Adsorbent Structures for the Removal of Phosphates and Ammonia from Wastewater and Methods of Use

Abstract

High surface area magnesium carbonate structures formed from a calcined slurry of magnesium carbonate powder and a binder and method for their use to adsorb aqueous phosphate and ammonia for recovery and repurposing as a fertilizer are disclosed. A binder is utilized to aid in the formation of useful structures. The binder significantly increase porosity and the available surface area for adsorption.

Description

TECHNICAL FIELD

The field of the technology generally relates to methods for separating and recovering phosphates and ammonia from water.

BACKGROUND

As the limiting nutrient in most waterways, increased phosphate (PO₄³⁻) concentrations can promote accelerated eutrophication, which has a range of environmental and economic impacts. Eutrophication leads to increased water treatment costs, decreased recreational value; but notably, the proliferation of algal blooms. Some of these blooms produce cyanotoxins like microcystins and cylindrospermopsin which can be detrimental to both human and aquatic health. Though chemical precipitation and biological treatments are commonly used methods for the remediation of PO₄³⁻, problems including costs, sludge production and stability/reliability issues have led to the research of alternative methods for the removal of PO₄³⁻ from waterways.

While viewed as a pollutant at excessive concentrations (i.e., >20 μg L−1), phosphate (PO4 3−), the primary species of phosphorus in the environment, is necessary for a range of industrial purposes including the production of agricultural fertilizers, animal feeds, and chemical pesticides. The Environmental Protection Agency (EPA) limit for acceptable phosphorus levels in water is only 0.1 mg/L or lower. Phosphate reserves are quickly declining, therefore the recovery and reuse of PO4³⁻ is an essential component of phosphate remediation. Adsorption is a technique which can both remove and recover PO4³⁻ from aqueous suspensions and has been extensively studied. Adsorbents ranging from modified iron oxide, to calcined waste eggshells, to magnesium modified corn biochar have been investigated for phosphate adsorption. However, adsorption of is problematic because desorption of can be difficult. The use of highly adsorptive fine powders which can desorb phosphate after remediation is a growing area of study, their removal from solution after adsorption is challenging. Therefore, the synthesis of highly adsorptive, inexpensive, granular sized sorbents which can safely recycle phosphate back into the environment in a controlled manner would be extremely beneficial to the problem of phosphate pollution, especially in agricultural endeavors.

Adsorption is a surface-based phenomenon resulting in the adhesion of an adsorbate on the surface of an adsorbent through covalent bonding and electrostatic interactions. Unlike chemical precipitation and biological removal processes, adsorption is unique in that it can remove contaminants over a wide pH range and at low concentrations. A wide variety of materials have been investigated for the adsorption of phosphate including metal oxides, waste materials, zeolites, and polymers. Lesser-studied materials for phosphate sorption are carbonates. Previous studies have explored the use of calcium carbonates (CaCO₃) as phosphate binders to decrease phosphate concentrations in aquatic environments.

SUMMARY

Structures made from metal carbonates having very low water solubility (e.g., 0.11 g/L at 25° C. for MgCO₃) are utilized to remove phosphates and ammonia from water. Powdered metal carbonates, e.g., alkaline earth metal carbonates such as MgCO₃ and lanthanoid carbonates such as La₂(CO₃)₃, are mixed with a binder and pressed into structures. The binder and any diluent are then removed by calcining the pressed structure which increases its porosity, thus increasing the surface area available for phosphate and ammonia adsorption. Naturally occurring carbonate structures may also be utilized and shaped accordingly where they have sufficient porosity and when the strength of the structure is not a significant consideration for the application in which the resulting structure is to be utilized.

The phosphate and ammonia adsorbent structures can be used as linings, channels, load bearing structures, or other constructs with surfaces that can be placed in contact with wastewater which could benefit from the capture of phosphates and ammonia. Moreover, the structures can be formed into pellets aggregated into a flow-through bed. Aggregates may be placed within porous housings for use in situ for low flow settling ponds and tanks or in high flow applications such as effluent stream. The aggregates may also be used to create adsorbent beds in flow-through arrangements such as pipes and columns. These porous bags of pelletized carbonates may also be placed within open cell foam structures to filter common debris (e.g., leaves, wood, and insects) that could potentially interfere with the porosity of the pellet bag or clog the pores of the aggregate adsorbent. The phosphorous and nitrogen can be reclaimed from the spent structures by using them as fertilizer.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a table depicting the resulting molar ratios achieved with various samples in Example 1 herein.

FIG. 2 demonstrates the adsorption capacity of the formed pellets under various experimental conditions.

FIG. 3 depicts a pelletized metal carbonate for use as an aggregate.

FIG. 4 depicts SEM images of pellets formed with and without cellulose, before and after phosphate adsorption.

FIG. 5a depicts XRD analysis of MgCO₃ calcined pellets MgCO₃ pellets formed with varying quantities of cellulose,

FIG. 5b is an XRD analysis of calcined MgCO₃ pellets formed with varying quantities of cellulose after adsorption of phosphates.

FIG. 6 is a graph of the adsorption capacity of example MgCO₃ pellets formed with varying quantities of cellulose.

FIG. 7 is a graph of BET surface area of example MgCO₃ pellets formed with varying quantities of cellulose.

FIG. 8 is a graph of the thermal stability of MgCO₃ pellets formed with varying quantities of cellulose.

FIG. 9 is a graph of the free phosphate concentration of a phosphate-water solution over time in the presence of MgCO₃ pellets formed with varying quantities of cellulose.

FIG. 10 graphs phosphate concentration change over time normalized by initial concentration.

FIG. 11 depicts an embodiment utilizing metal carbonate pellets as media within a porous boom suspended in flowing water with a cutaway view of a boom.

FIG. 12 depicts cross-sectional view of a porous boom for suspension in water filled with a metal carbonate aggregate and encased in an open foam housing.

FIG. 13 is a cross-sectional view of a wastewater pipe with a metal carbonate liner.

FIG. 14 is schematic view of a column packed with metal carbonate aggregate through which wastewater is pumped.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present application is directed to the manufacture and use of porous structures comprised of metal carbonates which act as phosphate and ammonia adsorbing substrates in aqueous media. High surface area structures are formed which possess nanopores, i.e., having pore radii of less than or equal to 1 nanometer (10 Å), to increase water permeability and the surface area of the substrate available for loading.

Alkaline earth metal carbonates such as MgCO₃ and lanthanoid carbonates such as La₂(CO₃)₃ have been determined to be useful metal carbonates. Strontium carbonate and zinc carbonate also possess similar characteristics for use as adsorbents. MgCO₃ is a preferred metal carbonate to act as a phosphate adsorbent because of its tendency to form magnesium ammonium phosphate (NH₄MgPO₄.6H₂O), i.e., struvite, in aqueous media having phosphate ions and ammonia, and forms magnesium phosphate pentahydrate (Mg(PO₃OH).3H₂O), i.e. newberyite, in aqueous media having phosphate ions but little to no ammonia. The formation of NH₄MgPO₄.6H₂O (struvite) and/or Mg(PO₃OH).3H₂O (newberyite) binds phosphate ions to MgCO₃ in the molar ratio of 1:1 and binds ammonia to struvite in the molar ratio of 1:1. This results in significant phosphate and ammonia loading onto a MgCO₃ substrate. Solid structures are preferred because they are more easily removed from an aqueous media than powders, but surface area and loading capacity per gram must necessarily be sacrificed to produce structures that can easily be manipulated and retrieved. In the following Example 1, of adsorbed PO4 and NH4 was determined using powdered MgCO3 as a control in comparison to a MgCO₃ pellet to ascertain the molar ratio of the adsorbates.

Sample Preparation:

• • i. Prepared 1000 ml of 4000 ppm PO₄³⁻ solution using NaH₂PO₄2H₂O (A herein).
  •  Mixed 6.57 g of A with 1 L of water:

$6.57gA\times \frac{94.971g{\mathrm{PO}}_4}{156.01gA}\times \frac{1}{1000L}=4.00g{\mathrm{PO}}_4/L\left(=4100\mathrm{ppm}\right)$

• • ii. Obtained 0.3228 g of sample (MgCO3).
  • iii. Measured 0.597 ml of 28% NH₄OH solution in water.

$0.597\mathrm{ml}\times \frac{28}{100}=0.167\mathrm{ml}{\mathrm{NH}}_4\mathrm{OH}$

• •  Density: 0.8 g/mL

$\mathrm{So},0.167\mathrm{ml}{\mathrm{NH}}_4\mathrm{OH}\frac{0.880g}{\mathrm{ml}}=0.147g{\mathrm{NH}}_4\mathrm{OH}$ $0.147g{\mathrm{NH}}_4\mathrm{OH}\frac{18.039g{\mathrm{NH}}_4}{35.04g{\mathrm{NH}}_4\mathrm{OH}}=0.0759g{\mathrm{NH}}_4$ $0.0759g{\mathrm{NH}}_4\frac{1\mathrm{mol}}{18.039g}=0.00421\mathrm{mol}{\mathrm{NH}}_4$

• • iv. Combined 100 ml of PO4 solution (in i), 1.0 g MgCO₃ sample (in ii), and 1.85 ml NH₄OH solution (in iii).
  •  PO₄:

$0.597\times \frac{28}{100}\mathrm{ml}{\mathrm{NH}}_4\mathrm{OH}\times \frac{0.880g}{\mathrm{ml}}\frac{18.039g{\mathrm{NH}}_4^{-}}{35.04g{\mathrm{NH}}_4{\mathrm{OH}}_4}\frac{1\mathrm{mol}}{18.039g}=0.042\mathrm{mol}{\mathrm{NH}}_4^{-}$ $100\mathrm{mL}{\mathrm{PO}}_4^{3-}\frac{4000\mathrm{mg}}{1000\mathrm{mL}}\frac{1\mathrm{mol}}{94.971g}=0.00421\mathrm{mol}{\mathrm{PO}}_4^{3-}$ $\begin{array}{c}{\mathrm{MgCO}}_3:0.3228g{\mathrm{MgCO}}_3\times \frac{1\mathrm{mol}}{84.3139g}=0.00383\mathrm{mol}{\mathrm{MgCO}}_3\\ \left(1.0g{\mathrm{MgCO}}_3\times \frac{1\mathrm{mol}}{84.3139g}=0.01186\mathrm{mol}{\mathrm{MgCO}}_3\right)\end{array}$

• •  NH₄:
  • v. Measured PO₄³⁻ concentration using DR1900 (max 2 ppm)
    • 1. In order to measure PO₄³⁻ using DR1900, one needs to dilute the solution down to 2 ppm (from 4000 ppm initial concentration) in 10 ml absorption bottle.
    • 2. Mixed 10 ml DI water with 0.005 mL solution, then measure.

EXAMPLE 1

FIG. 1 summarizes the resulting molar ratios achieved with various samples. In Example 1, the adsorption capacity, FIG. 2, was measured for samples under the following conditions:

• Condition 1: 1.0 g sample in 100 ml of 4000 ppm PO₄³⁻+0.0 ml NH₄OH (MgCO3 :PO4:NH4=2.8:1:0)
• Condition 2: 1.0 g sample in 100 ml of 4000 ppm PO₄³⁻+0.6 ml NH₄OH (2.8:1:1)
• Condition 3: 0.323 g sample in 100 ml of 4000 ppm PO₄³⁻+0.0 ml NH₄OH (1:1.1:0.0)
• Condition 4: 0.323 g sample in 100 ml of 4000 ppm PO₄³⁻+0.6 ml NH₄OH (1:1.1:1.1) The results demonstrated that, in the absence of NH₄, #2 shows higher adsorption capacity (Q_PO4) than sample 12. In the presence of NH₄, Q_{PO4, #2} increases, while Q_PO4,#12 decreases.

The adsorption capacity of several example samples was determined as shown in Table 1. Table 2 details the experimental conditions for each sample. Table 3 details the measured decrease of PO₄³⁻ in an aqueous solution over time for the samples of Example 1.

TABLE 1
Adsorptive Capacity
Sample Capacity†
A      324
B      388
C      318
D      818
E      157
F      54
G      73
H      19
†Capacity (qe) = (co-cf)*V/m

TABLE 2
Sample conditions in aqueous media of 4000 ppm PO43-
	Sample	Sample ID	Sample mass (g)	NH3OH (ml)†
	A	2	1.0031	0
	B	2	1.003	0.597
	C	2	0.3223	0
	D	2	0.3227	0.597
	E	12	1.0198	0
	F	12	1.014	0.597
	G	12	0.3158	0
	H	12	0.3242	0.597
	†Solution of 28% NH3OH and 72% H2O

TABLE 3
Concentration of PO43- in solution containing various MGCO3 samples
	Time (hrs)
Sample	0	1	2	3	24
A	1.965	1.26	0.065	0.88	0.34
B	1.965	0.035	0.025	0.04	0.02
C	1.965	1.865	1.665	1.635	1.453
D	1.965	0.985	0.71	0.785	0.645
E	1.965	1.115	1.885	1.305	1.165
F	1.965	1.855	1.765	1.825	1.69
G	1.965	1.955	1.405	1.76	1.85
H	1.965	2	1.805	1.855	1.935

EXAMPLE 2

In a further example, the adsorptive capacity in terms of the molar ratio of MgCO₃:PO₄³⁻:NH₄ is examined in baked and unbaked pellets. Samples, as summarized in Table 4, of MgCO₃ powder (control) were compared against calcined and uncalcined MgCO₃ pellets formed with cellulose as a binder. These samples were further compared against pieces of naturally occurring magnesium chalk (sample 13). All samples were immersed in 100 ml of a 4000 ppm aqueous PO₄³⁻ solution. All samples had a mass of 0.3277 g. The adsorptive capacity of the samples of Example 2 are described in Table 5.

TABLE 4
Example 2 Sample Characteristics
Sample	NH4	Molar ratio
2	0.597 ml	1.1:1.0:1.1
2	0	1.1:1.0:0.0
12-baked	0.597 ml	1.1:1.0:1.1
12-unbaked	0.597 ml	1.1:1.0:1.1
12-baked	0	1.1:1.0:0.0
13	0.597 ml	1.1:1.0:1.1
13	0	1.1:1.0:0.0

TABLE 5
Example 2 Sample PO43-Adsorptive Capacity
Code	Sample #	NH4	Molar ratio	Capacity †
A	2	0.597 m;	1.1:1.0:1.1	864
B	13	0.597 ml	1.1:1.0:1.1	579
C	13	0	1.1:1.0:0.0	182
D	2	0	1.1:1.0:0.0	260
E	12 baked @	0.597 ml	1.1:1.0:1.1	131
	400 C. (1~1.5 hr)
F	12 unbaked	0.597 ml	1.1:1.0:1.1	178
G	12 baked @	0	1.1:1.0:0.0	439
	400 C. (1~1.5 hr)
H	12 baked at	0.597 ml	1.1:1.0:1.1	361
	500 C. (1~1.5 hr)
† PO43- (mg) adsorbed/MgCO3 (g)

To partially compensate for the loss of available surface area for adsorption, these structures, as shown in FIG. 3 as pelletized embodiments, are formed in such a way as to increase porosity and thus increase the available surface area for adsorption. Surface area is further enhanced by increasing the density of smaller pores (e.g., nanopores) relative to the density of larger pores (e.g., micropores) in the structure. The smaller pore diameters permit a greater number of pores per given volume and thus results in an increase in available surface area for adsorption. The smaller pores also enhance structural integrity by minimally impacting the framework of the structure.

As shown in FIGS. 4a and 4b, the structure composition was examined using SEM-EDS performing both a line scan and cross-section scan of the pellet from each batch. The uniformity was determined by comparing the percentage of carbon (C), oxygen (O) and Mg present across each scan. The BET surface area was determined using a NOVA 2000e Surface Area & Pore Size Analyzer. Samples were first purged with nitrogen gas at 150° C. overnight before analysis. The surface morphology of the MgCO₃ structures was observed using SEM at an accelerating voltage of 30 kV. The crystal structure was determined using XRD with the 2-theta diffractometer under CuKα radiation and a wavelength of 1.54 μm. The XRD patterns were analyzed using JADE software.

FIG. 4c shows SEM images of the surface of (a) a calcined pelleted formed with no cellulose before phosphate adsorption, (b) a calcined pelleted formed with no cellulose after phosphate adsorption, (c) a calcined pelleted formed from a 10% cellulose-MgCO₃ mixture before phosphate adsorption, and (d) a calcined pelleted formed from a 10% cellulose-MgCO₃ mixture after phosphate adsorption. Many particulate aggregates were observed on the surface of the MgCO₃ pellets before phosphate exposure. To confirm phosphate adsorption on the surface of the pellet, the elemental composition was determined with EDS.

FIGS. 5a and 5b depict the XRD patterns for MgCO₃ pellets before and after an adsorption isotherm. Cellulose, periclase (MgO) and brucite (MgOH) are present in the pellets before adsorption. As shown in FIG. 5a, the pellets had both variations of magnesium present due to mixing magnesium carbonate with water and then calcining the pellets. After adsorption experiments were conducted, magnesium variations were detected mostly as hydromagnesite (Mg₅(CO₃)₄(OH)₂.4H₂O) with some remaining brucite and magnesium phosphate (as cattiite) as shown in FIG. 5b. The pellet with the most phosphate present was the 15% pellet as seen with the highest peak of cattiite. Finding magnesium phosphate after the adsorption experiments further confirmed that adsorption occurred and that the increased surface area from cellulose addition was providing additional adsorption capacity. A distinct peak was seen for phosphorus on the 10% cellulose pellet while the 0% cellulose pellet did not have such a pronounced peak, indicating that the increased cellulose content resulted in an increase in phosphate adsorption, as expected.

Various magnesium phosphates can form depending upon the pH and molar concentration and are listed below.

• • Monomagnesium phosphate (Mg(H₂PO₄)²)
  • Dimagnesium phosphate (MgHPO₄)
  • Magnesium phosphate tribasic (Mg₃(PO₄)²)
  • Amorphous magnesium phosphate.The XRD patterns for each sample before and after an adsorption isotherm showed that cellulose, periclase (MgO) and brucite (MgOH) were present for the pellets before adsorption. As shown in FIGS. 5a, the pellets had both variations of magnesium present due to mixing magnesium carbonate with water and then calcining the pellets. After adsorption experiments were conducted, magnesium variations were detected mostly as hydromagnesite with some remaining brucite and magnesium phosphate (as cattiite) as shown in FIG. 5b. The pellet with the most phosphate present was the 15% 1 pellet as seen with the highest peak of cattiite. Finding magnesium phosphate after the adsorption experiments further confirmed that adsorption occurred and that the increased

Analytical grade MgCO₃ powder was formed into pellets, 6 mm in diameter and 17 mm in length on average in one non-limiting embodiment, using flat die pellet mill. Varying amounts of a cellulose binder having an average particle size of 20 μm was used to optimize the pellet design.

In an exemplary experiment, cellulose was added in amounts from 0 to 20% by mass to slurries comprised of 55% MgCO₃ by mass and 45% deionized water by mass. The cellulose acts as a binder which can be removed by calcination. Polyvinyl alcohol or similar organic polymers are also useful for this purpose.

After shaping, the pellet structures, in an embodiment, are calcined at 300° C. to remove the cellulose for additional porosity without impacting the integrity of the magnesium carbonate structure. Cellulose content and calcination time were varied to evaluate the effect of these variables as follows: 0% cellulose calcined for 17 hours (0% 17), 5% cellulose calcined for 1 hour (5% 1), 10% cellulose calcined for 2 hours (10% 2), 15% cellulose calcined for 1 hour (15% 1) and 20% cellulose calcined for 2 hours (20% 2). Small pellet structures, e.g., cylindrical pellets having a diameter of approximately 10 mm or less, were also successfully formed from MgCO₃ without the need for a binder provided that the pellet can be formed without sacrificing too much surface area provided by the pore volume. In an embodiment, a slurry pre-mix is created mixing powdered metal carbonates with or without a binder.

The slurry pre-mix is diluted in deionized water or a similar diluent that can be volatilized and mixed to form a slurry. The slurry is then dried and subsequently ground into a powder. The carbonate-diluent mixture, or the carbonate-diluent-binder mixture are then pressed into a desired form. In an embodiment, cylindrical pellets are formed having a diameter of approximately 5 mm and a length of approximately 4 mm. Cylindrical pellets are particularly useful in that they can be packed together so as to permit maximum exposure of their outer surface which optimizes access to the pores extending through the structure so as to achieve a desired accessible active surface area for scavenging phosphates and ammonia.

When the slurry is compacted, the binder material acts to form carbonate free areas within the pre-calcined mixture. During calcination, the volatilization of the diluent and pyrolysis of any binder material in the slurry creates pores in the formed structure as the volatilized diluent and gaseous combustion by-products escape from within the pressed structure. The resulting structure possesses greater surface area and structural integrity than would otherwise be available from just a pressed powder. The pressure required to form a structure from the powdered carbonate alone would result in a lower available surface area due to the collapse of pores as the material is compressed. The formed structure is then calcined to remove the binder and any remaining diluent. Ideally, the binder is a material that can be removed through calcining while leaving little char. Cellulose is a non-limiting example of an acceptable binder material.

The mass % of cellulose as a binder in the slurry should be no more than 20%, preferably no more than 15%, and most preferably between about 95% and about 15%. Binder content is optimized to ensure that a sufficient surface area is formed from the resulting increase in porosity when the binder is removed by calcining while still achieving a desired structural integrity of the resulting structure that could otherwise be compromised from making the structure too porous. If the structural integrity is insufficient, the pores will collapse and reduce the surface area available for adsorbing phosphates and ammonia. In experiments, it was generally found that pellets of MgCO₃ formed from a slurry pre-mix equal to approximately 20% cellulose by mass lacked sufficient structural integrity to maintain a useful pore volume. FIG. 6 reveals a MgCO₃-cellulose ratio curve generated from experimental data using a cylindrical 6 mm×17 mm pellet with the capacity dropping significantly as 20% cellulose was reached. The 20% cellulose pellet also a stability in appeared to be structurally unstable in water, so no higher cellulose ratio was studied. FIG. 7 depicts the relationship between pre-calcined pellet cellulose content and calcined BET surface area.

Calcining times vary by binder material, structure size, and mass percent of binder and diluent. The cellulose in the aforementioned pellets can be burned off from the resulting slurry at temperatures at or above 200° C., more preferably at a temperature at or above 300° C., and most preferably at a temperature at or above 350° C. Smaller structures such as the aforementioned pellet formed with cellulose as a binder, for example, should be thoroughly calcined for 1 to 2 hours at the previously suggested temperatures. In an embodiment, the aforementioned pellets are calcined at a temperature of 300° C. throughout the structure for 2 hours. After 2 hours, enough cellulose has undergone pyrolysis to form a pellet having a porosity of approximately 70% to 80%. After approximately 80% porosity, the pellet will lose structural integrity and will be unable to maintain a preferred pore volume. As the cellulose undergoes pyrolysis, gaseous by-products form within the slurry and escape, leaving open pores. Ideally, the binder is selected and calcined so as to minimize the production of char or other combustion by-products that could block pores and reduce the available surface area for adsorption.

FIG. 8 demonstrates the thermal stability of the aforementioned experimental MgCO₃ pellets. The pellet formed with 0% cellulose had a peak for onset degradation temperature at 319° C. and is the baseline. The pellets formed with cellulose showed improvement in onset decomposition temperature. The onset decomposition temperatures for the pellets formed with 5%, 10% 15%, and 20% cellulose were 384, 399, 364 and 403° C., respectively. The cellulose provided binding and increased the thermal stability until a cellulose content of 15% was utilized and the increase in porosity slightly decreased the stability. However, the pellet formed with 15% had a higher onset degradation temperature than the pellet formed with 0% cellulose. The pellet formed with 20% cellulose showed an increase in thermal stability over the pellet formed with 15% cellulose due to char on the surface and in the pores.

Adsorption experiments conducted on the example pellets to determine the equilibrium time for the phosphate concentration remaining in the solution after pellets had reached adsorption capacity. FIG. 9 discloses how the phosphate concentration in an experimental solution changed with respect to time. FIG. 10 demonstrates how the phosphate concentration an experimental solution changed over time when normalized by initial concentration.

These substantially water-insoluble carbonate structures possess a relatively high surface area per given volume due to their porosity and work well with standing water as well as effluent streams in both uncontrolled water run-off and end-of-pipe applications in reducing the concentration of these contaminants in water and in reducing the environmental impact of human activities such as farming and mining. Circulating water across the pellets acts to increase the contact rate of a given volume of water with the substrate. The enhanced porosity of the structures greatly increases surface area through an increase in pore volume, and thus increases the residence time of contaminated water at the liquid-solid interface of the system where adsorption takes place.

As depicted in FIG. 11, pelletized metal carbonate structures 10 can contained within a porous housing 20 and placed in water, e.g., a porous mesh or a polypropylene bag. The pelletized structure is preferably a substantially cylindrical pellet although other shapes are also useful. As shown in FIG. 12, these porous housings 20 of pellets 10 may also be placed within an open cell foam casing 30 as a barrier to common debris (e.g., leaves, wood, and insects) that could potentially interfere with the porosity of the pellet housing 20. These carbonate structures can also be formed as other structures, FIG. 13, that are intended to come into contact with wastewater, e.g., liners. In a non-limiting example, a wastewater pipe 50 may utilize a metal carbonate liner 40. As shown in FIG. 14, the metal carbonate pellets 10 may also be used as media in a flow through column 60.

As phosphates are adsorbed by the carbonate pellets, newberyite (MgHPO₄(H₂O)₃) is formed. When ammonia is also present and bound to the pellet, struvite (MgNH₄PO₄(H₂O)₆). The contaminated pellets that contain captured phosphates and/or ammonia may be ground and utilized as a slow-release fertilizer, resulting in the conservation of phosphorous as a resource while contributing to the removal of phosphates from the environment through their capture from wastewater.

Desorption experiments were conducted to evaluate the potential to release the recovered phosphate. The concentration of phosphate that returned to the solution was measured and the desorption percentage of phosphate was calculated which confirmed the desirability of spent or loaded pellet for use as a slow-release fertilizer

Sample Characterization: The Brunauer, Emmett, and Teller (BET) surface area of the resulting adsorbent structure was determined using a Tristar 3000 porosimeter analyzer (Micromeritics). Prior to characterization, the samples were first outgassed by purging with nitrogen gas at 150° C. for 2 hours. The surface morphology of the various materials was characterized using an environmental scanning electron microscope. Elemental analysis of the samples was performed using Energy-dispersive X-ray spectrophotometer (EDS) installed in the ESEM. The crystal structure of the adsorbents was determined by X-ray diffraction (XRD) analysis using a 2-theta diffractometer at a wavelength of 1.54 μm and at 2-theta range 2-90° under CuK_α radiation. To gain further insights on the physical properties of the synthesized materials, high resolution-transmission electron microscopy (HR-TEM, model JEM-2010F, obtained from JEOL) was used with a field gun emission at 200 kV. Before analysis, the materials were dispersed by ultrasonication in 99.8% pure isopropyl alcohol for 20 min. Then, a single drop of the supernatant was fixed on a carbon-coated copper grid (LC325-Cu, EMS) and dried at room temperature prior to imaging. The obtained images were analyzed using ImageJ, an image processing software.

Adsorption Experiments: To evaluate the effectiveness of each adsorbent for the removal of phosphate, several adsorption experiments were conducted and their results compared. Variable dose isotherm experiments were conducted to determine equilibrium adsorption parameters. Varying masses of adsorbent, ranging from 0.15-1.5 g, were placed in 125 mL Nalgene polypropylene bottles with 100 mL of the phosphate stock solution. The solution was prepared by dissolving sodium phosphate monohydrate in deionized water (2 mM) with 15 mM MOPS buffer to maintain a constant pH (pH 7). The bottles were placed on a rotary shaker at 150 rpm for 2 weeks to ensure equilibrium was reached. After adsorbent saturation, samples were filtered using a 0.45 μm polypropylene syringe filter and analyzed for phosphate concentration remaining in solution.

Column tests were conducted in 80 cm height and 1.9 cm diameter Harvel plastic columns. Ten grams of adsorbent media was placed in the columns with sand and gravel above and below, as well as a stainless-steel sieve at the bottom end of the column to prevent washout. Using a peristaltic pump, the phosphate solution (at an initial phosphate concentration of 215 mg L⁻¹), was passed through the column at a rate of 2 mL min⁻¹ at room temperature. Similar to the isotherm experiment, solution pH was adjusted initially and buffered to remain constant. The column effluent samples were collected, filtered using a 0.45 μm polypropylene syringe filter, and analyzed for phosphate concentration at various time periods. All isotherm and column experiments were conducted once and sample measurements were analyzed in triplicate and averaged.

The phosphate concentration in all experiments was analyzed by a colorimetric measurement technique in which ammonium molybdate and potassium antimony tartrate react in an acidic solution with orthophosphate to form phosphomopydbic acid which can be reduced by ascorbic acid to form an intense blue color. The absorbance due to the blue complex was monitored at 880 nm using a UV-Vis spectrophotometry. This is based off the US EPA Method 365.1 for the determination of dissolved orthophosphate.

The BET surface area for each adsorbent was measured prior to and after phosphate adsorption, as illustrated in Table 2. The adsorbent with the highest BET surface area was the MgCO₃ pellet, which had a surface area of roughly 26 m² g⁻¹ prior to phosphate adsorption, while the other adsorbents had much lower surface areas of about 2 m² g⁻¹. Since adsorption is a surface-based process, higher surface areas should correlate to an increased adsorption capacity as there are an increased number of sites for the phosphate ions to adhere to the sorbent surface. Upon comparison of BET surface areas prior to and after phosphate adsorption, the used samples were found to have higher surface areas. This increase in surface area after adsorption indicates that the phosphate is adsorbed onto the material surface, forming a surface complexation, thus resulting in an increased surface area when compared to the unused sorbents.

SEM was conducted to evaluate the surface morphology of the different adsorbents before and after PO₄³⁻ adsorption as illustrated in FIG. 1. The different adsorbents yielded quite different surface morphologies, which may play a significant role in overall phosphate adsorption. For the CaCO₃ sample, seen in FIGS. 1 (a) and (b), the surface structure appears to form as a bulky, irregular crystal with particles ranging from nano- to micron-sized. The La₂(CO₃)₃ sample, illustrated in FIG. 1 (d), revealed the formation of aggregates ranging from 0.5 to 2.0 μm after PO₄³⁻ adsorption compared to the pellet before adsorption as seen in FIG. 1 (c). FIG. 1 (f) shows SEM images for the MgCO₃ adsorbent. This material had a sheet like structure, similar in appearance to the mineral selenite rose, with amorphous “sheets” averaging 2 μm in length.

FIG. 2 shows XRD patterns of MgCO₃, CaCO₃, and La₂(CO₃)₃ samples. The peaks of XRD spectra were identified using JADE software (MDI, Inc., Livermore, Calif.) with JCPDS 04-013-763 1 for hydromagnesite (Mg₅(CO₃)₄(OH)₂(H₂O)₄), 04-009-5447 for magnesium oxide (MgO), 04-010-3609 for lanthanite (La₂(CO₃)₃(H₂O)₈), 01-080-9776 for calcium carbonate (CaCO₃) and 00-036-0426 for dolomite (CaMg(CO₃)₂). As seen in FIG. 2 (a), raw MgCO₃ powder was already converted into hydromagnesite due to humidity in the air. It was partially converted into MgO during the heat treatment with cellulose for the pellet preparation. MgO was converted into hydromagnesite again during PO₄³⁻ removal processes. Unfortunately, the formation of newberyite (MgHPO₄(H2O)₃) was not observed, which may be due to concentrations below the detection limit. This may indicate that PO₄³⁻ adsorption occurs on the surface of pellets since the presence of phosphorus was detected by EDS analysis (see Figure S1). For lanthanum pellets, lanthanite (La₂(CO₃)₃(H₂O)₈) was observed in raw La₂(CO₃)₃ powders due to humidity in the air. However, lanthanite peaks were not detected in the sample calcined with cellulose but lanthanum remained as seen in Figure S1. Again, lanthanite formed after PO₄³⁻ adsorption. A similar phenomenon was observed in the MgCO₃ samples where no peaks corresponding to phosphorus containing lanthanum were detected. This may also be due to the surface-limited reaction for PO₄³⁻ adsorption. In this case, although the peak corresponding to phosphorus was detected in EDS analysis, the concentration of phosphorus could not be determined because of lower concentration of phosphorus on the surface of La₂(CO₃)₃ pellets as well as a masking effect due to gold coating for SEM analysis (see Figure S1). For CaCO₃ pellets, two compounds, CaCO3 and CaMg(CO₃)₂, were detected and these phases did not change during the entire preparation and treatment processes. This indicates CaCO₃ samples are very stable in water. Interestingly, no phosphorus containing forms in all three pellets were detected with XRD analysis. As discussed before, this is likely due to the surface-limited reaction for PO₄³⁻ adsorption and EDS analysis supported the findings.

FIG. 3 shows HR-TEM images of each sample. As seen in FIG. 3 (a), the measured lattice spacing in the MgCO₃ pellets before PO₄³⁻ adsorption were 0.270 and 0.211 nm, corresponding to (321) plane of Mg₅(CO₃)₄(OH)₂(H₂O)₄ and (400) plane of MgO, respectively. After PO₄³⁻ adsorption, the lattice spacing of 0.230 nm, which corresponds to (400) plane of Mg₅(CO₃)₄(OH)₂(H₂O)₄, was measured (see FIG. 3 (b)). These results were in good agreement with the results of XRD analysis showing the presence of both hydromagnesite and magnesium oxide in the pellet before adsorption process and MgO was converted into hydromagnesite after PO₄³⁻ adsorption. As seen in FIGS. 3 (c) and (d), the measured lattice spacing of 0.272 and 0.301 nm corresponding to (016) and (115) planes of La₂(CO₃)₃(H₂O)₈, respectively, indicated the presence of lanthanum carbonate in the pellets even though the XRD patterns were not clear after the pellet preparation using cellulose. For CaCO₃ pellets, lattice spacings of 0.303 and 0.153 nm were observed, which correspond to the (104) plane of CaCO₃ and (122) plane of CaMg(CO₃)₂, respectively. These results are also in good agreement with the XRD results. Unfortunately, no lattice spacing corresponding to phosphorus-containing compounds was observed in the analyzed area of each sample after PO₄³⁻ adsorption since a very limited area can be shown with HR-TEM analysis at very high magnification of 800,000.

Adsorption Results: The specific relationship between the equilibrium adsorbate concentration in solution and the amount adsorbed at the surface can be revealed by adsorption isotherms. The isotherm results for phosphate adsorption onto the La-, Ca-, and Mg—CO₃-based sorbents at a constant temperature of 21° C. were analyzed using the Langmuir and Freundlich isotherm models. The Langmuir adsorption equation is based on the assumptions that: (1) adsorption is limited to one monolayer, (2) all surface sites are equivalent (i.e. free of defects), and (3) adsorption to one site is independent of adjacent sites occupancy condition^[36]. The Langmuir isotherm is expressed as:

$q_e=\frac{q_{\max }{K}_L{C}_e}{1+K_L{C}_e}$

where q_e is the amount of adsorbate adsorbed per unit mass of adsorbent (mg/g), C_e is the amount of unadsorbed adsorbate concentration in solution at equilibrium (mg/L), q_max is the maximum amount of adsorbate per unit mass of adsorbent to form a complete monolayer on the surface (mg/g), and K_L is a constant related to the affinity of the binding sites (L/mg). In its linear form, the Langmuir equation can be expressed as:

$\frac{C_e}{q_e}=\frac{1}{q_{\max }}{C}_e+\frac{1}{K_L{q}_{\max }}$

A linear plot of specific adsorption against equilibrium concentration ((C_e/q_e) vs. C_e) as seen in FIG. 4 indicates that phosphate adsorption onto the La-, Ca-, and Mg—CO₃-based adsorbents obeys the Langmuir model. The Langmuir constants q_max and K_L, determined from the slope and intercept of the plot, are presented in Table 2. While the LaCO₃ and MgCO₃-based adsorbents had similar monolayer phosphate adsorption capacities (49.5 and 52.6 mg/g, respectively), the CaCO₃-based adsorbent had a much lower capacity for phosphate adsorption (18.7 mg/g). The dimensionless constant separation factor R_L^[38] can be used to express essential characteristics of the Langmuir isotherm according to the following equation:

$R_L=\frac{1}{1+K_L{C}_0}$

where C₀ is the initial adsorbate concentration (mg/L) and K_L is the Langmuir constant (L/mg). Values of R_L can indicate the favorability of adsorption; that is, for favorable adsorption, 0<R_L<1; for unfavorable adsorption, R_L>1; R_L=1 for linear sorption; and for irreversible adsorption, R_L=0^[35]. Values of R_L, documented in Table 2, were in the range of 0-1, suggesting favorable adsorption of phosphate onto the La-, Ca-, and Mg—CO₃-based adsorbents.

The Freundlich isotherm, applicable for non-ideal adsorption on heterogeneous surfaces with multi-layer sorption, is expressed as:

q_e=K_FC_e^1/n

where K_F is the adsorption capacity of the adsorbent (mg/g (L/mg)^1/n) and n indicates sorption favorability, with values of n in the range 1<n<10 indicating favorable sorption. As values of n approach 1, the impact of surface heterogeneity can be assumed less significant and as n approaches 10, surface heterogeneity becomes more significant. Typically, adsorption capacity of an adsorbent increases as the values of KF increase. The Freundlich constants K_F and n can be determined by the linearized form of the Freundlich equation:

${\log q}_e={\log K}_F+\frac{1}{n}{\log C}_e$

The linear plot of the Freundlich isotherm for phosphate adsorption onto phosphate the La, Ca-, and Mg—CO₃-based adsorbents is shown in FIG. 5. The Freundlich constants were determined from the slope and intercept of the plot and are documented in Table 2.

Isotherm results best followed the Langmuir model, which assumes the formation of a monolayer of adsorbate on the adsorbent. According to the Langmuir isotherm, the Mg—CO₃-based adsorbent proved to have the highest adsorption capacity, followed by the La—CO₃-based adsorbent while the Ca—CO₃-based adsorbent was not as effective at removing phosphate. The increased phosphate removal for the MgCO₃ material is likely due to its increased BET surface area.

Column experiments were conducted to evaluate the phosphate adsorption as would be seen in an industrial-scale fixed bed adsorber. The breakthrough curves were constructed by plotting the ratio of PO₄³⁻ concentration at time t to the initial influent concentration (C/C₀) versus time (t). FIG. 6 shows the typical “S” shape of the breakthrough curves indicating the effects of mass transfer parameters as well as internal resistance within the column. Phosphate adsorption was initially high, decreasing with time until fully saturated. Breakthrough for LaCO₃ and CaCO₃ occurred at 30 min while, for MgCO₃, the time to reach breakthrough was 1 hr. Yet, after 7 hr of operation, the CaCO₃ adsorbent was 95% saturated while LaCO₃ and MgCO₃ were only 73 and 74% saturated, respectively. Though the time to reach breakthrough was twice as long for the MgCO₃ sorbent compared to the LaCO₃ sorbent, the LaCO₃ sorbent proved to have the greatest phosphate column capacity as well as having a longer operation time to reach 95% saturation (36 hr compared to 30 hr), indicating that the LaCO₃ adsorbent was the best sorbent for phosphate adsorption in continuous column experiments.

The cumulative adsorption capacity of the columns for phosphate adsorption was determined and illustrated in Table 3. Cumulative column adsorption capacity for LaCO₃, CaCO₃, and MgCO₃ was 20.1, 13.0, and 17.8 mg/g, respectively. These results show that the phosphate adsorbent capacity of the adsorbents in columns were lower when compared to batch experiments. However, the adsorbent mass differed between experiments and this is the likely reason for differing values of adsorbent capacity. Also, batch experiments were conducted using 0.1 L of phosphate solution while the continuous column experiments passed around 5.0 L of phosphate solution through the sorbents.

Claims

1. A water contaminant adsorbing structure comprising a water-permeable aggregate of a substantially water insoluble metal carbonate substrate formed into a user desired shape, wherein said substrate adsorbs at least one of phosphate and ammonia, and said aggregate has a multiBET surface area of at least 20 m2/g and a nanopore volume of at least 7.5e−7 m3/g.

2. The structure of claim 1, wherein said aggregate has a multiBET surface area of at least 25 m2/g and a nanopore volume of at least 9e−7 m3/g.

3. The structure of claim 2, wherein said aggregate has a multiBET surface area of at least 30 m2/g and a nanopore volume of at least 1e−6 m3/g.

4. The structure of claim 1, wherein said metal carbonate is at least one of magnesium carbonate and lanthanum carbonate.

5. The structure of claim 1, wherein said structure is produced by the process of: a. creating a slurry by mixing a diluent and at least one of a powdered metal carbonate and a binder-metal carbonate mixture;b. forming preliminary structure by forming said slurry into a shape;c. calcining said preliminary structure so that only said metal carbonate substantially remains.

6. The structure of claim 5, wherein said slurry is formed by the process of: a. adding 10% to 50% water by mass to said powdered metal carbonate to create a pre-slurry of a desired consistency;b. partially drying said pre-slurry;c. grinding said pre-slurry into a granular paste; andd. shaping said structure from said granular paste.

7. The structure of claim 6, wherein said water is deionized water.

8. The structure of claim 6, wherein said binder is selected from the group consisting of cellulose and organic polymers.

9. The structure of claim 1, wherein said structure is shaped as a pellet.

10. The structure of claim 9, wherein said pellet is cylindrical.

11. A method of removing contaminants from water comprising placing a water-permeable, contaminant adsorbing structure in water contaminated with at least one of phosphates and ammonia, wherein said structure is formed from a substrate that adsorbs at least one of phosphates and ammonia, wherein said substrate is an aggregate of a substantially water insoluble metal carbonate characterized by having a multiBET surface area of at least 20 m2/g and a nanopore volume of at least 7.5e−7 m3/g.

12. The method of removing contaminants from water of claim 11, wherein said structure is selected from the group consisting of liners, screens, blocks, and ducts.

13. The method of removing contaminants from water of claim 11, wherein said structures are a placed within a water-permeable housing which retains said structures when said housing is placed in water.

14. The method of removing contaminants from water of claim 13, wherein said structures are pellets.

15. The method of removing contaminants from water of claim 14, wherein said pellets are cylindrical.

16. The method of removing contaminants from water of claim 11, wherein said metal carbonate is at least one of magnesium carbonate and lanthanum carbonate.

17. The method of removing contaminants from water of claim 11, wherein said structures are produced by the process of claim 5.

18. The method of removing contaminants from water of claim 17, wherein said structures are produced by the process of claim 6.

19. The method of removing contaminants from water of claim 18, wherein said binder is selected from the group consisting of cellulose and organic polymers.

20. A fertilizer comprising at least one granulated metal carbonate structure onto which at least one of phosphates and ammonia are adsorbed in the process of claim 10.

21. The fertilizer of claim 20, wherein said metal carbonate structure is formed by process of claim 5.

22. The fertilizer of claim 21, wherein said metal carbonate structure is further formed by process of claim 6.