Innovative Integrated Technique for Nutrient Acquisition

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Innovative integrated technique for nutrient acquisition: Simultaneous recovery of carbon and nitrogen source from the anaerobic fermentation liquid of food waste Tao Zhou, Shuya Wu, Lianghu Su, Jianying Xiong, and Youcai Zhao ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.8b02336 • Publication Date (Web): 09 Jul 2018 Downloaded from http://pubs.acs.org on July 14, 2018

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ACS Sustainable Chemistry & Engineering

Innovative integrated technique for nutrient acquisition: Simultaneous recovery of carbon and nitrogen source from the anaerobic fermentation liquid of food waste

Tao Zhoua, Shuya Wua, Lianghu Sub, Jianying Xiongc, Youcai Zhaoa,d*

a

The State Key Laboratory of Pollution Control and Resource Reuse, School of

Environmental Science and Engineering, Tongji University, 1239 Siping Road, Shanghai 200092, China b Nanjing Institute of Environmental Sciences, Ministry of Environmental Protection, Nanjing 210042, China c Shanghai Municipal Engineering Design Institute (Group) Co., Ltd., 901 North Zhongshan Road (2nd), Shanghai, China d Shanghai Institute of Pollution Control and Ecological Security, 1515 North Zhongshan Rd. (No. 2), Shanghai 200092, PR China

*Corresponding author. Tel.: +86 13917048171. Postal mail: The State Key Laboratory of Pollution Control and Resource Reuse, School of Environmental Science and Engineering, Tongji University, 1239 Siping Road, Shanghai 200092, China E-mail: [email protected] (Youcai Zhao)

ACS Paragon Plus Environment

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Abstract Anaerobic fermentation liquid from food waste contains abundant nutrients, such as volatile fatty acids (VFAs) and NH3-N, which are promising nutrient sources for plant growth. Here, we report a novel integrated technique for the simultaneous recovery of C and N sources from anaerobic fermentation liquid. VFAs and NH3-N extracted from anaerobic fermentation liquid were synthesized and converted into layered double hydroxides (LDH) and NH4HCO3, respectively, for subsequent use as soil amendments to provide external C and N to plants. The X-ray diffraction, Fourier-transform infrared spectroscopy, and scanning electron microscopy/energy dispersive X-ray spectroscopy results suggested that VFAs were successfully intercalated within the LDH, and the composite (VFA-LDH) exhibited good sustained release performance. In the VFA-LDH synthesis process, the concentrations of chemical oxygen demand, VFAs, and NH3-N were reduced by 49.4%, 27.9%, and 43.6% in the anaerobic fermentation liquid, respectively. In addition, VFA-LDH and NH4HCO3 promoted increased soil fertility, including organic matter content, cation exchange capacity, and N and P contents. Overall, the developed technique exhibits great potential as an anaerobic fermentation liquid treatment and nutrient acquisition method.

Keywords Anaerobic fermentation liquid, Carbon and Nitrogen recovery, VFA-LDH, NH4HCO3, Soil amelioration


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Introduction Sustainable treatment of high-concentration organic wastewater is one of the major issues of this century 1. Moreover, recovering nutrients from organic wastewater is crucial in meeting economic development and energy demands to feed a growing population.1-2 Food waste, one of the main types of municipal solid waste, readily corrupts garbage

3-4

and causes secondary pollution.5-6 Because of the high value

potential of food waste, many resource techniques have been proposed and adopted.7-10 Among them, anaerobic fermentation is a particularly promising technology that can achieve both reduction and resource recovery of organic matter (OM) .3,

10-12

However, the generated anaerobic fermentation liquid has been a

troublesome problem. Anaerobic fermentation liquid from food waste contains abundant nutrients, such as C (e.g., volatile fatty acids (VFAs) and alcohols) and N (e.g., NH4+). For instance, the concentrations of VFAs and NH3−N were as high as 30.0 g/L and 2.2 g/L, respectively,13-15 which have proven to be a promising source of nutrients for plant and microbial growth,16-17 e.g., the C and N sources donator for soil fertility promotion.7 However, the high salinity and low pH, as well as heavy metals and other hazardous matter18-19 in anaerobic fermentation liquid limit its application due the salinisation and acidification of soil.20 Moreover, it can be difficult to control the efficient dosage and application rate of external C and N sources if the direct irrigation of fermentation liquid.21 For these reasons, efficient separation, and secure



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methods for utilizing anaerobic fermentation liquid from food waste have gained increasing attention. Extracting VFAs and NH3-N from anaerobic fermentation liquid could solve these problems

simultaneously.

Layered

double

hydroxide

(LDH),

known

as

hydrotalcite-like compounds or anionic clays, are a family of compounds which commonly adopt the general formula M2+1-xM3+x(OH)2(An-)x/n·mH2O (where M2+ and M3+ are metal cations, and An- is an interlayer anion).22-23 The isomorphic substitution of M2+ by M3+ results in a high charge density of the layers, which should be balanced by exchangeable interlayer anions.24-25 Researches have shown that LDH has been prepared successfully with various targets, such as agrochemicals,26 phosphate and nitrate ions,27 as well as volatile organic acids.28 Such studies have suggested the feasibility of extracting VFAs from anaerobic fermentation liquid via LDH synthesis. Meanwhile, to address high concentrations of NH3-N in anaerobic fermentation liquid, efforts have been made to recycle and reuse N, for example via the gas-stripping method for NH3-N recovery.29-31 Generally, the NH3-N recovery efficiency is greatly affected by pH and temperature when using the gas-stripping method.30, 32 During LDH synthesis, two essential conditions, a fermentation liquid pH of ~9.5 and temperature of ~70°C are ideally required for gas-stripping. Such a method could simultaneously recover C and N from anaerobic fermentation liquid, which could be reused to improve soil fertility and enhance plant growth.28,

33

Furthermore, the

combination of LDH synthesis and gas-stripping for C and N extraction could


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decrease the effects of organic pollutants on the environment. On this basis, to achieve the ambitious goal of combining pollutant reduction with nutrient recovery, we created a novel integration method for the simultaneous recovery of C and N. The objectives of this work were to (1) investigate the feasibility of VFA-intercalated LDH (VFA-LDH) synthesis and NH4HCO3 conversion using real anaerobic fermentation liquid from food waste, (2) evaluate C and N recovery and the release performance of VFA-LDH, and (3) analyze the C and N use rates for soil improvement.

Materials and methods Materials The acetic acid, propionic acid, butyric acid, and valeric acid used for gas chromatography (GC) analysis were chromatographically pure and purchased from Sino Pharm (Shanghai, China). Sodium hydroxide, hydrochloric acid, and salts such as Mg(NO3)23H2O and Al(NO3)39H2O were of analytical grade and purchased from Aladdin Chemistry Co., Ltd (Shanghai, China). Food waste was collected from a student canteen at Tongji University and included cooked rice, vegetables, meat, eggs, animal bones, clamshells, etc. After removing the animal bones and clamshells, the food waste was slurried with deionized water (mass ratio, 1:1) by a beater. Then, the prepared food waste slurry (6.0 L) was fermented in an automatic control reactor (effective volume: 7.0 L) without sludge inoculation at 35°C for 3 days at pH 5.0–6.0 (adjusted with 4 M NaOH and 4 M HCl). The



fermentation liquid was obtained after separating the fermentation residue through analytical filter paper. The characteristics of the anaerobic fermentation liquid are shown in Table 1. Table 1 Characteristics of the anaerobic fermentation liquid from food waste

Parameter

Value

Chemical oxygen demand (mg/L) NH3-N (mg/L) Volatile fatty acids (mg/L) pH Total P (mg/L) Na (mg/L) K (mg/L) Cl- (mg/L) Color

14,800~16,100 680~800 2,510~2,735 3.8~4.5 145~172 500~570 180~240 1,200~1,500 Canary yellow

C and N recovery Recovery of C was realized via the synthesis of VFA-LDH, which was prepared with anaerobic fermentation liquid from food waste via coprecipitation under a controlled pH. The experimental procedure was carried out based on references27-28 with some modifications, and the details were as follows: Fermentation liquid (5.0 L) was placed in the automatic control reactor, and the pH was adjusted to the expected pH (~9.5) with 4 M NaOH or 4 M HCl. Then, 0.30 mol of Mg2+ solution (150 mL) and 0.15 mol of Al3+ solution (150 mL) were added to the reactor dropwise under constant agitation (100 rpm) in an N2 atmosphere. All operations were conducted at room temperature. Once precipitate began to appear, the suspension was treated hydrothermally at 70°C for 24 h. After hydrothermal treatment, the precipitate was washed with deionized water three times and collected via centrifugation. The resulting solid was dried in a vacuum oven at 50°C for 36 h to yield VFA-LDH. The


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VFA-LDH sample was ground into powder for further use. Fig. 1 presents a schematic illustration of the VFA-LDH synthesis procedure.

Fig. 1 Schematic illustration of the volatile fatty acid-layered double hydroxide synthesis and NH4HCO3 conversion process.

Recovery of N was realized via two stages, including the adjustment of pH (~9.5) and temperature (70°C) in solution. In the ionization equilibrium between NH4+ and NH3, pH and temperature are two key influence factors. After NH3 escaped from the solution, NH3 steam entered into the NH3 recovery system, which consisted of a water-bath condenser, three mouth flask (100 mL) with 50 mL water. Then, CO2 was introduced to react with the NH3, forming NH4HCO3. The resulting NH4HCO3 was crystallized and precipitated out, although a small portion remained dissolved in the original solution. Characterization of VFA-LDH The crystallinity of VFA-LDH was measured using X-ray diffraction (XRD; D8 Advance Sol-X; Bruker Co., USA), and the patterns with CuKa radiation (1.5418 Å)



at 40 kV and 40 mA were recorded in the 2θ region from 5° to 80°. Fourier-transform infrared spectroscopy (FTIR; Nicolet 5700 FT-IR spectrometer) was used to qualify the chemical bonds between the functional groups of VFA-LDH. The samples were subjected to wave number analysis within the range of 500–4000 cm-1 at a resolution of 4 cm-1. The morphologies and microstructures of the samples were characterized using a scanning electron microscopy/energy-dispersive X-ray spectroscopy and mapping system (SEM-EDS; Inspect F; FEI Co., USA). Thermal analysis (TGA-DTG) was recorded on a thermal analysis system (Q600 SDT; TA Instruments, USA) over a temperature range of 50−800°C with a heating rate of 10°C/min in an N2 stream. Elemental analysis of C, O, Mg, Al, and P was performed using an elemental analyzer (Vario EL III, Germany). VFA-LDH release test We performed OM release tests of VFA-LDH synthesized from anaerobic fermentation liquid in deionized water and a Na2CO3 solution (1.0 g/L) at 30°C in a water bath with a magnetic stirring speed of 100 rpm. VFA-LDH particles (1.0 g) were added to both the deionized water (500 mL) and Na2CO3 solution (500 mL). The VFA-LDH release performance was evaluated with COD analysis. Application of C and N sources to soil The C and N source application treatment was conducted as follows. First, 2.0 g of VFA-LDH was thoroughly mixed with 200 g of dry soil in replicate, which were adjusted to a 50 wt.% water holding capacity with a NH4HCO3 solution (1.0 g/L) and then covered with perforated parafilm to prevent water loss via evaporation. Next, the


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soil samples were incubated for one month at 30°C in an automated illumination incubator (GZX-300BS-III; CIMO Medical Instrument Manufacturing Co., Ltd., Shanghai, China). The control experiment was conducted without the addition of VFA-LDH and moistened with deionized water. The other conditions were the same as in the treatment experiment. At the end of the incubation, the soil was sampled for the analysis of soil properties. The pH, OM, cation exchange capacity (CEC), total N (TN), total P (TP), total Ca (hereafter, Ca), total Mg (hereafter, Mg), and exchangeable K and Na of soil samples were determined following standard analytical methods.34 Sampling and analytical methods For the VFA measurement, the samples from the reactor were centrifuged at 4,000 rpm/min for 10 min, and then filtered through a 0.45-µm cellulose nitrate membrane filter. VFA concentrations were determined by GC (6890 N; Agilent, USA) with injector and flame ionization detector temperatures of 200°C and 220°C, respectively, and equipped with a 30.0 m × 0.53 mm × 1.00 µm DB-WAX-etr polar column (Agilent). The VFA yield was calculated as the sum of the measured acetic acid, propionic acid, n-butyric acid, iso-butyric acid, n-valeric acid, and iso-valeric acid. The molecular weight distribution (MWD) was determined with a GFC analyzer (LC-10AD; Shimadzu). A TSK gel column (G4000PWXL; Tosoh Co., Japan) heated to 40°C and maintained with a thermostat control (CTO-10ASvp) was used with Milli-Q water as the mobile phase at a flow rate of 0.5 mL/min. The samples were filtered through a 0.22-µm hydrophilic filtration membrane and diluted to the approximate appropriate concentrations using Milli-Q water before injection (60 µL)



and analyzed using a refractive index detector (RID-10A). The COD and NH3-N tests were conducted using a closed reflux method and Nessler’s reagent photometry method, respectively.35 Meanwhile, pH was monitored with a pH meter (PHS-3C; Shanghai Rex, China).

Results and discussion Physical and chemical properties of the C and N products Fig. 2a shows the XRD pattern of VFA-LDH. The sharp characteristic reflection corresponded to a well-crystallized layered phase with a pattern at 2θ = 11.4°, and the result was similar to that reported by Benício.

27

In addition, the results showed the

presence of two other basal reflections (006) and (009) at 2θ = 22.0 and 34.2°, respectively, suggesting that VFAs (e.g., acetic acid, propionic acid, butyric acid, and valeric acid) were intercalated into the interlayer galleries of Mg/Al-LDH.28 Carbonate was also observed, corresponding to a series of small peaks at high 2θ values, suggesting that high concentrations of carbonate existed in the anaerobic fermentation liquid during the VFA-LDH synthesis process. The carbonate anion has a strong affinity with LDH and readily forms CO32--LDH.36 Fig. 2b presents the FT-IR spectra of VFA-LDH. The sharp peak at 3457.4 cm-1 was attributed to the O–H stretching vibration, indicating the presence of water or hydroxyl groups in the interlayer of the VFA-LDH. The concomitant appearance of peaks at 1648.8 cm–1 and 1560.2 cm–1 manifested the formation of COO− stretching vibrations, which confirmed the intercalation of VFAs in Mg/Al-LDH. The sharp peak at the 1350.4 cm–1 peak corresponded to carbonate group vibrations,37-38 which was also revealed in the XRD analysis. In addition, bands around 1050.2 cm-1 and 663.4 cm-1 could be assigned to the P-O and Mg/Al-OH translation modes of the LDH layers, respectively.27


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(a)

(b) Trancemittance (a.u.)

Intensity

(003)

(006) (009+012) (015)

10

20

30

(111)

(018)

40

50

60

70

1050.2 1648.8 1560.2

663.4 1350.4

786.8

3457.4

80

3500

3000

2500

2000

1500

1000

Wavenumber (cm-1)

2θ/degree

(c)

(d)

O

Element

wt%

C O Mg Al P

15.51 56.64 16.46 9.65 1.42

Mg

Al

P

C

Ca 0

(e)

100

1

2

3

keV

4

(f)

0.00 ※

90

-0.10 70 -0.15 60

※ NH4HCO3

Intensity

Weight (%)

-0.05 80

d (Wt%) / d (Temp)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


※

※ ※

-0.20

50 100

200

300

400

500

600

700

※

10

20

o Temperature ( C)

※

※

※

※ ※

※※

30

40

50

2θ/degree

Fig. 2 (a) X-ray diffraction, (b) Fourier transform infrared spectroscopy, (c) scanning electron microscopy, (d) energy-dispersive X-ray spectroscopy, and (e) thermal analysis of volatile fatty acid-layered double hydroxide; (f) X-ray diffraction analysis of N product

Figs. 2c and 2d present the SEM images and XRD spectrum of VFA-LDH. The surface topography revealed considerable surface roughness, indicative of cracked crystals of LDH platelets. EDS analysis confirmed the elemental composition of VFA-LDH, and revealed the dominant presence of C, O, Mg, Al, and P in the material. The high content of Mg (16.46 wt.%) and Al (9.65 wt.%) in VFA-LDH confirmed



that the VFA-LDH was mainly composed of Mg/Al-OH. The chemical formulas showed that the LDH had an Mg/Al ratio of 1.92 (the expected Mg/Al ratio is 2), indicating that VFAs were successfully intercalated into Mg/Al-LDH.39 The C content of the synthesized LDH was 15.51%, and could be ideal to serve as a C source. Furthermore, the presence of P in VFA-LDH indicated the possibility that P species (from the anaerobic fermentation liquid) prevailed in the interreaction process. The thermal behavior of VFA-LDH was examined by primary TGA-DTG analysis (Fig. 2e). The TGA curve presented three weight loss events. The first step occurred from 50 to 240°C, with a mass loss of ~17.6%. Therein, two steps could be segmented, below 100°C and from 100 to 240°C, which were mostly due to the release of adsorbed water and intercalated water molecules, respectively.40 The third step occurred from 240 to 490°C with a mass loss of 28.7%, which was associated with the removal of water molecules via layer dehydroxylation.41 The third step was observed in the range 490 to 800°C with a mass loss of 3.6%, which may have represented the release of CO2 from VFA-LDH decomposition,27 and was confirmed by the XRD results. From the X-ray diffraction analysis of N product (Fig. 2f), it can be seen that the major crystalline phases was NH4HCO3. The result indicated the successful conversion of NH3 into NH4HCO3 with the excess CO2. Variations in the properties of anaerobic fermentation liquid Fig. 3 presents the concentrations of COD, VFAs, and NH3-N, as well as the MWD. The COD concentration (Fig. 3a) of anaerobic fermentation liquid decreased from 15,400 to 7,800 mg/L after treatment, with a total removal efficiency of about 49.4%. As shown in the XRD and FTIR analyses, some VFAs intercalated into the interlayer galleries of Mg/Al-LDH, which contributed to the COD removal. Moreover, during


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VFA-LDH synthesis, a portion of macromolecular OM was flocculated and precipitated, also contributing to the COD reduction. The NH3-N concentrations in the fermentation liquid before and after treatment are presented in Fig. 3b. The NH3-N concentration decreased from 710.9 to 401.1 mg/L, with an NH3 removal efficiency of about 43.6%. NH3-N in the fermentation liquid mostly existed in the form of NH4+. The transformation of the NH4+ into NH3 depended on the ionization equilibrium between NH4+ and NH3. The transformation process is shown in Eq. (1). Increases in pH and temperature would accelerate the time to equilibrium and push the equation to the right, thereby increasing the proportion of free NH3. The correlations among free NH3 concentrations [NH3-N], total NH3 concentrations [NH4+ + NH3], pH, and temperature are expressed in Eq. (2) .42 NH4++OH- ⇌ NH3+H2O

ሾܰ‫ܪ‬ଷ ሿ =

ேுయ ାேுరశ ൣಹశ൧ ଵା ಼ೌ

(1)

ሾேு ାேு శ ሿ

ర = ଵାଵ଴య(೛಼ೌష೛ಹ)

(2)

pKa = 4×10-8×T3+9×10-5×T2-0.0356×T+10.072 where, [NH3] is the concentration of free NH3, mg/L, [NH4++NH3] is the total NH3 concentration (the sum of free NH3 and fixed NH3 concentrations) (mg/L), and pKa is the acid dissociation constant of NH3 related to temperature. The NH3 from the anaerobic fermentation liquid was transferred into the NH3 recovery system and interacted with CO2, forming NH4CHO3. The reactions are shown in Eqs. (3–5). Crystallized and precipitated NH4OH was recovered for further use. NH3 + H2O ⇌ NH3·H2O ⇌ NH4+ + OH-

(3)

CO2 + H2O ⇌ H2CO3 ⇌ HCO3- + H+

(4)

NH4+ + HCO3- ⇌ NH4HCO3

(5)

Fig. 3c shows the changes in VFAs and individual organic acid concentrations over



time in the reactors. Acetic acid, propionic acid, butyric acid, and valeric acid were the main VFA species present in the fermentation liquid. The initial VFA concentration in the fermentation liquid was 2,638.5 mg/L, which decreased to 1,902.8 mg/L after treatment. The large variations in VFAs were driven by acetic acid and propionic acid, with rates of decrement of 26.8% and 58.4%, respectively. From the FTIR results, the variations in VFAs in the fermentation liquid indicated that all VFAs (especially acetic acid and propionic acid) were successfully intercalated into Mg/Al-LDH. Fig. 3d shows the molecular weight of OM in both pre- and post-treatment samples. The MWD in the influent was about 1,092 kDa, but only 324 kDa after treatment. This could be explained by the fact that high-molecular-weight compounds were more readily removed by coprecipitation or acted as a source of C for VFA-LDH than low-molecular-weight OM.

15000

700

(a)

(b)

600

NH 3 -N (mg/L)

C O D (m g/L)

12500 10000 7500 5000

500 400 300 200

2500

100 0

0

Before

2500

VFA concentration (mg/L)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(c)

2000

Before

After

Valeric acid

Butyric acid

Propionic acid

Acetic acid

After

(d) After

Before

Mn/100kDa

1500

Before After

1000

10.92 3.24

500

0

Before

After

4.0

6.0

8.0

10.0

12.0

14.0 min

Fig. 3 Variation in (a) chemical oxygen demand (COD), (b) NH3-N, (c) volatile fatty acids (VFAs), and (d) the molecular weight of anaerobic fermentation liquid. Note: “Before” refers to the raw


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fermentation liquid, and “After” refers to the fermentation liquid after removing C and N.

OM release performance of VFA-LDH and pH variation in different solutions Fig. 4 shows the release profiles of VFA-LDH and the pH variation in Na2CO3 solution (1 g/L) and deionized water. The Na2CO3 solution had a higher COD release content than deionized water (Fig. 4a), indicating that the alkaline environment enhanced the OM release of VFA-LDH. In addition, the release performance in the Na2CO3 solution exhibited a gradual two-step release behavior, a rapid release stage in the first 5 h, followed by a relatively slow period over the next 5–48 h, which was due to the different incorporation of VFAs and Mg/Al-LDH. A previous study showed that the rapid release stage is related to physical adsorption of VFAs on Mg/Al-LDH surface, while the subsequent slow release is attributed to the successful interreaction of VFAs and Mg/Al-LDH.43 The release tests indicated that a slow dosing rate of OM could be achieved via the synthesis of VFA-LDH tablets, which could be used to improve soil fertility and enhance plant growth. Fig. 4b presents the pH variation in Na2CO3 solution and deionized water. The results showed that the pH of the Na2CO3 solution was higher than that of deionized water due to the alkalinity of the Na2CO3 solution. The pH decreased gradually (from 12.02 to 11.11) in the Na2CO3 solution over the soaking time, indicative of slow OM release. However, in deionized water, the pH was 9.55~9.76 during the first hour, and remained fairly static thereafter. We speculate that application of VFA-LDH would promote an appropriate soil pH. Research has shown that an increase in pH could help increase nutrient concentrations in the soil solution and enhance the nutrient element availability to plants.44-45 Hence, the increase in pH via the addition of VFA-LDH could create an excellent environment for the high use efficiency of C from VFA-LDH.



12.5 160

(b)

12.0

(a)

In Na2CO3 solution

11.5

140

11.0 10.5

120

pH

COD (mg/L)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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10.0 9.5

In deionized water

100

In Na2CO3 solution In deionized water

80

9.0 8.5 8.0 7.5

60

7.0 0

5

10

15

20

25

30

35

40

45

50

0

5

10

15

Time (h)

20

25

30

35

40

45

50

Time (h)

Fig. 4 Release profiles of (a) volatile fatty acid-layered double hydroxide based on chemical oxygen demand (COD) and (b) pH variation in the Na2CO3 solution (1.0 g/L) and deionized water.

Variation in soil fertility after VFA-LDH and NH4HCO3 application To evaluate the effects of VFA-LDH and NH4HCO3 on soil amelioration after application, soil fertility was measured. The results are shown in Table 2. All fertility-related parameters increased after the application of VFA-LDH and NH4HCO3. For example, the OM contents were 18.40 and 26.91 g/kg in the original soil and treated soil sample, respectively, which was attributed to the increase in dissolved OM.46 The abundant N and P contents in the NH4HCO3 and VFA-LDH supplied the soil with high N and P contents (TN: from 0.548 to 1.070 g/kg, TP: from 0.648 to 0.786 g/kg), which are the major nutrients required for plant growth. After application, the CEC of soil increased slightly, which indicated that the high ion content led to the presence of an excess of base ions.47 Furthermore, VFA-LDH and NH4HCO3 treatment produced a slight increase in soil pH. Generally, a higher pH is beneficial for soil recovery and can help improve the maintenance of nutrients in the soil solution.45, 48 Because of the Na, Ca, and Mg contents in the VFA-LDH, the contents of these


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ions also increased in the treated soil. It is important to note that excess Na+ in soil could cause the soil structure to disintegrate and degrade the soil physical properties.34, 49

However, the suitable content of Na+ in VFA-LDH in this study avoided excess ion

inputs into the soil. Table 2 Soil characteristics before and after the application of volatile fatty acid-layered double hydroxide and NH4HCO3

Untreated soil

Treated soil

pH

6.79

7.63

Organic Matter (g/kg)

18.40

26.91

Cation exchange capacity (cmol(+)/kg)

5.78

6.15

Total N (g/kg)

0.548

1.070

Total P (g/kg)

0.648

0.786

Ca (g/kg)

2.400

4.600

Mg (g/kg)

0.288

0.600

Exchangeable K (cmol/kg)

0.285

0.315

Exchangeable Na (cmol/kg)

0.355

0.435

Environmental effects assessment and economic evaluation of C and N recovery Finally, we considered both the environmental and economic effects of the proposed C and N recovery method. In the raw anaerobic fermentation liquid, C and N existed as NH3-N and VFAs, which were converted into VFA-LDH and NH4HCO3, respectively, after treatment. The decrease in COD and NH3-N in fermentation liquid could mitigate the effects of organic pollutants on the environment and the wastewater treatment load. In addition, in the recovery of C and N, the main costs include electricity, and N2 and CO2 consumption. However, the obtained VFA-LDH and NH4HCO3 could counteract some of this cost. Moreover, the use of VFA-LDH and NH4HCO3 would benefit plant growth and environmental protection efforts, which may be sufficient to outweigh the cost. Fig. 5 presents a schematic diagram of the environmental and economic effects.



Fig. 5 Schematic diagram of the environmental and economic effects. COD: chemical oxygen demand; MWD: molecular weight distribution; VFAs: volatile fatty acids.

In the present VFA-LDH and NH4HCO3 production system, not all C and N in the anaerobic fermentation liquid was recovered. This indicates room for further optimization for engineering applications. For instance, the enhancement of N recovery by negative-pressure air-stripping pretreatment could be investigated. Furthermore, the effects of ions on the recovery efficiency should be studied due to the heterogeneous concentration of salts (Na+, K+, and Cl-), Ca2+, SO42-, and PO3- in fermentation liquid. Nonetheless, the simplicity and low cost of the process could incentivize further investigations to improve the C and N recovery from anaerobic fermentation liquid of food waste.

Conclusions Anaerobic fermentation liquid of food waste is rich in C and N, such as VFAs and NH3-N. In this study, C recovery from anaerobic fermentation liquid was achieved via the synthesis of VFA-LDH. Simultaneously, N was obtained via crystallization into NH4HCO3 under a high pH and temperature. The resulting VFA-LDH exhibited good (i.e., slow) release performance, and could serve as an external nutrient source in tandem with NH4HCO3 to improve soil fertility, including the OM, N, and P contents, and CEC. Moreover, the recovery efficiencies of COD, VFAs, and NH3-N were 49.4%, 27.9% and 43.6%, respectively, from the treated anaerobic fermentation liquid. This technology shows beneficial environmental effects and may be economically


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feasible, and the results presented in this work suggest that VFA-LDH in tandem with NH4HCO3 has a great potential to be applied as fertilizer for plant growth. To improve the potential applicability of this technology, VFA-LDH application in different soil types and with different plants should be further investigated. Moreover, there are many opportunities for improvement to C and N recovery before commercializing the technology.

Acknowledgements This work was financially supported by the National Natural Science Foundation of China (No. 51678419), and the Fundamental Research Funds for the Central Universities (No. 22120170266).



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TABLE OF CONTENTS (TOC) GRAPHIC.

Synopsis C and N sources, the potential soil amendments, were recovered simultaneously from anaerobic fermentation liquid via an integrated technique.


Innovative Integrated Technique for Nutrient Acquisition

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