BRPI1003414A2 - phosphogypsum as an adjunct to the aluminum industry by-product for use as an adsorbent and softener in contaminated areas - Google Patents

Abstract

Phosphogypsum as an adjunct to the aluminum industry by-product for use as an adsorbent and contaminant in contaminated areas. mud ", for use as adsorbent and softener for areas contaminated with cations and anions, such as: Zn, Cu, Cd, Pb, As, P and others. Because it is Ca and 5 enriched alkaline material and contains high concentrations of oxides, the by-product of the aluminum industry when reacted with phosphogypsum is a matrix that, besides retaining high amounts of toxic elements, provides, when applied to contaminated soils with the potential to generate acid drainage, lower contaminant toxicity and a better environment for plant growth and development. Its application in contaminated areas up to 12%, based on weight, and incorporation into the soil or mining tailings, promotes the retention of cationic and anionic contaminants and neutralization of acidity. Inorganic elements, being sorbed in a solid matrix, will become unavailable for plant absorption, transfer to the food chain or leachate to the water table, which substantially reduces environmental contamination and water eutrophication in the event of phosphor.

Description

Phosphogypsum as an adjunct to the aluminum industry by-product for use as an adsorbent and softener

CONTAMINATED

In the present invention, it is proposed to use phosphogypsum as an adjunct to the aluminum industry by-product, also called red mud or "red mud", for use as an adsorbent for inorganic cations and anions and to soften contaminated areas. Because it is an alkaline material rich in oxides, the red mud promotes an increase in pH contributing to alleviate the problems of acid mine drainage and is an oxide matrix with potential for sorption of inorganic contaminants. Sorption is a broad term used to refer to adsorption, precipitation and complexation, when it becomes difficult to ascertain the mechanism by which retention is occurring, which is quite common in soils due to its complexity and the heterogeneity of correlated factors. The reaction of the byproduct of the aluminum industry with phosphogypsum makes it rich in calcium and sulfur enhancing, respectively, the mechanisms of retention of anions and inorganic cations by the formation of ternary complexes. It is also noteworthy that the calcium and sulfur present in phosphogypsum are plant nutrients and when added to the aluminum industry byproduct, it increases its efficiency as a softener in the remediation processes of contaminated areas, making it the most appropriate medium for better plant development. .

The use of the aluminum industry by-product as adsorbent and soil softener promotes the sorption of trace elements, reducing their concentration in the solution and making them unavailable and less toxic. This product acts as a filter by removing Zn, Cu, Cd, Pb, As and other elements present in water, preventing their diffusion and transfer to other ecosystem components, as well as eutrophication by removing excess P in these systems. When used as a soil softener, this by-product alters some environmental conditions (eg, pH increase) favoring the retention of toxic elements, besides the material itself being a potential adsorptive matrix for these contaminants avoiding their absorption by plants and leaching. groundwater, thus contributing to the revegetation of the area and diffusion of contamination. As an indirect effect, there is an area revegetated with the soil protected against the action of wind and rain. Among the benefits of revegetation in relation to the actions of rain are protection against the impacts of rain tastes by reducing soil particle breakdown and drag, surface runoff, greater balance between infiltration and evapotranspiration rates, longer weather. of residence and accumulation of water in the soil and the greater cycling of the elements from subsurface layers to the surface. Regarding social benefits, there is revitalization of areas with negative stigmas due to pollution and valuation of properties in the involved watershed.

At present, the by-product of the aluminum industry, or red mud, is deposited in dams with double waterproofed bottom with compacted clay and PVC membranes, generating a very high cost not only for its construction, but also for maintenance and monitoring. These dams require huge areas for their construction, rendering them unusable, thus generating environmental liabilities. This is one of the main problems caused by aluminum production and the disposal of this waste is a potential risk due to its caustic nature and its high stored volume. With respect to phosphogypsum, part of its production has been marketed for use in agriculture. However, this demand is well below the amount generated, also resulting in huge stacks of stockpiled material that need to find viable alternatives for its use. As proposed in this invention, the use of phosphogypsum as a red mud adjuvant for use as an adsorbent and softener in contaminated areas minimizes the impact of separate activities by reusing and recovering materials that would be deposited in tailings dams, reducing containment dams and areas intended for its construction and the reduction of the toxicity of the contaminant in water bodies and soil.

Several patents reveal the potential of some materials to reduce the acidity of aqueous solutions. "KR2005054135-A and KR576731-B1" patent numbers, which refer to the treatment of water by a porous alkaline hematite-containing agent for the removal of organisms from red tide and water acidity control, differ from this invention mainly due to processing form for use of the by-product which is washed and dried at a temperature of 280-300 ° C and then macerated and the target object to be treated. Patent number "W0200234673-A" utilizing the neutralized bauxite (red mud) processing waste for treating acidic waters (eg acid mine drainage) containing dissolved inorganic substances differs from this invention in the way it is used to treat the by-product. . The treatment is done by adding certain amounts of calcium and magnesium ions and the product is added directly to the water. In this invention a residue called phosphogypsum is used to react with the by-product of the aluminum industry. Although there is similarity of calcium in the reaction, it comes from different sources and the control of acid mine drainage is proposed by mixing the softener directly into the sterile. W0200206158-A uses red mud and its derivatives to remove impurities such as calcium, magnesium and heavy metals from seawater or brine for the production of purified sodium chloride. This patent differs from the present invention by proposing the use of red mud only to reduce impurities contained in seawater, providing crystallization of sodium chloride. The main focus of this patent regarding the use of red sludge as adsorbent is to obtain purified sodium chloride and not to purify water. As for its use in remediation, it is a material that has already been used for water purification after undergoing a recovery process, which makes it different in character from those of the present invention. Several surveys reveal attempts to use various wastes to remove impurities from aqueous solutions. Patent "JP55132633" entitled "Arsenic Adsorbent" aims at the removal of arsenic from aqueous solutions using granulated red sludge with activated components. JP62014984, which proposes a method for phosphorus removal by adsorption, aims to remove phosphorus from sewage. In the case of the patent "KR20030090547", which deals with an industry wastewater treatment agent using red mud and a sample preparation method, the aim is to clean wastewater, with the benefit of reducing of the volume of sludge generated. These patents differ from this invention by the treatments or reactions employed in the by-product of the aluminum industry prior to their use for removal of elements from aqueous effluent solutions or bodies of water. In addition, it is proposed in this invention to apply the by-product to the soil so that it can retain the elements present in the soil solution while retaining them in a solid matrix.

Patent "HU9802690" proposing a method for hazardous waste decontamination aims to correct the pH of the waste and dispose it between layers of red mud. This patent differs from the present invention in that it provides an alternative for disposing of contaminated tailings, whereas in this invention the use of red mud as a contaminant soil softener is envisaged. It is an object of this invention to use the mud's own pH value to raise the pH of contaminated soil by incorporating it into the soil. In addition, it is a technology that is less environmentally aggressive and that, in addition, aims to use a waste to remediate contaminated areas, avoiding disposal in dams or landfills.

There are several techniques for removing pollutants from water and for remediation of contaminated soil. Decontamination of wastewater requires filters consisting of materials with high contaminant holding capacity, similar to this proposed adsorbent. Regarding the techniques used for remediation of contaminated areas, some of them are considered very aggressive to the environment, very expensive and not always technically feasible. Stabilization, which aims at the use of a soil softener enabling the growth and development of plants, has been widely used. In addition, the use of industrial waste, as proposed in this invention, in place of commercial softeners, may be a smart alternative by reducing the expense of buying commercial softener and enabling the transformation of two wastes into reuse by-products. in the ground.

Using the aluminum industry by-product and its transformation products after reaction with phosphogypsum is an economically viable and environmentally sound alternative. Its use as an adsorbent filter or as a softener in contaminated areas, especially in those with acid drainage problems, raises the pH of the medium and favors the sorption of inorganic contaminants. In addition, the presence of oxides in the aluminum industry by-product - a term used to refer to iron and aluminum oxides, oxyhydroxides and hydroxides - and calcium and sulfur from its reaction with phosphogypsum. Oxides are a matrix with a high potential to siphon off contaminants, keeping them strongly retained and thus reducing the environmental risks arising from their desorption. Specifically dealing with iron oxides present in very significant quantities in red mud, they can also increase their power to siphon elements through redox cycles (oxy-reduction reactions), which is of great importance to oxide soils (highly weathered soils that contain present in their constitution kaolinite and very significant iron and aluminum oxides in tropical climate areas). In addition, calcium and sulfur, considered plant nutrients, may favor adsorption by the formation of ternary complexes and supply part of plant requirements.

The important thing is that the contaminants being trapped in a solid matrix will not be able to be absorbed by plants and transferred to the food chain, nor will they contaminate aquifers. In addition, the alkalinity of the adsorbent associated with the presence of calcium will allow carbon sequestration. This can occur in two ways, namely directly by the alkaline reaction of Ca present in the gypsum-treated by-product with CO2 present in the soil in concentrations up to 10 times higher than the atmosphere by the possible formation of carbonates and, in a similar way. indirectly, by growing plants in contaminated areas, thus contributing to greater environmental sustainability.

Another major advantage of this invention relates to the use of a technology that aims to address several distinct problems. Firstly by adding value to two wastes, one from the aluminum industry and the other from the phosphate fertilizer industry and secondly by finding a use purpose for the benefit of water pollution and remediation of areas contaminated by various activities, especially those with the potential to generate acid mine drainage. Added to this are the reduction in the number of dams and areas intended for their leasing in order to reduce the potential risk of contamination by the large volume of material stored and the reduction of contaminant toxicity through a less aggressive process to the environment. most economically viable.

The by-product of the aluminum industry, called red mud or "red mud", is generated during bauxite processing for aluminum extraction via the Bayer process. It is estimated that during the bauxite refinement to produce one ton of alumina, approximately 1 to 1.5 tons of red mud will be generated. In general, the byproduct has a pH in water of 10.48 ± 0.03, determined according to EMBRAPA (1997) (pH in water in the adsorbent: water ratio of 1: 2.5) and a mineralogy consisting of hematite. , goethite, maghemite, quartz and gibbsite, determined by X-ray powder diffraction analysis, based on the methodology recommended by Jackson (1979), using a range of 4 to 40 ° 2Θ per second. Sulfur attack results showed the presence of 15.76% SiO2, 31.68% Al2O3, 15.53% Fe2O3, 3.06% TiO2, 0.20% P2Os. Semitotal Zn, Cu, Cd, Pb and As concentrations (mg kg "1) by microwave digestion (USEPA 3051 A method) were 26.46, 1.39, 0.03, 85.72 and 19.41 respectively, its zero charge point (PCZ), determined by measuring electrophoretic mobility using the Zeta-Meter System 3.0 device, was 5.26 ± 0.28, and the null saline effect point (PESN) determined by Titration curves were 8.02 ± 0.21.

Phosphogypsum is a byproduct produced in large quantities in the production of phosphoric acid by the phosphate fertilizer industry. In this case, the phosphate concentrate is attacked with concentrated sulfuric acid in an aqueous medium at a temperature of approximately 70 ° C. Thus, the waste called phosphogypsum is generated in large proportions. The term phosphogypsum is often referred to in the literature as by-product plaster, chemical plaster, plaster residue, agricultural plaster and synthetic plaster, and also because it contains phosphorus in its composition (0.7 to 0.9%), can be called phosphogypsum. . It is estimated that, for each ton of phosphorus, 9.8 tons of dry phosphogypsum are produced by the hemihydrate process and by the dihydrate and hemihydrate processes, 11.2 tons of dry phosphogypsum are produced. It is estimated that the generation of phosphogypsum in the world is around 100 to 280 million tons per year and in Brazil is 4.5 χ 106 tons per year. Of this total obtained in the country, 1.7 χ 106 tons per year are used in agriculture as a soil conditioner and 0.7 χ 106 tons per year are used in the cement industry, with the remaining (2.1 χ 106 tons) stored in outdoor batteries. Finally, worldwide, including Brazil, there is an imbalance between the use of phosphoric acid and the consumption of phosphogypsum, resulting in huge accumulated amounts of this material, which also generates a large environmental liability.

In general, phosphogypsum has a pH in water of 5.08 ± 0.05, determined according to EMBRAPA (1997) (pH in water in the byproduct: water ratio of 1: 2.5) and a mineralogy consisting of plaster and anhydrite, determined by X-ray powder diffraction analysis, based on the methodology recommended by Jackson (1979), using a range of 10 to 60 ° C per second. Sulfur attack results showed the presence of 1.86% SiO2, 0.91% Al2O3, 1.20% Fe2O3, 2.66% TiO2, 1.70% P2O5. The semitotal concentrations of Zn1 Cu1 Cd1 Pb and As (mg kg "1) by microwave digestion (USEPA 3051A method) were 4.68, 2.14, 0.04;

1.43 and 0.21, respectively.

For the preparation of the material used as adsorbent, the phosphogypsum (G) was subjected to reaction with the aluminum industry by-product (LA) in the following proportions based on weight: 100% G, 75% G + 25% LA, 50% G + 50% LA, 25% G + 75% LA and 100% LA. Doses between 0% to 25% phosphogypsum (e.g., 1%, 2%, 5%, 10% and 25%) were also tested for reaction with the aluminum industry by-product. The materials were weighed, considering the appropriate proportions to obtain 100 g of final weight. Immediately thereafter, they were homogenized in 500 mL flasks, 100 mL of Milli-Q water (Millipore de-distilled and deionized water) (1: 1 adsorbent: water ratio) added and allowed to react for 72 hours. alternating 12 hours of rest and 12 hours of agitation. After the reaction period, a pH and electrolyte conductivity reading of the supernatant was performed. The solid material was air dried, macerated in agate and passed through a 1 mm diameter sieve to obtain a more homogeneous material. For the preparation of the soil softener, the phosphogypsum (G) was added to the aluminum industry by-product (LA) and then applied to the soil to process the reaction in the soil due to the higher CO2 input in this system. .

The tests for by-product evaluation as adsorbent and softener of contaminated areas consisted, respectively, of adsorption and desorption tests, with the objective of evaluating the potential of the material to retain contaminants, and of experiments in a greenhouse, containing contaminated soil in the soil. which were cultivated Brachiaria brizantha cultivar marandu and mining sterile with potential for acid mine drainage generation. In these two greenhouse tests, vessels with leachate collectors were used. The adsorption and desorption tests were divided into three stages: In the first stage, referring to the pH adjustment, in triplicate, 0.3 g of the adsorbents previously air dried and sieved 1 mm in diameter were weighed. which were suspended in 20 mL of 0.01 mol L "1 Ca (NO3) 2 or 0.03 mol L" 1 NaCl (adsorbent ratio: 1:67 solution). For this purpose, predetermined amounts of 1 mol L1 HNO3 and saturated Ca (OH) 2 solution were used to adjust the Ca (NO3) 2 solution, and HC11 mol L1 and 1 mol L1 NaOH to adjust the solution. NaCl1 which were calculated by regression equations fitted to titration curves. After addition of acid or base, 12 hours of rest and 12 hours of stirring were alternated, with pH readings taken daily until stability was achieved. The pH was considered stable when the variation of its value was less than 0.2 units, which occurred around 5 days. Part of the tests were carried out in Ca (NO3) 2 for the adsorption of divalent metals (Zn, Cu, Cd and Pb), as Ca2 + tends to be the main base cation in the soil mixtures complex and NO3 'has a small capacity. form ionic pair with metals in solution. In the other cases (As and P), we chose to use NaCI as a background electrolyte to prevent precipitation of arsenate and phosphate.

In the second step, regarding adsorption, immediately after stabilizing the pH of the solution with the adsorbent, 10 mL of Zn (N03) 2.4H20, Cd (N03) 2.4H20, Cu (N03) 2.3H20, Pb solution were added. (NO3) 2, H3PO4 and Na2HAsO4JH2O (adsorbent ratio: final solution 1: 100 or 10 g L '). To determine the maximum sorption capacity of the aluminum industry byproduct using the Langmuir equation, the solution concentrations were: 0; 0.15; 0.30; 0.75; 1.50; 2.25; 3.00 and 4.50 mmol L -1 for Zn, Cu, Cd and Pb and 0.30, 0.75, 1.50, 2.25, 3.00, 4.50, 6.00 and 9.00 mmol L "1 for As and P. For adsorption in the different proportions of the phosphogypsum aluminum industry by-product mixture, concentrations were 1 mmol L" 1 for cations and 2 mmol L'1 for anions. It is noteworthy that such concentrations have equivalent amounts in mmol L charge 1 for comparison between cations (divalent) and anions (predominantly in monovalent form at the studied pH, as revealed by ionic speciation). After the adsorbate was added, the samples remained for 72 hours, alternating 12 hours of stirring and 12 hours of rest for processing. Then, the samples were centrifuged for 20 minutes at 3,000 rpm and the supernatant collected to determine the concentration of the elements to calculate their adsorbed amount. This was calculated by the difference between the amount added and the remainder in the equilibrium solution.

In the third desorption step, to the remaining adsorption residue was added 30 ml of 0.01 mol L "Ca (NO3) 2, to promote cation desorption, or 30 ml 0.03 mol L" NaCl 1, in order to promote desorption of the adsorbed anions. The samples remained for a further 72 hours, alternating 12 hours of shaking and 12 hours of rest, and then centrifuged for 20 minutes at 3,000 rpm. The supernatant was collected to determine the concentration of the elements used to calculate the concentration. amount desorbed. In this calculation the metal remaining in the equilibrium solution from the previous adsorption is discounted. Concentration determinations used in calculating adsorbed and desorbed quantities were made by flame atomic absorption spectrophotometry and graphite furnace for readings in the concentration range in mg L "1 and pg L" 1, respectively.

The greenhouse test, in which two arsenic contaminated and brachiaria-cultivated soils were used, consisted of the following procedures:

The following were used as substrate: a sandy soil, Spodic Ortic Quartzaric Neossol (RQo), containing 840 g kg "1 sand, 90 g kg" 1 silt and 70 g kg "1 clay and a clayey, typical dystroferric Red Latosol ( LVdf) containing 140 g kg "1 sand, 110 g kg" 1 silt and 750 g kg "1 clay, both contaminated with arsenic. The organic matter content (dag kg ") of LVdf and RQo was 2.8 and 1.2, respectively. The final concentration of As in these soils was 150 mg dm" 3, reference value for intervention in areas industrial facilities in Brazil.

The design was completely randomized with 5 repetitions. Were used as witnesses, the two types of soil without contamination and the contaminated soils without any kind of softener. For comparison purposes, the contaminated soils were used as a softener, a synthetic limestone made with the mixture of CaCO3 and Mg (CO3) 4Mg (OH) 2-SH2O maintaining the Ca: Mg ratio of 3: 1. The amount of limestone was calculated to raise soil pH to 6.5 by regressions determined in pre-experiments. The by-product of the aluminum industry and the product of its reaction with

75% LA + 25% G phosphogypsum was used in the proportions of 0.5%, 1.0% and 2.0%, based on volume, corresponding to 10, 20 and 40 tons per hectare. These proportions were calculated considering that the softener will be incorporated into the soil at a depth of 0.20 m. The soils used, as well as the softeners, were previously air dried and sieved with 4 mm diameter mesh.

For the assembly of the test, the pots were made with bottles of polyethylene terephthalate (PET), with 2 dm3 of volumetric capacity, which were cut approximately in half, in order to obtain the pot where the soil was placed, in the upper part. and a leachate collector at the bottom. The bottle caps were perforated and, before being filled with soil, a glass wool was placed to prevent the soil from passing through the holes, allowing the collection of a leachate with less particulate matter. Five repetitions were made for each treatment, making a total of 90 plots. The amount of soil used in each pot was 1.5 kg for sandy soil (Quartzarenic soil) and 1.1 kg for clay soil (Red Latosol), both occupying a volume of approximately 1.05 dm3.

After the experiment was set up, an excess water depth was applied so that it was possible to determine the weight of the vessels after drainage, considering them with a humidity level close to 100% of the field capacity. This made it possible to monitor and replenish water to maintain soil moisture at approximately 50 to 60% of field capacity over 30 days to process the reactions of the softeners with the substrates. The excess drainage water used to determine the field capacity was returned to the soil keeping the system closed to control the losses of the elements present in this leachate. At the end of the incubation period, 20 cm3 of substrate were removed for characterization and then Brachiaria was sown.

brizantha cultivate marandu. Single samples containing 20 cm3 of substrate collected from the five

Vessels belonging to each treatment were air-dried and passed through 2 mm sieves for further fertility analysis, according to Vettori (1969) and EMBRAPA (1997), as well as for the determination of available and semitotal amounts of As by Mehlich 1 extractor ( EMBRAPA, 1997) and the US Environmental Protection Agency's USEPA 3051A method (USEPA, 1998), respectively. In these samples, the "Toxicity Characteristic Leaching Procedure" (TCLP) leaching test was also performed by the USEPA 1311 method (USEPA, 1992). Initially, the test was performed to choose the recommended fluid for extraction in which fluid 1 - solution containing glacial acetic acid and sodium hydroxide with a pH of 4.93 ± 0.05 was determined. After the extraction solution was determined, 10 g of soil was weighed and 200 ml of fluid 1 was added and then the samples were agitated using a rotary shaker at a speed of 30 ± 2 rpm for 18 ± 2 hours at a temperature of 23 ± 5 ° C. After the stirring period, the samples were filtered and the As concentrations determined using a graphite furnace atomic absorption spectrophotometer (EAA).

concentration range of pg L ").

To ensure homogeneous seed germination, dormancy was broken using concentrated sulfuric acid for 2 minutes and then washed with running water and dried in the shade for subsequent sowing. The choice of Brachiaria brizantha was due to its very aggressive growth, providing rapid ground cover. After germination, thinning was done, leaving six plants per

vase.

Three fertilizers were made via fertigation, the first at 10 days after sowing and the other two at 25 and 45 days. The nutrient doses applied were as follows: 300 nitrogen, 200 phosphorus, 300 potassium, 75 calcium, 30 magnesium, 50 sulfur, 0.5 boron, 1.5 copper, 5 iron, 10 of manganese and 0.1 molybdenum (values in mg dm "). With the exception of nitrogen and potassium, which were splitted in three applications, the other nutrients were applied in the first fertilization.

During the experiment 3 leachate collections were made, the first with 20, the second with 40 and the third with 60 days after sowing. Before each collection the soil moisture was raised to 100% of the field capacity and then the desired amount of water was collected for leachate collection. The collected volumes were measured, followed by pH and electrolytic conductivity readings. An aliquot of the leachate was filtered at 0.45 pm (soluble fraction) to determine arsenic concentration using a flame atomic absorption spectrophotometer and / or with a graphite furnace.

The plants were collected at 60 days after sowing and had their aerial part separated from the roots. The shoot was washed only with deionized water and the roots washed with deionized water for soil removal, dipped in 0.1 mol L1 HCl solution and then rinsed with deionized water. The shoot and roots were dried in a forced air oven at a temperature of approximately 60 ° C. After drying, both parts were weighed separately for the evaluation of root and shoot dry matter production, and then, the shoot was ground for arsenic determination by HNO3 digestion in a microwave oven. In the digestion extracts, the arsenic concentration was determined using a flame atomic absorption spectrophotometer or a graphite furnace, for readings in the concentration range in mg L "1 and pg L" 1, respectively.

The other greenhouse test, in which we used sterile arsenic contaminated mining and with potential for acid mine drainage generation, consisted of the following procedures:

The aluminum industry by-product (100% LA) and the aluminum industry by-product plus 25% phosphogypsum (75% LA + 25% G) were mixed with the sterile substrate in the following proportions: 0%, 1%, 3 %, 6%, 9% and 12%. These materials, together with a control soil collected in a preservation area near mining, were incubated in plastic bags for a period of 30 days, during which time they were kept with moisture close to the field capacity, so that if the reactions of the softeners were processed with the sterile substrate. In order to evaluate the potential of the mitigators to reduce the acidity caused by acid mine drainage, the pH values were determined every five days and the pH stabilized 30 days after the beginning of the incubation. These pH values were determined in extracts prepared with 10 cm3 of substrate for 25 mL of distilled water (1: 2.5 soil to water ratio).

In the extract where the pH reading was performed, an electrolytic conductivity (EC) reading was also performed to verify the influence of the softeners on the salt content of the substrate. At the end of the incubation period, 1 dm3 of the samples was taken from the plastic bags and placed in polyethylene terephthalate (PET) bottles for leachate collection, according to the procedure described in the previous test.

The adsorption and desorption tests simulating the ionic strength of oxide soils found a high adsorption capacity and low desorption capacity of inorganic cations and anions. These results make it possible to verify the ability of the aluminum industry byproduct and its phosphogypsum mixture to remove these elements from the solution while retaining them in the non-exchangeable solid matrix. The calculated maximum adsorption capacity of the aluminum industry by-product at pH 5.5 followed the ascending order Cd <Zn <Cu <Pb <As <P and were equal to 9.05; 36.63; 85.47; 95.24; 212.77 and 256.41 mmol kg -1, respectively. These adsorbed amounts represent in relation to the amount added (150 mmol kg "for divalent cations and 300 mmol kg" 1 for anions) a removal rate equal to 6% Cd, 24% Zn, 57% Cu, 63 % Pb, 71% As and 85% P.

When comparing the aluminum industry by-product with the adsorbents from its phosphogypsum reaction, the adsorber with the highest retention capacity was the one containing 25% phosphogypsum and 75% by-product from the aluminum industry. For an added amount of 33.33 mmol kg "1 for Cd and Pb and 66.66 mmol kg" 1 for As and P and with a pH of the solution 5.5, the by-product of the pure aluminum industry ( 100% LA) presented, respectively, an adsorbed amount of Cd, Pb, As and P equal to 8.58; 29.85; 56.22 and 63.98 mmol kg "1 and the adsorbent containing 25% gypsum and 75% aluminum industry by-product (25% G + 75% LA) equal to 10.23; 31.66; 48.98 and 61.71 mmol kg -1. The removal rates in relation to the adsorbed amount of the pure aluminum industry by-product (100% LA) and the adsorbent containing 25% phosphogypsum and 75% by-product of the aluminum industry (25% G + 75% LA) was 26%. and 32% Cd, 92 and 97% Pb, 87 and 76% As and 99 and 95% P, respectively.

It is observed that the reaction of the aluminum industry byproduct with phosphogypsum reduces the alkalinity of the adsorbent, favoring the adsorption of anions, such as arsenic. When the aluminum industry by-product is compared with the adsorbents from its phosphogypsum reaction at a ratio of 0% to 25% by weight, there is an increase in the maximum adsorption capacity of 3.7 times at the highest proportion of phosphogypsum. This increase can be attributed in part to ternary complex formation and pH reduction. The pH of the suspension was monitored and its value decreased from 10.1 in the pure aluminum industry by-product (100% LA) to 8.8 in the adsorbent containing 25% phosphogypsum and 75% by-product of the aluminum industry ( 25% G + 75% LA), ie a reduction of 1.3 pH units. In this case, the higher adsorption of the anion is due to the reduction in the negative charge density on the surface caused by the greater proximity of the pH to the adsorbent's zero charge point and due to the formation of ternary complex with calcium.

The greenhouse experiment using pots containing arsenic contaminated soils and different doses of the aluminum industry byproduct and its phosphogypsum reaction product allowed to evaluate and quantify the dose and the softener that provided lower concentration of As in the TCLP test and in the Potted leachate, higher root and shoot dry matter yield and lower As and dry matter production. The concentration of As in the TCLP leach test extract and the pot leach test reduced in the soils where the softeners were applied up to 2%. The application of the aluminum industry byproduct and the softener containing 25% phosphogypsum and 75% aluminum industry byproduct (25% G + 75% LA) at up to 2% by weight resulted in an increase in soil pH. and leachate, increase in the production of dry matter of root and shoot of brachiaria and reduction in the amounts of As in dry matter of root and shoot. It should be noted that the product containing% phosphogypsum and 75% by-product of the aluminum industry (25% G + 75% LA) was superior to the by-product of the pure aluminum industry (100% LA) for use as a softener due to the additional effect. of calcium and sulfur supply to plants making the most suitable medium for their development.

The greenhouse experiment using the byproduct of the aluminum industry and the product of its reaction with phosphogypsum allowed to evaluate its efficiency to neutralize the acidity verified in the sterile substrate. Both softeners raised the initial sterile pH from 2.1 to around 7.0 when applied at a 12% weight-based ratio. Thus, the efficiency of these materials for use as an adsorbent and mitigating acidity and contamination by cationic and anionic contaminants is verified.

Claims (12)

1. "Phosphogypsum as an adjunct to the aluminum industry by-product for use as an adsorbent and softener in contaminated areas", with an oxide mineralogy consisting of hematite, goethite, maghemite, quartz, gibbsite, gypsum and others characterized by high pH value for its use as adsorbent for the retention of cationic and anionic contaminants of aqueous solutions such as Zn, Cu, Cd, Pb, As, P, among others without restriction, and for neutralization and / or filtration of contaminated water and treatments. of effluents. 2. "Phosphogypsum as an adjunct to the aluminum industry by-product for use as an adsorbent and softener for contaminated areas", with an oxide mineralogy consisting of hematite, goethite, maghemite, quartz, gibbsite, plaster and anhydrite and others, with a high pH value for its use as a softener for the remediation of areas contaminated or polluted with cationic and anionic contaminants, such as Zn, Cu, Cd, Pb, As, P, among others, without restriction, and for the recovery and / or revegetation of degraded areas. with or without potential to generate acid drainage. 3. "Phosphogypsum as an adjunct to the aluminum industry by-product for use as an adsorbent and softener for contaminated areas" with oxide mineralogy consisting of hematite, goethite, maghemite, quartz, gibbsite, plaster and anhydrite and others, and high pH value its use as a softener for the remediation of, without restricting, contaminated or polluted with cationic and anionic contaminants such as Zn, Cu, Cd, Pb, As, P1 and others, without restriction, and for the recovery and revegetation of degraded areas with or without potential to generate acid drainage. 4. "Phosphogypsum as an adjuvant to the aluminum industry by-product for use as an adsorbent and contaminant in softening areas" according to claims -1 to 3, characterized by its use as an adsorbent, softener and conditioner after reaction of the by-product of the aluminum with phosphogypsum, air drying, grinding and screening to obtain homogeneous material. 5. "Phosphogypsum as an adjuvant to the aluminum industry by-product for use as an adsorbent and softener in contaminated areas" according to claims -1 to 4, characterized in that it is used as a powder in aqueous solutions in the range of up to 100 g L "1, calculated on the basis of weight or volume and with subsequent filtering or not. 6. "Phosphogypsum as an adjunct to the aluminum industry by-product for use as an adsorbent and contaminant in contaminated areas" according to claims -1 to 4, characterized in that it is used in the form of dust, applied to the soil surface up to 12%, calculated on the basis of soil weight or volume and incorporated to a depth of 0.3 m. 7. "Phosphogypsum as an adjunct to the aluminum industry by-product for use as an adsorbent and softener for contaminated areas" according to claims -1 to 4 and 6, characterized by its use in soil as a conditioner or as a softener for substrates for the development of plants for revegetation or production. 8. "Phosphogypsum as an adjuvant to the aluminum industry by-product for use as an adsorbent and contaminant in contaminated areas" according to claims -1 to 7, characterized in that it is used to raise the pH of aqueous solutions and soils, tailings and other materials. from industrial or mining activities and areas with acid drainage generation problem. 9. "Phosphogypsum as an adjuvant to the aluminum industry by-product for use as an adsorbent and contaminant in contaminated areas" according to claims -1 to 7, characterized in that it is used in aqueous solutions and in the soil as an interactive medium for various reactions such as as adsorption, precipitation and oxidation. 10. "Phosphogypsum as an adjunct to the aluminum industry by-product for use as an adsorbent and contaminant in contaminated areas" according to claims -1 to 7, characterized in that it is used in aqueous and soil solutions as an interactive means for carbon sequestration by carbonate formation and others. 11. "Phosphogypsum as an adjuvant to the aluminum industry by-product for use as an adsorbent and softener of contaminated areas" according to claims -1 to 7, characterized in that it is used as a reducing agent for the concentration of cationic and anionic contaminants such as Zn, Cu, Cd, Pb, As, P1 among others, without restricting, in water bodies and soil solution and reducing the bioavailability or availability of these elements. 12. "Phosphogypsum as an adjuvant to the aluminum industry by-product for use as an adsorbent and contaminant in contaminated areas" according to any of claims 1 to 7, characterized in that it is used as a reducer for the diffusion of pollutants from Zn, Cu, Cd. , Pb, As, among others, without restricting, in solution and soils.