O   1.36                    
For example, if iron nanoparticles are added to biosolids containing nickel (Ni), Ni(II) can be reduced and precipitated out of water as summarized in equation (10):
 
       Ni 2+ +Fe→Ni+⇓Fe 2+   (10)
 
     The reduced toxic metals have low solubility in water and thus are less likely to leach into groundwater and runoff into surface water. Bioavailability and biotoxicity of the reduced metals are expected to be lower too. According to one exemplary embodiment of the invention, the oxidizable iron particles reduce and immobilize or eliminate toxicity associated with metal ions selected from the group of metals consisting of: cadmium, copper, lead, nickel, cobalt, mercury, chromium, and combinations of metal ions. 
     According to one exemplary embodiment, reduction and immobilization of chromium is used as an example to demonstrate the ability of iron nanoparticies for metal ion stabilization. 
     Chromium is one of the most commonly used metals and also one of the frequently detected inorganic contaminants in soil and water. Chromium has high and acute toxicity to humans, animals, plants, and microorganisms, and is classified as a potential carcinogen. 
     Chromium in natural waters exists primarily in +3 and +6 valence states. Hexavalent chromium, Cr(VI), such as chromate (2CrO 4   2− ) is highly soluble and mobile in aquatic systems. On the other hand, trivalent chromium [Cr(III)] is relatively stable and has low solubility (&lt;10 −5  M) in aqueous solutions over a wide pH value range. Hexavalent chromium can be reduced to trivalent chromium by iron nanoparticles as shown in equation (11): 
       2CrO 4   2− +3Fe+10H + →2Cr(OH) 3 ⇓+⇓2H 2 0  (11)
 
     Table 8 presents results of hexavalent chromium reduction by iron nanoparticles from a laboratory experiment with contaminated groundwater and soil samples. Groundwater and soil samples were collected from an industrial site in New Jersey. The groundwater contained 42.83±0.52 mg Cr/L, and the soil had 3,280±90 mg Cr/kg. This study shows that one gram of nanoparticles can reduce 84.4-109.3 mg Cr(VI) in the groundwater and 69.28-72.65 mg Cr(VI) in soil/groundwater slurries, respectively. This reduction capacity is 50-70 times greater than that of microscale under similar experimental conditions. 
     
       
         
           
               
             
               
                 TABLE 8 
               
             
            
               
                   
               
               
                 Reductive capacity of Cr(VI) by Iron 
               
            
           
           
               
               
               
            
               
                   
                 Type of Fe 
                   
               
               
                   
                 particles 
                 mgCr(VI)/g Fe 
               
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Groundwater 
                 Micron-sized Fe 
                 1.53-1.75 
               
               
                   
                   
                 Nano-sized Fe 
                  84.40-109.30 
               
               
                   
                 Soil (in distilled 
                 Micron-sized Fe 
                 1.26-1.33 
               
               
                   
                 water) 
                 Nano-sized Fe 
                 64.16-67.67 
               
               
                   
                 Soil/groundwater 
                 Micron-sized Fe 
                 1.07-1.12 
               
               
                   
                   
                 Nano-sized Fe 
                 69.28-72.65 
               
               
                   
               
            
           
         
       
     
     Iron Nanoparticles for Dechlorinating Chlorinated Aliphatic Compounds 
     The environmental chemistry of metallic or zero-valent iron has been extensively studied. One of the best-documented examples is dechlorination and hydrogenation of chlorinated hydrocarbons (RCI), as summarized in equation (12): 
       RCl+Fe 0 +H + →Fe 2+ +Cl −   (12)
 
     Research at Lehigh University has examined a large number of chlorinated compounds (Table 9). Most of them can be quickly dechlorinated by the iron nanoparticles. 
     According to an exemplary embodiment, the invention provides a method for dehalogenating one or more halogen containing compounds comprising the steps of combining the biosolids composition with iron particles having diameters between 1 to 200 nm, and lowering or eliminating the halogen content, as compared to an untreated biosolids composition. Treatment of hexachlorocyclohexanes (“HCHs”), one of the most widely used pesticides, serves as an example of the application of iron nanoparticles for treatment of persistent organic contaminants. Chlorinated pesticides have been widely used as insecticides, fungicides, and herbicides. These pesticides have been discovered to have harmful side effects as they do not readily degrade in nature and tend to accumulate in fatty tissues of most mammals. Perhaps the most infamous of all chlorinated pesticides is DDT. Other compounds may include hexachlorobenzene, chlordane, and dieldrin. These compounds are all amenable for degradation by iron nanoparticles. 
     HCHs are a well-known and widely studied class of organochlorine pesticides.  FIG. 8  summarizes structures of environmentally significant HCH isomers, including the two HCH enantiomers. Four isomers, namely gamma, alpha, beta, and delta HCHs ( FIG. 8 ), are of major environmental concern due to their toxicity, sorption and bioconcentration potential, and relative stability within the environment. 
     In a laboratory study, groundwater and aquifer samples from a site contaminated by HCHs were exposed to the nanoscale iron particles in batch reactors. The total HCH burden in site groundwater was approximately 700 μg/L.  FIG. 9  summarizes the effectiveness of annoscale iron particles in removing HCHs from solution as a function of dosage. In general, batch experiments with 2.2-27.0 g/L iron nanoparticles showed that more than 95% of the HCHs were removed from solution within 48 hours ( FIG. 9 ). The observed pseudo first-order rate constants (k obs ) were in the range of 0.04-0.65 hr −1 . 
     
       
         
           
               
             
               
                 TABLE 9 
               
               
                   
               
               
                 Common environmental contaminants that can be degraded by the 
               
               
                 nanoscale iron particles. 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
            
               
                   
                 Chlorinated Methanes 
               
               
                   
                 Carbon tetrachloride (CCl 4 ) 
               
               
                   
                 Chloroform (CHCl 3 ) 
               
               
                   
                 Dichloromethane (CH 2 Cl 2 ) 
               
               
                   
                 Chloromethane CH 3 Cl 
               
               
                   
                 Chlorinated Benzenes 
               
               
                   
                 Hexachlorobenzene (C 6 CL 6 ) 
               
               
                   
                 Pentachlorobenzene (C 6 HCl 5 ) 
               
               
                   
                 Tetrachlorobenzenes (C 6 H 2 Cl 4 ) 
               
               
                   
                 Trichlorobenzenes (C 6 H 3 Cl 3 ) 
               
               
                   
                 Dichlorobenzenes (C 6 H 4 Cl 2 ) 
               
               
                   
                 Chlorobenzene (C 6 H 5 Cl) 
               
               
                   
                 Pesticides 
               
               
                   
                 DDT (C 14 H 9 Cl 5 ) 
               
               
                   
                 Lindane (C 6 H 6 Cl 6 ) 
               
               
                   
                 Organic Dyes 
               
               
                   
                 Orange II (C 16 H 11 N 2 NaO 4 S) 
               
               
                   
                 Chrysoldin (C 12 H 13 ClN 4 ) 
               
               
                   
                 Tropaeolin (C 12 H 9 N 2 NaO 5 S) 
               
               
                   
                 Trihalomethanes 
               
               
                   
                 Bromoform (CHBr 3 ) 
               
               
                   
                 Dibromochloromethane (CHBr 2 Cl) 
               
               
                   
                 Oichlorobromomethane (CHBrCl 2 ) 
               
               
                   
                 Chlorinated Ethenes 
               
               
                   
                 Tetrachloroethene (C 2 Cl 4 ) 
               
               
                   
                 Trichloroethene (C 2 HCl 3 ) 
               
               
                   
                 cis-Dichloroethene (C 2 H 2 Cl 2 ) 
               
               
                   
                 trans-Dichloroethene (C 2 H 2 Cl 2 ) 
               
               
                   
                 1,1-Dichloroethene (C 2 H 2 Cl 2 ) 
               
               
                   
                 Vinyl Chloride (C 2 H 3 Cl) 
               
               
                   
                 Other Polychlorinated Hydrocarbons 
               
               
                   
                 PCBs 
               
               
                   
                 Pentachlorophenol (C 6 HCl 5 O) 
               
               
                   
                 Other Organic Contaminants 
               
               
                   
                 N-nitrosodiumethylamine (NDMA)  
               
               
                   
                 (C 4 H 10 N 2 O) 
               
               
                   
                 TNT (C 7 H 5 N 3 O 6 ) 
               
               
                   
               
            
           
         
       
     
     Reducing Pathogens Using Metal Nanoparticles 
     Pathogenic organisms in biosolids may be excreted by human beings and animals who are infected with disease or who are carriers of a particular infectious disease. The pathogenic organisms in biosolids can be classified into four broad categories: bacteria, protozoa, helminthes, and viruses. Examples of pathogenic organisms found in biosolids are listed in Table 3. 
     Federal regulations for biosolids management (Part 503 pathogen-reduction requirements) are divided into Class A and Class B categories. The goal of Class A requirements is to reduce the pathogens in the biosolids (including  Salmonella  sp. Bacteria, enteric virus, and viable helminth ova) to below detectable levels. When this goal is achieved, Class A biosolids can be land applied without any pathogen-related restrictions on the site. The goal of the Class B requirements is to ensure that pathogens have been reduced to levels that are unlikely to pose a threat to public health and the environment under specific use conditions. The Part 503 has strict restrictions for the use of Class B biosolids, therefore, effort has been focused on producing relatively clean Class A biosolids. 
     According to an exemplary embodiment, the invention provides a method for stabilizing and reducing or eliminating one or more pathogens in a biosolids composition comprising the step of: combining a biosolids composition with iron particles having diameters between 1 to 200 nm to form a stabilized biosolids composition, as compared to an untreated biosolids composition. According to a separate embodiment, the invention provides a method for removing or eliminating odors associated with one or more pathogens in a biosolids composition comprising the step of: treating a biosolids composition with iron particles having diameters between 1 to 200 nm, reducing or eliminating amounts of odorous biological and organic compounds, as compared to an untreated biosolids composition. The presence of iron nanoparticles should have both direct and indirect impacts on microorganisms. Direct impacts might include uptake, transformation and accumulation of iron nanoparticles by microorganisms. Direct exposure of large dose of iron nanoparticles can cause detrimental effect on microorganisms as iron nanoparticles can penetrate cell membranes and bind to proteins and DNAs. According to an exemplary embodiment, the oxidizable iron particles react with one or more pathogens in the biosolids composition to minimize, de-activate or control odors associated with said one or more pathogens. According to a separate exemplary embodiment, the oxidizable iron particles reduce and immobilize or eliminate odor-producing hydrogen sulfide in pathogens. According to another embodiment, the oxidizable iron particles increase biosolids pH relative to an initial biosolids pH to inhibit growth of one or more pathogens. 
     Indirect impact entails the reduction of organisms as a result of rapid changes in environmental conditions (e.g., low E h ). The reactions of iron with water and other oxidants in water (e.g., oxygen, nitrate, sulfate etc.) produce a rapid decrease in solution standard potential (E h ) and increase in pH. A drastic change in the water chemistry including the depletion of dissolved oxygen, could lead to death or at least slow the growth of many microorganisms, especially aerobic microorganisms which respire an dissolved oxygen in water. According to an exemplary embodiment, a method is included for increasing, controlling or maintaining pH of a biosolids composition comprising the step of combining the biosolids composition with iron particles having diameters between 1 to 200 nm, as compared to an untreated biosolids composition. 
     The presence of iron and generation of dissolved iron also accelerates the aggregation of soil particles and thus dewatering. This hardens biosolids, reduces the biodegradability of biosolids, and diminishes the attractiveness to potential vectors. 
     Iron can be added to the biosolids during the sludge treatment ( FIG. 10 ) or after the treatment at the land application location.  FIG. 10  is a schematic diagram of an iron particle of the invention feeding and miring system in combination with conventional lime stabilization. The iron can be added as dry powder or dispersed in water as a liquid slurry. The slurry is then mixed with the biosolids. 
     The amount of iron needed per unit of biosolids mass depends on the solid concentration of the sludge, the amount of sulfur and other elements in the biosolids, and also on the size and activity of the iron nanoparticles. The proper dose can be determined by appropriate laboratory tests. 
     Iron nanoparticles have several advantages over conventional methods for biosolids stabilization. Iron nanoparticles are multi-functional with respect to treatment of a large variety of pollutants. Iron nanoparticles have been shown to be effective for the transformation of a very large number of organic and inorganic pollutants in groundwater and soil. They are also effective for the reduction of odor-producing compounds, immobilization of heavy metal ions, and extermination of disease-causing pathogens in biosolids. 
     Iron nanoparticles have high surface reactivity and rapidly react and stabilize biosolid compositions. In a typical wastewater treatment plant, final blending and processing of concentrated biosolids takes from minutes to a few hours. A highly reactive stabilizing reagent is therefore desired. Because of their extremely small size, iron nanoparticles can penetrate the intra-aggregate pores of biosolids and have much better access and availability toward pollutants within the solid phase. The high reactivity can be attributed to; (1) high specific surface, and (2) high reactivity per unit surface area. 
     Iron nanoparticles have a low adverse environmental impact and are benign to the environment. Iron is the fifth most used element of the periodic table in daily activities; only hydrogen, carbon, oxygen and calcium are typically consumed in greater quantities. Iron is at the active center of many biological molecules and is therefore considered an essential element for life. The end products of iron nanoparticle reactions are iron hydroxides and oxides with no evidence suggesting any negative impact. 
     Iron nanoparticles are simple to use and are attractive from an economics standpoint. The application processes using nanoparticles are highly portable, and can be combined to various procedures already in place (e.g., heat treatment, biological digestion, lime neutralization etc.). For example, many wastewater treatment facilities have facilities for lime addition and mixing so no new equipment is needed to deploy iron nanoparticles. Little capital investment is therefore required for such conversions.