The present invention relates to improved processes for preparing modified biochar/coal lignite, and to the use of such products for phosphorous remediation and as soil adsorbents/fertilizers.
Commercial fertilizer and manures are known for aiding plant growth and development, but their continued usage has been linked to environment and economic challenges.
Eutrophication, resulting from the nutrient enrichment of waters, is a current global crisis (Nixon, 2012). Phosphorous, a limiting plant nutrient widely applied in agriculture worldwide, often contaminates stormwater run-off. Environmental aqueous phosphate in concentrations as low as 100 μg/L can stimulate undesirable algal and plant blooms. These blooms lead to water quality deterioration, dissolved oxygen deficiency, biodiversity abatement, and economic losses (Zanchett and Oliveira-Filho, 2013). Annually, the US spends $2.2 billion to combat freshwater eutrophication (Dodds et al., 2009). Phosphorus regulation is essential to mitigate eutrophication. The WHO and EPA recommend that phosphorus levels should be lower than 10 mg/L in natural waters (US EPA, 2015), while phosphate concentrations should be below 0.10 mg/L in rivers and streams and below 0.05 mg/L in lakes and reservoirs, according to the Australian water quality guidelines (Huang et al., 2017). Phosphate recycling and recovery will eventually be necessary for problematic phosphorus depletion.
Phosphorous is discharged into surface waters from point source wastewater and nonpoint source run-off. Typically, municipal wastewater has approximately 5-20 mg/L total phosphorus (organic and inorganic) concentration before treatment (Hasson et al., 2016). Inorganic phosphorus mainly exists as PO43−, HPO42−, and H2PO4−, depending on the solution pH (Yin et al., 2017). Traditional precipitation, reduction, flotation, coagulation, flocculation, and membrane filtration methods have many drawbacks, including initial investment, chemical consumption, efficiency, simplicity, and scalability (Leo et al., 2011). Efficient, practical, green, and restorable techniques have been developed for contaminant removal (Bombuwala Dewage et al., 2018). After adsorptive recovery, adsorbed phosphates have been reused as soil fertilizers, making sorption technology more ecofriendly (Mosa et al., 2020). Phosphate recovery from wastewater treatment plants (WWTP) can theoretically replace 40-50% of total phosphate application needs.
Coal seams are abundant, and various grades of coal can be employed as a support/sorbent to remove contaminants from aqueous solutions (Simate et al., 2016). Aqueous Cu2+, Pb2+, and Ni2+ ions were fixed by surface carboxyl and hydroxyl groups (˜60-90%) on low-rank coal lignite because of its elevated cation exchange capacity and surface complexation ability (Pehlivan and Arslan, 2007). However, these oxygenated active centers have low affinities for anionic pollutants (Qi et al., 2011; Zhang et al., 2010). Introduction of Al3+, Ca2+, Bi3+, Fe3+, and Mg2+ to cation-deficient adsorbent surfaces previously raised phosphate removal (Fang et al., 2020; Karunanayake et al., 2019; Yang et al., 2018; Zhang et al., 2013; Zhou et al., 2013).
Yao et al., 2013 found that spent Mg-enriched tomato leaf char sorbents were successfully reused even after 10 sorption cycles and employed to treat phosphorus deficiency in soils. Equal amounts of weakly-bound P (˜3.2% of the amount of PO43− on this char) were released to water each day (average/day) over 11 consecutive days. Therefore, it behaved as a slow-release fertilizer (Yao et al., 2013). These modified, environmentally friendly sorbents could sequester C in soils and are more beneficial than commercial adsorbents. Colloidal and nano-sized MgO and Mg(OH)2 surface particles on anaerobically digested sugar beet biochar improved mononuclear and polynuclear phosphate adsorption (Zhang et al., 2012). MgO-modified peanut shell biochar adsorbed 20% more PO43− than raw peanut shell biochar due to the presence of MgO active surface sites. The application of P-laden MgO-biochar to coastal alkaline soils improved the available P (phosphorous) and P uptake by field rice plants, which increased rice yields (Wu et al., 2019). Ca2+ was released from alkaline Ca-doped biochar (Ca(OH)2: biosolids, 20 wt. %) at pH=4, into a solution where it reacts with phosphate to form insoluble brushite (CaHPO4), which precipitated (Antunes et al., 2018). The maximum phosphate removal capacity of this biochar was 79 mg-P/g (at initial pH=3). Elevated phosphate sorption was achieved by a novel Mg(OH)2/ZrO2 composite (MZ) resulting from both i) ligand exchange between the ZrO2 and MgHPO4 and ii) reaction between Mg(OH)2 and phosphate forming MgHPO4 and Mg3(PO4)2 (Lin et al., 2019). Dissolved Mg2+ originating from the Mg(OH)2 also synergistically enhanced the phosphate-binding on the ZrO2 component in MZ. Ca2+-Mg2+ pre-loaded (19 wt. % Mg2+ and 19 wt. % Ca2+/biomass) on corncob biochar had a very high (326 mg/g) Langmuir phosphate uptake capacity.
However, there still remains a need for new methods to modify cheaper and widely available lignite to remove aqueous phosphates.
The present inventor has surprisingly found that locally abundant raw lignite (RL) may be converted into a chemically and thermally modified lignite (CTL) by reacting (e.g., via a surface deposition treatment) with Mg2+ (e.g., using MgSO4, MgO, Mg(OH)2) and Ca2+ (e.g., using CaSO4, CaO, Ca(OH)2) followed by pyrolysis (e.g., at 600° C.). Without wishing to be bound by theory, the inventor theorizes that CTL prepared by such a process promotes phosphate removal form a solution due to the formation of precipitates (such as Mg3(PO4)2, MgHPO4, Ca3(PO4)2, and CaHPO4) formed upon reaction of solution phosphate with Mg2+ and Ca2+ ions released from the CTL.
Lignite-based adsorbents' high abundance, physico-chemical properties, and low-costs are attractive for traditional water treatment. These can be utilized to reduce eutrophication in natural waters by remediation of point and nonpoint sources of P.
A variety of CTLs have been developed to remove phosphate fertilizers from agriculture run off and the spent sorbent can then be recycled as a slow-release fertilizer.
Unwashed CTL could also be utilized to lower soil acidity, enhance soil fertility, and can be readily produced at a large scale in a few steps.
Accordingly, in one aspect, the present invention relates to a process for preparing a modified biochar/coal lignite (e.g., a chemically and thermally modified lignite, CTL). In one embodiment, the process comprises
In one embodiment, steps (i) and (ii) are performed sequentially. Step (ii) may be performed before step (i). In one embodiment, steps (i) and (ii) are performed simultaneously.
In one embodiment, the process further comprises
In certain embodiments, the source of Mg2+ ions is selected from MgSO4, MgO, Mg(OH)2, MgCl2, or any combination thereof.
In certain embodiments, the source of Ca2+ ions is selected from CaSO4, CaO, Ca(OH)2, CaCl2), or any combination thereof.
In certain embodiments, the source of K+ ions is selected from KOH, KCl, or any combination thereof.
In one embodiment, step (iii), if performed, is conducted at about 105° C.
In one embodiment, step (iii), if performed, is conducted for about 4 hours
In one embodiment, pyrolysis step (iv) is conducted at about 600° C. In certain embodiments, pyrolysis step (iv) is conducted under an intern atmosphere (e.g., nitrogen, argon).
In certain embodiments, step (v), if performed, involves washing the modified lignite with water (e.g., distilled water).
In one embodiment, step (iii), if performed, is conducted at about 80° C.
In certain embodiments, step (ci), if performed, is conducted for between about 12 and about 24 hours, such as between about 18 and about 24 hours. In one embodiment, step (vi), if performed, is conducted overnight (e.g., for about 18 hours).
In additional embodiments, the process further comprises
In one embodiment, the modified lignite has a particle size of less than about 150 microns. In one embodiment, the modified lignite has a particle size of between about 150 microns and about 300 microns. In one embodiment, the modified lignite has a particle size of greater than about 300 microns.
In a second aspect, the present invention relates to a process for reducing the amount of phosphate in a solution (such as a water source, e.g., wastewater or contaminated water, such as mining wastewater). In one embodiment, the process comprises
In a third aspect, the present invention relates to a process for removing agricultural waste (e.g., phosphates) from a solution (such as a water source, e.g., wastewater or contaminated water, such as mining wastewater). In one embodiment, the process comprises contacting the solution with a modified lignite (e.g., a modified lignite prepared by a process according to any of the embodiments described herein, thereby producing phosphate laden modified lignite.
In one embodiment, the process of the second and third aspects further comprise a fourth aspect, regenerating the modified lignite. In certain embodiments, the regeneration process comprises
In additional embodiments, the process of aspect 2 (or aspect 3) followed by the process of aspect 4 is conducted more than one time, such as two, three, four or five times (i.e., the process in which modified lignite adsorbs phosphate then is regenerated is repeated more than one time).
In a fifth aspect, the present invention relates to a process for preparing a fertilizer and/or soil amendment (e.g., a slow release fertilizer and/or soil amendment). In one embodiment, the process comprises
Accordingly, in a sixth aspect, the present invention relates to a process for preparing a modified biochar/coal (e.g., a modified biochar/coal useful as a fertilizer/soil amendment/soil additive). In one embodiment, the process comprises
In certain embodiments, the source of phosphorous is selected from (NH4)2HPO4.
In an eighth aspect, the present invention relates to a fertilizer and/or soil amendment (e.g., a slow release fertilizer and/or soil amendment) prepared by a process according the any of the embodiments described herein. In one embodiment, and of the modified biochar/coal lignites described herein is in the form of pellets.
As used herein the following definitions shall apply unless otherwise indicated.
As used herein, the term “thermally modified lignite” or “TL” refers to raw lignite that has been pyrolyzed, but not chemically treated (with, e.g., Mg2+ and Ca2+).
As used herein, the term “chemically and thermally modified lignite” or “CTL” refers to raw lignite that has been modified according to any of the processes described herein.
As used herein the terms “treating” refers to a reaction in which a raw lignite is surface modified with one or more specified ions via surface deposition.
Coal surface area per unit weight depends on its source and rank and are typically approximately 100 m2/g for lignite (Mohan and Pittman, 2006). A very low lignite surface area (SBET, 1 m2/g) was also reported (Milicevic et al., 2012). Specific surface areas for RL, TL, and CTL were calculated through Brunauer-Emmett-Teller (BET) theory and are shown in Table 1. CO2 and N2 were employed as adsorbates for the BET surface area determinations. BET using N2 can be inaccurate for samples with higher micropore contents (<1.2 nm) because the slow rate of N2 diffusion blocks pore filling at 77K (de Jonge and Mittelmeijer-Hazeleger, 1996). In contrast, CO2 fills micropores far faster because of its far higher thermal energy at 0° C. (McLaughlin, 2012). Specifically, the BET surface areas using N2 were 2.9, 46, and 21 m2/g (Table 1) for RL, TL, and CTL, respectively (particle size, 150-300 μm) (Table 1). The corresponding surface areas using CO2 were significantly larger (35, 127, and 120 m2/g for RL, TL, and CTL, respectively), indicating the abundance of narrow micropores in these samples.
RL's low surface area (35 m2/g) increased to 127 m2/g in TL after heating at 600° C. under N2. When the lignite is pyrolyzed, the moisture, volatiles and decomposing matter are evaporated. This out-gassing leads to new pore formation, or opening of closed pores, creating higher surface area materials. The average pore volume increased, and the average pore radius decreased slightly in TL versus RL (Table 1). A 40.5% weight loss occurred after RL's thermal treatment at 600° C. (yield of TL was 59.5%). The CTL surface area tripled versus RL (120 vs. 35 m2/g) due to fine MgO/Mg(OH)2, CaO/Ca(OH)2, and CaCO3 particle formation, possibly due to loss of tightly held water and lignite structural changes. The oxides form the corresponding hydroxides on water washing. These surface deposits close some CTL pores, reducing total pore volume relative to TL. Mg/Ca compound existence on the CTL surface was observed from SEM/EDX observations (
aAdsorbent surface areas were measured at the particle size 150-300 μm. When the particle size of all three adsorbents decreased to <150 μm, their BET surface areas (using N2) increased (RL = 2.9 m2/g, TL = 120 m2/g, CTL = 60 m2/g).
bRL, TL, and CTL uptake capacities (qe) at the particle size 150-300 μm were obtained
c Phosphate removal capacity was divided by the absorbent surface area to obtain specific sorption ability (mg/m2).
dMicropore volumes using CO2 were negligible for all three adsorbents.
eDFT theory accurately describes the pores in micro- and mesopore range.
fO content presented here does not reflect the oxygen associated with their inorganic constituents.
Coals contain micropores (<1.2 nm), mesopores (1.2-30 nm), and macropores (>30 nm) (Simate et al., 2016). Pore sizes obtained from the NL-DFT method were presented in Table 1. DFT theory accurately describes the pores in the micro and mesopore range. NL-DFT treats the sample as an effective porous material, where heterogeneity is approximated by a distribution of pore sizes.
Thus, heterogeneity due to the chemical groups on the surface, pore shape variations, pore networking, and blocking effects is not accounted for explicitly (Fraissard and Conner, 1997; Inagaki, 2006). Pore size distributions of RL, TL, and CTL were obtained using both N2-DFT and CO2-DFT. RL has a wide pore size distribution (2-25 nm). TL has a higher mesopore fraction than RL, distributed from 1.8 to 2.3 nm by N2-DFT. CTL is highly microporous, with pores narrowly distributed around 1.8.
CO2-DFT found average pore diameters ranging from 0.49 to 0.54 nm for these three lignite adsorbents (Table 1). Phosphate anions have diameters of 0.223 nm, which increase to 0.339 nm with its water hydration shell (Zhong et al., 2015). Thus, a portion of micropores in all three adsorbents have access to hydrated phosphate.
The PZC of TL (˜9.4) versus RL (˜3.9) (Table 1) reflects the presence of basic oxides, hydroxides, and carbonates formed during the 600° C. lignite pyrolysis. The high TL porosity was caused by mass loss. The following extensive washings removed many basic oxides, hydroxides, and some carbonates from TL. The PZC of CTL increased to 11.8. The abundant silica was detected in both CTL and TL is from original lignite ash (19.9%, SiO2), which is in good agreement with XRD analysis. Lignites are carbonaceous with 20-25% fixed carbon (Bowen and Irwin, 2008).
RL contains 39.4% C (Table 1). Heat treatment of RL reduced the C percentages remaining in TL (26.5%) (Table 1) while increasing the Al (0.3% to 2.3%) and Si (9.3% to 35.7%) contents in TL vs. RL. Organic matter gasification during thermolysis reduced carbon levels. The ash content of washed CTL (SiO2, 54.4%, Al2O3, 12.1%, CaCO3, 3.75%, and MgO, 4.8%) totals 75.0% (Table 1), is consistent with the ash content (˜74.6%) determined by TGA analysis run at 0-1000° C. under O2 (heating rate 10° C./min) (
After complete acid digestion of washed CTL, Mg (2.9%) and Ca (1.5%) weight percentages were determined using atomic absorption spectroscopy (AAS). Bulk Ca and Mg percentages were smaller than the amounts detected using SEM/EDX studies (Mg=2.8% vs. Ca=5.9%), and the percentages quantified using XPS (Mg=12.4% and Ca=4.8%). Thus, Ca and Mg species are more concentrated on the top ˜8 nm of the CTL sample. After washing and drying, RL (1 atm, several days, 80° C.) and TL moisture contents were similar (˜3.2%) (Table 1). The inherent moisture of coal can be either the moisture within the micropores and microcapillaries while deposited in the ground (interior adsorbed water) or surface-bound water (Karthikeyan et al., 2009). CTL has a lower moisture content (˜2.5%) than TL and RL (˜3.2% in both) after heating the samples for 2 h in a hot air-oven at 105° C.
The high background intensity in the RL XRD spectrum indicates an extensive amorphous carbon nature (
Other CTL peaks at 21.2°, 39.7°, 50.5°, and 60.0° accredited to Mg(OH)2 (Zhang et al., 2015). The peaks at 2θ=36.6°, 43.4°, 62.6°, and 74.7° can be indexed to the 111, 200, 220, and 311 planes of face-centered cubic surface MgO. The tiny 43.3° peak indicated traces of MgO (˜50.5 nm) exist on the CTL. MgO formation on the CTL is caused by the dehydration of Mg(OH)2 (ΔH=+81 J/mol) (Mastronardo et al., 2016). If MgSO4 was present, it could decompose to MgO under the reducing atmosphere.
After P removal at pH 2.2, peaks located at 2θ=29.8° and 39.7° (CaCO3 and Mg(OH)2) suffered significant intensity reduction, and the peaks centered at 60° and 36°-37° (Mg(OH)2) and 47°-48° (Ca(OH)2) vanished in P-laden CTL (
Scanning electron microscopy (SEM) analyses examined the morphology and chemistry changes after thermal and chemical modifications to RL (
RL exhibits abundant C, O, with Al, Si, Mg, Ca, K, and Fe in the surface region. SEM-EDX analysis of CTL after exposure to pH=2.2 DI water (25 mL, 200 rpm, 24 h, 25° C.), filtering and drying showed smaller Ca, Mg, Si, K, and O atomic percentages (
Tunneling electron microscopy (TEM) images of CTL showed MgO clusters (black) dispersed on the char matrix (grey) (
Efficient sorbents have a high adsorbate affinity at low adsorbate concentrations (Wu et al., 2020). Sorption affinities of RL, TL, washed CTL, and unwashed CTL were tested at low phosphate concentration (0.4 ppm) (
When the particle size decreased from >300 to <150 μm, phosphate adsorption increased by 37% (RL), 80% (TL), and 33% (CTL) (adsorbent dose 50 mg, 25 mL of 50 ppm [phosphate], 24 h, pH 5.5, 25° C.). The <150 μm particle size had a higher phosphate removal and was selected for adsorption isotherm experiments. Interestingly, RL surfaces have a higher specific sorption ability per unit surface area (6.0 mg/m2) than CTL (0.6 mg/m2) and TL (0.01 mg/m2) (adsorbent dose 50 mg, 25 mL of 50 ppm [phosphate], 24 h, pH 2.2, 25° C., particle size, 150-300 μm) (Table 1). When RL was ground to smaller particle size, <150 μm RL's sorption ability was lower (1.0 mg/m2). This is due to the increase of its surface area (2.9 m2/g) at the size <150 μm. RL can be used for water treatment with lower production costs than CTL, as it requires no pre-treatment, and is plentiful and cheap. CTL's higher capacity (11.6 mg/g) (Table 1) is due to the dissolution of Ca2+ and Mg2+ ions, which precipitates its phosphate and hydrophosphate salts.
The CTL P uptake initially increased rapidly, and >80% of the maximum adsorption capacity (17.9 mg/g) was adsorbed within 5 h (adsorbent dose 50 mg, 25 mL solution volume, 50 ppm phosphate concentration, 25° C.). This rise was due to the presence of un-occupied adsorption sites on the CTL surface at the beginning. However, CTL P removal equilibrium was achieved after ˜20 h, similar to phosphate adsorption into MgO-digested sugar beet tailings biochar (Yao et al., 2011) and Mg-enriched tomato tissue biochar (Yao et al., 2013). Rapid initial and slow subsequent uptake suggest that precipitation is not the only removal path. The reported abundant nano-CaO and MgO (PZC>10) surface species and a very high BET specific surface area of Ca—Mg/biochar (487.5 m2/g) accelerated the P binding equilibrium to within 30 min. (Fang et al., 2015). The slow P removal kinetics by washed CTL can be due to the much smaller quantities of MgO and Mg(OH)2 (after washing), which reduces the amount and rate of phosphate uptake by precipitation and adsorption. The relatively low CTL surface area (BET-N2, 21 m2/g and BET-CO2, 120 m2/g) reduces the extent of physisorption. The calculated qe of CTL (18.9 mg/g) is close to the experimental value (17.9 mg/g). CTL phosphate removal follows the pseudo-second-order kinetic model (R2=0.99), suggesting chemical bond formation. Similar trends were observed in Ca-doped biochar (Antunes et al., 2018) and MgO-modified diatomite (Xie et al., 2014). Phosphate binding onto Mg-enriched tomato tissue could be better described by an nth order model and followed multiple mechanisms (Yao et al., 2013).
Phosphate adsorption into TL (k2=0.3 g/mg min) is faster than CTL (k2=0.04 g/mg min). As the contact time increases, TL's kinetic curve exhibited a rapid phosphate uptake and plateaued ˜5 h, with a maximum phosphate absorbance of 1.9 mg/g. Pseudo-second-order kinetic model describes TL phosphate removal well.
Phosphate sorption by RL is only weakly pH-dependent (
Leaching of Mg2+ and Ca2+ from washed CTL was investigated at pH 2.2, 7, and 10 in the presence/absence of dissolved phosphate (0, 25, and 100 ppm) (25 mL solution volume, 25° C.). After reaching equilibrium with either phosphate-containing or phosphate-free solutions, the final pHs had risen (
In the presence of the dissolved phosphates, pollutant species (speciation quantities which depend on the pH), Mg3(PO4)2, CaHPO4, MgHPO4, and Ca3(PO4)2 were deposited onto the CTL, as demonstrated by XRD/XPS analysis. When the phosphate concentration rose to 25 ppm, Ca2+ precipitates as less soluble Ca3(PO4)2 (Ksp=2.1×10−33), giving less measured Ca2+ leaching (2.2 mg/L) at pH 7 (
Partial dissolution of Mg(OH)2, MgO, Ca(OH)2, and CaCO3 sites on the CTL surface is followed by precipitation of Ca2+ and Mg2+ phosphates/hydrophosphates and sorption occurs at protonated surfaces which attract HPO42−/PO43−. This gives high CTL P removal efficiency (19.9%) at pH=2.2 (
At low pH (approximately 2), TL has greater phosphate adsorption than RL, due to the higher Al (0.9% vs 0.3%) and Mg contents (0.5% vs. 0.3%), and higher surface area. Al content on biochar improved phosphate adsorption (Yin et al., 2018). Lower surface area and a surface-cation-deficiency account for the lower P-binding of RL. A mild increase in RL adsorption at pH 11.5 may be caused by some surface OH groups ion exchanging with phosphate ions.
Isotherm studies were conducted at the optimal pH (pH 2.2) and the environmentally relevant pH level (pH 7) (
The isotherm studies were conducted for pyrolyzed and washed CTL, and pyrolyzed but unwashed CTL at pH 7, 25° C. (
Thermodynamic parameters (ΔG, ΔH, and ΔS) were calculated for all isothermal studies performed at pH 2.2. Phosphate sorption was spontaneous on RL, TL, and washed CTL (negative ΔG values) and all ΔH values were endothermic (positive). ΔH values were RL (308.9 kJ/mol) TL (241.5 kJ/mol), and washed CTL (100.4 kJ/mol) were consistent with chemisorption (greater than 40 kJ/mol) and not physisorption (less than 20 kJ/mol). This is consistent with the kinetic analysis. Positive values of ΔS (RL [1.11], TL [0.91], and washed CTL [0.43] kJ/mol) revealed increased randomness in the uptake processes.
Washed and unwashed CTLs' recycling and use as a fertilizer was investigated after adsorbing phosphates. The phosphate adsorption-desorption was studied under 1000 ppm phosphate solution and 1.5 g of CTL dose, at pH 7, 25° C. (
XRD patterns of P-laden unwashed CTL and washed CTL illustrate the crystallographic structures formed upon P uptake (
Batch desorptions were carried out by stirring the P-laden washed CTL and P-laden unwashed CTL with 10 mL of 0.5 M HCl. The amount desorbed by 0.5 M HCl from the washed CTL was 0 mg/g in the first cycle because phosphate precipitation as Ca2+/Mg2+ salts is more referable at acidic pH (
Phosphate uptake of unwashed CTL (108.8, 98.3, 96.7, and 92.0 mg/g) is far better than the washed CTL (102.2, 87.6, 94.9, and 90.0 mg/g) on four regeneration cycles (
Phosphate desorption kinetics of P-laden washed CTL was investigated at different pH levels (6.5, 7.0, and 7.5) using deionized water, and the data were fitted using a second-order kinetic model (
At low pH, Ca/Mg phosphates tend to precipitate and becomes unavailable to plants. When the initial pH rose from pH 6.5 to 7.5, the equilibrium P concentrations in DI water also rose after 20 days where the final pH values were, 9.8, 9.7, and 9.4, respectively (
Phosphate-binding interactions on CTL were further characterized by XPS before and after P removal. CTL high resolution (HR) C1s XPS spectrum before P uptake contained five deconvoluted peaks assigned to C—C/C—H (284.3 eV), C—O (285.2 eV), C═O (286.2 eV), COOR (287.1 eV), and CO32- (290.0) (
After phosphate uptake at pH 2.2, the low-resolution CTL survey spectrum exhibited a new 134.9 eV peak due to surface phosphate precipitation. The higher P atomic percentage of CTL (7.7%) versus RL (2.0%) and TL (4.3%) after phosphate uptake demonstrated CTL's greater phosphate sorption ability. There are two key processes involved in the phosphate uptake on Mg—Ca rich biochar; surface adsorption of phosphates (Yao et al., 2011) and Mg(H2PO4)2, MgHPO4, Ca(H2PO4)2, and CaHPO4 precipitation (Yao et al., 2013). However, the phosphate surface adsorption did not play a major role on CTL, as presented by SEM/EDX analysis; precipitation dominated.
The P-laden CTL Mg1a spectrum had four key peaks at 1302.6 eV, 1303.9 eV, 1304.9 eV, and 1305.8 eV (
CTL has a very high PZC (˜13), and both MgO and Mg(OH)2 have PZCs around pH 12. When the solution pH is below the PZC, the adsorbent surface is positively charged; MgO, and CaO (if present) can acquire surface hydroxyls, whereas Mg(OH)2 can be protonated (Yao et al., 2011). At low pH, HPO42−/PO43− electrostatically interact with protonated Mg(OH)2 and MgO sites on CTL. Therefore, electrostatic interactions promote phosphate removal. Around a pH of approximately 4, P salts precipitate as MgHPO4, Mg3(PO4)2, and CaHPO4. Lin et al., 2019 reported a similar Mg—P formation on the Mg(OH)2/ZrO2 surface during phosphate uptake.
After CTL's phosphate uptake, the M-OH surface region's oxygen percentage (for M=Al3+ or Si4+) decreased (from 3.5% to 2.6%). The phosphate binding caused a drop of M-OH oxygen percentage on CTL, consistent with SEM/EDX studies. The ratio between M-OH of the adsorbent before phosphate exposure versus the P-laden adsorbent M-OH can be 0.5 (monodentate complex) or 2 (bidentate complex). Here, that ratio is 1.3 (3.5%/2.6%), which is within the permitted range. Mononuclear monodentate, mononuclear bidentate and binuclear bidentate phosphate complexes can potentially form Al and Si bound surface hydroxyls on CTL, in agreement with Li et al., 2013. However, this inner sphere chemisorptive complexation is only a small fraction of the overall CTL phosphate uptake.
The HR—XPS Ca2p spectrum of P-laden CTL contains Ca2p3 peaks at 347.7 eV (CaO, CaHPO4, Ca3(PO4)2) and 348.5 eV (Ca3(PO4)2, CaHPO4), and a Ca2p1 peak at 349.5 eV (CaCO3). This indicates the existence of Ca2+ on the surface and possible Ca2+/phosphate interactions (
Overall, CTL's Mg2+ and Ca2+ contents greatly exceeded RL's Mg (12.4% vs. 0.5%) and Ca (4.8% vs. 1.0%) and produced high phosphate uptake. After removing P from water, the P surface region percentage from XPS quantifications was highest in CTL (7.7%) vs TL and RL (4.3% vs. 2.0%). The HR P2p XPS spectrum's peaks were assigned to the 1.0% MgHPO4 (132.9 eV), 4.5% Mg3(PO4)2 (133.9 eV), and 2.2% Ca3(PO4)2, CaHPO4 (135.0 eV). Ca2+ or Mg2+/HPO42− complexes are thermodynamically more stable than H2PO4− and interact with the positively charged adsorbent surfaces. Precipitation of CaHPO4, Ca3(PO4)2 MgHPO4, and Mg3(PO4)2 on the CTL surface increases surface P percentages as described above. In summary, CTL phosphate remediation proceeds largely via precipitation of Ca2+ and Mg2+ salts originally released by CTL (Equations 1-4). At high pH (>9) speciation favors PO43, so Ca3(PO4)2 was precipitated (Equation 4). The electrostatic interaction of protonated surfaces with HPO42− and PO43− species contributes CTL's P uptake under environmental pH levels (pH=6-9).
Mg(OH)2+HPO42−→MgHPO4 (1)
3Mg(OH)2+2PO43−→Mg3(PO4)2 (2)
Ca(OH)2+HPO42−→CaHPO4 (3)
3Ca(OH)2+2 PO43−→Ca3(PO4)2 (4)
Turning to
The weightometer 104 measure the weight of the material and outputs a mass flow rate (rotations per minute) to the control system 106. The weightometer (measures weight) and has an integrator disc that integrates both variables perfectly because the integrator registering disc instantaneously reacts to variations of weight, and corresponds at all times to, the true position of the scale beam; also, the true speed of the conveyor is at all times transmitted to the integrator belt. For these reasons, the remarkable accuracy of weighing with the weightometer is consistent and assures accuracy. The control system 106 receives the initial moisture content and final moisture content from the moisture sensor 102, and receives the mass flow rate from the weightometer 104. Based on that information, the control system 106 determines the weight of the material and a data signal processor 108 sends a control signal with a desired mass flow rate to adjust the speed of the spray system, i.e., how much liquid is to be sprayed on the material. This provides feedback to the spray system to adjust the flow setpoint. That information also includes residence time, which can be provided to a solar dryer. The mass flow rate is weight measured and adjusted to the speed of the conveyor belt.
Thus, the wetness or dryness of the material is defined by the moisture content. The weight of the material is influenced by the amount of water in the product, which in turn influences the flow. Thus, the weightometer 104 determines the flow of the material to the spray system and the amount of liquid to be sprayed on the material. The flow is based on the amount of water, which is based on the weight. The control system 106 gives the feedback to the spray system. This method provides an environmentally sound determination of putting the exact amount of liquid on the material from the spray system to the material.
The Moisture Meter Process includes Conveying, Measuring, Regulating. The Moisture Meter measures the water in the material to output an electrical signal. The weightometer (measures weight) and has an integrator disc integrates both variables perfectly because the integrator registering disc instantaneously reacts to variations of, weight) and corresponds at all time to, the true position of the scale beam. The mass flow (weight measured by the weightometer equipped with an integrator disc which tell the speed of the conveyor belt (adjusting the speed according to weight). Distributed control continuously monitors and adjust the process of the spray system to the exact amount of Liquid 1 needed.
It is noted that the distributed control system 106 can include a processing device to perform various functions and operations in accordance with the disclosure. The processing device can be, for instance, a computer, personal computer (PC), server or mainframe computer, or more generally a computing device, processor, application specific integrated circuits (ΔSIC), or controller. The processing device can be provided with one or more of a wide variety of components including, for example, wired or wireless communication links, input devices (such as touch screen, keyboard, mouse) for user control or input, monitors for displaying information to the user, and/or storage device(s) such as memory, RAM, ROM, DVD, CD-ROM, analog or digital memory, flash drive, database, computer-readable media, floppy drives/disks, and/or hard drive/disks. All or parts of the system, processes, and/or data utilized in the system of the disclosure can be stored on or read from the storage device(s). The storage device(s) can have stored thereon machine executable instructions for performing the processes of the disclosure. The processing device can execute software that can be stored on the storage device. Unless indicated otherwise, the process is preferably implemented automatically by the processor substantially in real time without delay.
Turning to
Starting with preparation of the material, the material is retrieved from the storage 122, such as a silo (
As further shown in the system 150 of
The system is environmentally friendly and cost effective using the exact quantity of spray and not overdrying the product in the solar dryer so less degrade in the product.
Lignite was provided by the Mississippi Lignite Mining Company (Red Hills Mine, Ackerman, MS, USA). Raw lignite (RL) was washed thoroughly with deionized water to remove extraneous materials such as dirt, sand, and other impurities, followed by oven drying at 80° C. for 48 h (1 atm, air). The dried lignite was ground into fine particles using a high-speed multifunctional grinder (CGOLDENWALL, China, 2500 W, 36000/min, model no: HC150T2) and sieved to 150-300 μm. A high SiO2 fraction was (19.9%) in this lignite's ash (total 25.0%). All chemicals used, including magnesium sulfate, calcium sulfate hemihydrate, potassium hydroxide, concentrated sulfuric acid, ammonium molybdate, and ascorbic acid, were analytical grade and purchased from Sigma Aldrich.
Preparation of Ca2+ and Mg2+ Loaded Lignite Adsorbent (CTL)
RL (100 g), washed and dried, was treated (impregnated) with a single solution of MgSO4 and CaSO4 formed by combining two solutions prepared separately. A 10% aqueous MgSO4 solution (10 g of MgSO4 [0.083 mol] dissolved in 100 mL of water, [1.992 g of Mg]) was prepared. Then a 10% aqueous solution of CaSO4·½ H2O (10 g of CaSO4·½ H2O [0.069 mol] dissolved in 100 mL of water, [2.76 g of Ca]) was made and added to the MgSO4 solution. Next, a 1.5 M aqueous KOH (350 mL, 29.4 g of KOH, 13.9% wt. of K) was added to the combined MgSO4 and CaSO4·½H2O solution to adjust the pH to ˜13.9. RL (100 g) was stirred in the Ca2+— Mg2+ and KOH containing solution for 1 hour and aged for 24 hours. Then, the resulting slurry was transferred into watch glasses. These slurries were then oven-dried (1 atm, 105° C., 4 hours) and vacuum oven-dried (0-4.9 atm, 60° C. for overnight). The dried material weighs 139.2 g. This was then pyrolyzed at 600° C. in a muffle furnace under nitrogen at a 20° C./min ramp rate to 600° C., followed by holding at 600° C. for 1 hour. This temperature was chosen according to Takaya et al., 2016. The resulting solid (wt. 89.0 g) was washed with DI water, oven-dried (1 atm, 80° C. overnight), giving a solid (53.9 g). This weight difference showed that substantial amounts (35.1 g) of soluble Ca2, Mg2, and K+ compounds were removed. This resulting CTL was crushed to particle sizes smaller than 0.3 mm. CTL was then sieved into three particle sizes (<150 μm, 150 μm-300 μm, and >300 μm) and stored in air-sealed containers for future characterization and adsorption experiments. An as-received raw lignite sample (100 g) was identically pyrolyzed at 600° C., without adding any Ca/Mg chemicals, generating thermally-treated lignite (TL) (59.5 g) to compare with CTL and RL, after a wt. loss of 40.5 g.
Surface areas, DFT pore sizes, pore volumes, and micropore volumes of RL, TL, and CTL were determined. The surface areas were measured using N2 and CO2 physisorption using the BET method run on a Micromeritics Tristar II Plus surface analyzer. Scanning electron microscopy (SEM) was performed using a Carl Zeiss EVO50VP Variable Pressure Scanning Electron Microscope with an accelerating 15 kV voltage. A JEOL 2100 200 KV TEM with Oxford X-max 80 EDS detector was used to evaluate the CTL's inner morphology. Surface region (depth of 3.1 μm) elemental distribution was determined by Energy-dispersive X-ray spectroscopy using a Bruker Quantax 200 X Flash EDX Spectrometer System (LN2-free high-speed 30 mm2 SDD Detector) under a magnification 150×, employing an interaction diameter of ˜3.8 μm. Surface chemistry was studied using X-ray photoelectron spectroscopy to elucidate elements present and their oxidation states to a maximum detection depth of 80 Å. XRD analysis was performed on RL, TL, and CTL to a penetration depth of 0.5 mm and a spot size of 1 cm2. An ECS 4010 elemental combustion system (Costect Analytical Technologies Inc.) was used to analyze the C, H, and N composition. The samples were oven-dried for 2 hours at 105° C. before assessing their moisture contents. The samples were heated in air in a muffle furnace at 750° C. for 4 hours in an uncovered porcelain dish to determine their ash contents. Organic oxygen percentage was calculated by (100−[C+H+N+ash]). NaCl solutions, adjusted from pH 2-12 using 1 M HNO3 and 1 M NaOH, were used with a pH meter to determine the adsorbents' point of zero charges (PZC). Total Mg and Ca loadings of CTL were determined using AAS after complete acid digestion with 1:1 95% H2SO4/70% HNO3 (50 mL).
Unless otherwise specified, a 0.05 g adsorbent dose, 50 ppm phosphate concentration, and 25 mL solution volume were used in batch experiments without a pH adjustment (pH=5.5). This initial pH changed due to leaching of Ca2+/Mg2+ from the CTL, as presented in section 3.4.2. Batch experiments were conducted in a Thermo Forma Orbital Shaker (200 rpm, 25±0.5° C.) for 24 h to achieve equilibrium. The vials were removed after the shaking period, and the suspensions were filtered through Whatman 1001-110 Qualitative filter papers (11.0 cm diameter, pore size, 11 μm). Three replicates of each experiment were performed. Solution pH was determined before and after adsorption. The residual phosphate concentrations in the filtrates were determined calorimetrically by following the reduction of the blue-colored molybdenum phosphate complex at 830 nm using a Shimadzu, UV-2550 double beam Spectrophotometer. The analysis was conducted according to the ascorbic acid method of Lozano-Calero.
Phosphate sorption versus pH was determined by varying the solution pH from 2.2 to 11.5 by dropwise addition of 1 M HCl or 1 M NaOH. Kinetic experiments employed samples containing 50 ppm phosphate concentrations, collected at preselected times (5 min. up to 24 h). Adsorption isotherm experiments were conducted using 25-1000 ppm phosphate solutions under the optimum adsorption pH (˜2.2) and practically important pH level (pH-7) at 25, 30, and 40±0.5° C. for 24 h.
Calcium and Magnesium Leaching from CTL
A control experiment was conducted to investigate Ca2+ and Mg2+ leaching into DI water at pH 2.2. Washed CTL (0.1 g) was added into 50 mL DI water (without phosphates) at pH 2.2. This suspension was stirred for 24 hours at 25° C. (200 rpm), filtered, and the filtrates were quantified using AAS for leached Ca and Mg amounts.
The regeneration tests for P-laden washed CTL and P-laden unwashed CTL were conducted using an aqueous NaOH stripper and performed according to Du et al., 2019 with a minor modification. CTL (1.5 g) was first equilibrated with 750 mL of 1000 ppm phosphate solution in a mechanical shaker (200 rpm, 25° C., 24 hours) at pH 7. After phosphate uptake, the suspension was filtered, and the P-loaded CTL was washed with DI water (˜50 mL) to remove traces of unadsorbed P and only H-bonded phosphate on the CTL surface. After oven-drying (1 atm, 2 hours, 105° C.), P-loaded CTL was desorbed using a 1 M NaOH (10 mL, 25° C.) stripping treatment while stirring in a single batch. The filtrates were analyzed for released phosphate concentrations using the same colorimetric technique as previously described. Four adsorption-desorption cycles were performed. Since NaOH was not a potent phosphate stripping agent, both washed CTL and unwashed CTL sorbents were subjected to acidic stripping (
A desorption kinetic study was conducted on P-laden washed CTL. Initially, phosphate was adsorbed onto CTL (0.6 g) from a solution (100 ppm, 300 mL) in a plastic bottle during vigorous shaking for 24 h (pH 7, 25° C., 200 rpm). This suspension was filtered; the P-loaded CTL was washed with DI water (˜150 mL) to remove unadsorbed P and then oven-dried (1 atm, 105° C.) overnight. A series of 100 mL DI water samples (pH=6.5, 7.0, and 7.5) were prepared, and P-loaded CTL (0.15 g) was added to each. Samples (1 mL aliquots) were removed on consecutive days, and leached phosphate concentrations were determined as described in section 2.3. The pH of the DI water was also measured each day.
The following references may be pertinent to this disclosure.
The description of the present embodiments of the invention has been presented for purposes of illustration but is not intended to be exhaustive or to limit the invention to the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art. As such, while the present invention has been disclosed in connection with an embodiment thereof, it should be understood that other embodiments may fall within the spirit and scope of the invention.
All patents and publications cited herein are incorporated by reference in their entirety.
This application claims the benefit of U.S. Provisional Application No. 63/282,052, filed Nov. 22, 2021, the entire contents of which is hereby incorporated by reference.
At least some aspects of this invention were made with Government support from the National Science Foundation INFEWS REU Program under Grant No. 1852527. The Government has certain rights in this invention.
Filing Document | Filing Date | Country | Kind |
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PCT/US2022/080337 | 11/22/2022 | WO |
Number | Date | Country | |
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63282052 | Nov 2021 | US |