Banana Peel Derived Chitosan-Grafted Biocomposite for Recovery of NH₄⁺ and PO₄^3–

Abstract

Biomass-derived adsorbents afford accessible and inexpensive harvesting of nitrogen and phosphorus from wastewater sources. Human urine is widely accepted as a rich source of nitrogen and phosphorus. However, direct use of urine in agriculture is untenable because of its unpleasant smell, pathogen contamination, and pharmaceutical residues. In this work, we have grafted chitosan onto dried and crushed banana peel (DCBP) to generate the biocomposite DCBP/Ch. A combination of FTIR, TGA, XRD, FESEM, EDX, and NMR analyses were used to characterize DCBP/Ch and reveal condensation-aided covalent conjugation between O–H functionalities of DCBP and chitosan. The adsorption performance of DCBP/Ch toward NH₄⁺ and PO₄^3– is in sync with its attractive surface porosity, elevated crystallinity, and thermostability. The maximum adsorption capacity of DCBP/Ch toward NH₄⁺/PO₄^3– was estimated as 42.16/15.91 mg g^–1 at an operating pH of 7/4, respectively, and ranks highly when compared to previously reported bioadsorbents. DCBP/Ch performs admirably when tested on artificial urine. While nitrogen and phosphorus harvesting from human urine using single techniques has been reported previously, this is the first report of a single adsorbent for recovery of NH₄⁺ and PO₄^3–. The environmental compatibility, ease of preparation, and economic viability of DCBP/Ch present it as an attractive candidate for deployment in waste channels.

1. Introduction

The unfettered worldwide use of synthetic fertilizers has created immense burden on the energy sector and nonrenewable resources such as phosphate rock and natural gas.¹⁻⁵ NH₃/NH₄⁺ and PO₄^3– are the primary components of fertilizers that continue to rely on energy-intensive methods of production.^1,6 In this context, animal wastes have been envisaged as alternatives to synthetic fertilizers since these are abundant sources of nitrogen and relatively of phosphorus compounds. Generally, an adult healthy human excretes about 550 L of urine annually which is equivalent to 4 kg of nitrogen, 0.4 kg of phosphorus, and 0.9 kg of potassium.⁷ Despite containing as many as 3000 separate chemical species, the dominant components are water (95%) and urea (2%). Ions such as NH₄⁺, Na⁺, K⁺, Ca²⁺, Mg²⁺, Cl^–, SO₄^2–, NO₃^2–, and PO₄^3– are present in variable proportions.^8,9 NH₄⁺ and PO₄^3– are also part of waste effluents from industries such as fertilizers, synthetic polymers, and refrigerant gas.^10,11 Notably, excessive amounts of NH₄⁺ and PO₄^3– in the environment are understood to pollute the soil, water, and air.^1,4,12−17 High amounts of aqueous NH₄⁺ and PO₄^3– can cause eutrophication in water bodies, creating challenges for aquatic life.^18,19 The recovery of NH₄⁺ and PO₄^3– from domestic and industrial wastes can address environmental safety concerns while offering potential strategies for reducing dependence on synthetic fertilizers. In this context, the direct application of human urine as a fertilizing agent is severely constrained by the presence of pathogens, challenges associated with storage and transport, and social taboos.^9,20 WHO advises comprehensive sanitization of human urine with storage for at least six months, before being used as a fertilizer. Several distinctive processes have emerged for recovery of NH₄⁺ and PO₄^3– from human urine that preclude long-term storage. Techniques such as ammonia stripping,^21,22 chemical precipitation with NH₄MgPO₄·6H₂O,²³⁻²⁵ biological denitrification,²⁶ and electrochemical conversion^27,28 offer different routes to access NH₄⁺ and PO₄^3– in urine. These methods vary in the efficiencies of NH₄⁺ and PO₄^3– retrieval and the extent of energy or chemical/material investment necessary. Some of these methods may also ultimately contribute to transfer of pollutants. The safe and commercially viable recovery of NH₄⁺ and PO₄^3– from human urine could be considered as a less explored paradigm of sustainable technologies. In this regard, generic methods that are associated with the separation of ions from specific media could be adapted suitably. Coagulation/flocculation, photocatalysis, ultrafiltration, oxidation, Fenton-biological treatments, nanofiltration, membrane separation, and adsorption are among such methods for the separation of ionic substances.^29,30 Not surprisingly, adsorption is an attractive method owing to its simplicity, adaptability, and speed of operation.³¹⁻³⁴ The separate adsorptive removal of NH₄⁺ and PO₄^3– has been previously reported using low cost natural polymers, agricultural wastes, zeolites, clays, and chemically modified inorganic materials.³⁵⁻⁴⁶ The simultaneous adsorption of NH₄⁺ and PO₄^3– from urine or industrial effluents is relatively unexplored.

We have recently reported on the use of dried banana peel powder as an effective adsorbent for NH₄⁺.⁴² While banana peel compositions vary based on the climate and region of cultivation, components with significant presence include pectin (10–21%), cellulose (7.6–9.6%), hemicelluloses (6.4–9.4%), and lignin (6–12%).⁴⁷ Amino acids such as leucine, valine, phenylalanine, and threonine; dietary fiber; and chlorophyll pigments are other major components.⁴⁷⁻⁵⁰ Banana peel is a rich source of phenolic compounds including HCA, F, Fols, and CA.⁵¹ Interestingly, almost every part of the banana tree, such as the leaves, pseudostems, stalk, trunks, inflorescence, and piths, is fit for human consumption. The banana peel comprises nearly 35–50% of the total mass of the fruit and is almost universally consigned to waste.^47,52 Approximately 36 million tons of banana peels are generated worldwide every year and considering their current fate offer an attractive biomass for suitable chemical manipulation. In this regard, the natural polysaccharide chitosan has been used for calibrating the adsorption behavior of biomass.⁵³⁻⁵⁵ Chitosan is a prominent naturally occurring polycation, whose charge density is a function of degree of acetylation and pH of solution.⁵⁶ Therefore, chitosan is pH sensitive, i.e., readily dissolves at acidic pH and is insoluble at alkaline pH.⁵⁷ The prevalent protonated −NH₂ functionalities of chitosan render it soluble in acidic pH and insoluble in alkaline pH.⁵⁶⁻⁶⁰ We were interested in combining the pH-responsiveness and stability of chitosan with the adsorptive capabilities of banana peel powder.

In this work, we have developed a pH-responsive chitosan-grafted biocomposite of banana peel powder and have applied it for adsorptive exclusion of both NH₄⁺ and PO₄^3–. To the best of our knowledge, this is the first report of a biocomposite that is suitable for adsorbing both NH₄⁺ and PO₄^3– by variation of the pH of the medium.

2. Experimental Section

2.1. Materials

Banana peels, chitosan (molecular weight = 310 kDa, viscosity = 1000 cP in CH₃COOH solution, and degree of deacetylation >75%), CH₃COOH, CH₃COONa, NH₄Cl, Na₂SO₄, KCl, NaCl, CaCl₂, MgSO₄·7H₂O, NaH₂PO₄·2H₂O, Na₂HPO₄·2H₂O, uric acid (C₅H₄N₄O₃), creatinine (C₄H₇N₃O), sodium citrate (Na₃C₆H₅O₇·2H₂O), potassium oxalate (K₂C₂O₄·H₂O), and urea (CO(NH₂)₂) were purchased from Sigma-Aldrich and used without any modification/purification. All experiments in aqueous media were carried out by using MQW.

2.2. Preparation of DCBP/Ch

For synthesizing DCBP/Ch, first, banana peels were collected from waste stuffs of Indian Institute of Technology-Gandhinagar, washed thoroughly by MQW to remove superficial dirt and sticking impurities, cut into small pieces, sun-dried for 1 week, and finally oven-dried at 60 °C for 2 days. Such dried peels were crushed by using a mortar–pestle to obtain the DCBP powder (Scheme 1), which was used as the primary raw material for synthesizing DCBP/Ch. In the next stage, DCBP/Ch was synthesized by grafting chitosan within the microstructures of DCBP by using the optimum recipe (Table 1). Briefly, 3 mL of glacial CH₃COOH was dissolved in 100 mL of MQW and preheated to 40 °C, followed by the gradual addition of 4 g of oven-dried chitosan with constant stirring at 200 rpm. To this viscous chitosan solution, 1.5 g dried of the previously processed DCBP was added with constant stirring at 200 rpm and 25 °C. Stirring was continued for 6 h to ensure maximum grafting of chitosan with DCBP. The as-obtained DCBP/Ch was filtered under suction, washed thoroughly with 3:1 MQW:CH₃OH solution, dried in a hot-air oven for 6 h, and then stored in an airtight container for subsequent experiments.

Scheme 1. Synthetic Methodology of DCBP/Ch.

Table 1. Systematic Optimization of the Synthesis Parameters to Obtain the Optimum DCBP/Ch.

^#Not applicable.

2.3. Characterization

Structures of chitosan, DCBP, DCBP/Ch, N-DCBP/Ch, and P-DCBP/Ch were characterized by FTIR (Spectrum-2, Singapore, PerkinElmer), TG (TGA 4000, PerkinElmer), XRD (D8 Discover, Bruker), FESEM/EDX (JSM7600F, Jeol), ¹H NMR (Ascend NMR-500 MHz, Bruker), DLS (Nano ZS, Malvern Instruments), and UV–vis (Specord 210 Plus, AnalytikJena) analyses. All chemical structures and graphical presentations were generated by ChemDraw Ultra 12.0 and Origin 9.0 software, respectively.

2.4. Adsorption Methodology

Isothermal adsorption experiments were conducted by dipping 0.025 g of dry DCBP/Ch within 50 mL of NH₄⁺/PO₄^3– solution within 5–30/1–10 mg L^–1 at the pH_o = 7/4. Adsorption isotherm and kinetics data were determined by measuring unadsorbed NH₄⁺/PO₄^3– concentrations (mg L^–1) at different time intervals by ammonia electrode (Hanna HI4101) and ICPMS (Nexion 2000B ICP-MS, PerkinElmer). The adsorption capacities (ACs, mg g^–1), rate constants, and thermodynamics parameters were estimated by methods reported elsewhere.^61,62 Briefly, adsorption capacity at time t (q_t, mg g^–1) and equilibrium (q_e, mg g^–1) were calculated by using eqs 1 and 2, respectively.

1

2

Here, C₀, C_t/C_e, V, and m_s represent the initial concentration, residual concentrations at time t/equilibrium, volume of adsorbate solution, and mass of adsorbent, respectively. Separate calculations were performed from each replicate of adsorption experiments.

2.5. Preparation of ArUr

The principal components of human urine are provided in Table S1. In 100 mL of MQW, all the components listed in Table S1 were dissolved in isothermal conditions (37 ± 0.2 °C) by using a magnetic stirrer-cum-hot plate at 250 rpm. The final solution was stored in a tightly sealed flask. The ArUr thus prepared was used within 24 h. The ingredients selected for preparation of the artificial urine include only those substances that are present at relatively high concentrations that do not vary significantly in human urine.⁴²

3. Results and Discussion

3.1. Characterization to Confirm the Formation of DCBP/Ch from DCBP and Chitosan

3.1.1. FTIR Analyses of Chitosan, DCBP, and DCBP/Ch

FTIR characterization was performed on DCBP and is displayed in Figure 1a. The band at 1740 cm^–1 is attributed to the C=O str. of −COOH/–COOCH₃.⁶³ The presence of cellulose, hemicellulose, and lignin in DCBP can be inferred from the bands at 2921/2852 and 1368–1145 cm^–1 attributable to C–H str. and bending, respectively.^33,63,64 In our previous work we have described the FTIR spectra of BNPP formulation.⁴² Modifications in the process of drying and crushing deployed in the current work likely impose subtle variations in the behavior of chemical functionalities in FTIR. For instance, the greater population of O–H functionalities in DCBP compared to BNPP could be understood from the shifting of broad mutually hydrogen-bonded O–H···N–H str. from 3286 cm^–1 in BNPP⁴² to 3315 cm^–1 in DCBP⁶⁴⁻⁶⁹ as well as the appearance of O–H str. of amino acids at 2970 cm^–1 in DCBP.⁶³ Notwithstanding these subtle variations in functional group composition of DCBP, the intimate association of chitosan with DCBP in DCBP/Ch could be readily inferred from multiple FTIR bands. Notably, sharpening and shifting of mutual hydrogen-bonded O–H···N–H str. was observed from 3361 and 3315 cm^–1 of chitosan and DCBP, respectively, to 3449 cm^–1 in DCBP/Ch.⁶⁴⁻⁶⁹ The O–H str. of amino acids at 2970 cm^–1 in DCBP remain unaltered in DCBP/Ch, and the nonavailability of this peak in chitosan indicates the presence of a DCBP moiety in DCBP/Ch. The insertion of DCBP in DCBP/Ch was also understood from C–H str. of alkane and C=O str. of −COOH/–COOCH₃ at 2921/2852 and 1740 cm^–1 in DCBP and 2925 and 1739 cm^–1 in DCBP/Ch, respectively, and the absence of the corresponding peaks in chitosan.^33,63,64 Conversely, CH₂str. of the pyranose ring of chitosan at 2866 cm^–1 could be observed at 2865 cm^–1 in DCBP/Ch and substantiate the presence of chitosan in DCBP/Ch.⁶⁴ The O–C coupled association of chitosan with DCBP in DCBP/Ch is further confirmed from the emergence of a peak at 1119 cm^–1 in DCBP/Ch.⁷⁰ This band was attributed to the newly generated >CH–O–CH₂– linkage in DCBP/Ch. The disappearance of C–OH def. of chitosan at 1420 cm^–1 in DCBP/Ch indicated the conversion of C–OH into C–O–C moieties.⁶⁴ The FTIR analysis of DCBP and DCBP/Ch is detailed as part of Table 2.

Figure 1.

FTIR analyses of (a) chitosan, DCBP, and DCBP/Ch and (b) DCBP/Ch, N-DCBP/Ch, and P-DCBP/Ch.

Table 2. FTIR Analyses.

Chitosan	DCBP	DCBP/Ch	N-DCBP/Ch	P-DCBP/Ch	Assignment
3361	3315	3449	3297	3314	mutual OH···NH hydrogen bonding in chitosan and O–H group of polymeric compounds in DCBP64−69
–	2970	2970	–	–	O–H str. of amino acid groups66
–	2921, 2852	2925	2924	2922	C–H asym. str. of >CH–33,64
2866	–	2865	2862	2860	CH2str. of pyranose ring64
–	1740	1739	1738	1738	C=O str. of −COOH/–COOCH3 indicating the presence of carboxylic acids and/or their esters63
1625	–	–	–	1624	N–H def. in amine63
–	–	–	1621, 1530	–	C=O asym. str. of –COO–70,87
1588	1594	1596	–	1589	N–H bending(63)
–	1460	1462	–	1460	aromatic ring vibration of lignin63
–	–	–	1453	–	C=O sym. str. of –COO–87
1420	–	–	–		C–OH def.(64)
1380	–	1374	1375	1375	C–H twisting of >CH264
–	1368–1145	1364–1147	1314–1146	1315–1142	C–H bending of crystalline cellulose and C–H bending of cellulose, hemicelluloses, and lignin polymer63
1263	–	–	–	–	–CH2OH (side chain) of chitosan64
1154	–	1155	1152	–	asym. str. of C–O–C and C–N str.(67)
–	–	1119	1121	1123	>CH–O–CH2–, originated from grafting70
–	1046	1047	1047	1047	C–N str. of DCBP68
1065, 1026	–	1030	1066–1037	1068–1035	skeletal vibration of C–O str.(67)
–	–	–	–	988	P–O–H bond70,88,89
–	809–567	–	889, 780, 719	885–765	C–H vibration of carbohydrates69
713–560	–	–	549, 521	519	skeletal mode of pyranose ring64

3.1.2. TG Analyses of Chitosan, DCBP, and DCBP/Ch

Thermogravimetry is an important tool for the study of biopolymeric composites via a scrutiny of changes in thermal stabilities. We performed TG analyses of chitosan, DCBP, and DCBP/Ch to observe the relative change in thermal stability of DCBP/Ch because of grafting. The thermograms of chitosan, DCBP, and DCBP/Ch (Figure S1a) indicate three major decomposition ranges. Of these, the first range between 30 and 105 °C corresponds to elimination of volatile water.⁶⁴ This faster degradation rate for DCBP in this stage corresponds to a lower extent of hydrogen bonding within hydrophilic functionalities, such as −COOH and O–H. The second range in the thermograms between 238 and 386 °C represents the loss of H₂O during anhydride formation. The higher population of −COOH groups in DCBP accompanied by lower hydrogen bonding likely leads to rapid degradation in this region. Notably, the thermal stability of DCBP/Ch is significantly higher compared to DCBP, confirming the association of chitosan with DCBP. Moreover, the slower skeletal decomposition and higher residue in DCBP/Ch (24.57 wt %) than that of DCBP (21.72 wt %) indicates an improvement in structural stability of DCBP/Ch because of grafting-related covalency and hydrogen bonding.⁷¹

3.1.3. XRD Analyses of Chitosan, DCBP, and DCBP/Ch

The grafting of chitosan with DCBP is expected to alter Miller planes of DCBP. Observing the changes in the crystallinity of DCBP after chitosan grafting can indicate association between the two components. The sharp peak at 2θ = 10.15° (Figure 2a) observed for DCBP is attributed to the hydrogen-bonding-aided orderly arrangement of Miller indices. In contrast, two major peaks were observed at 2θ = 11.08° and 20.65°, for DCBP/Ch. The peak at 2θ = 11.08° is attributed to hydrogen-bonded orderly arrangement of planes from DCBP and chitosan. The peak at 2θ = 20.38° originates from the polymer-specific peak of chitosan. The shifting of the weak shoulder at 2θ = 38.01° of chitosan in XRD of DCBP/Ch indicates association of chitosan with DCBP.

Figure 2.

XRD analyses of (a) DCBP, chitosan, and DCBP/Ch and (b) DCBP/Ch, N-DCBP/Ch, and P-DCBP/Ch.

3.1.4. FESEM and EDX Analyses of Chitosan, DCBP, and DCBP/Ch

The grafting of chitosan with DCBP could affect the surface morphology of the latter. FESEM images of DCBP suggest a porous and fibrous surface morphology (Figure 3a). In contrast, chitosan displayed a mostly dense and homogeneous surface (Figure 3b). Notably, the fibrous/porous and dense/homogeneous characteristics of DCBP and chitosan, respectively, coexist in DCBP/Ch (Figure 3c,d). These results are consistent with the FTIR analysis and together with the XRD and TGA results justify intimate association of chitosan with DCBP in DCBP/Ch. From the EDX plot of DCBP/Ch (inset of Figure 3c), distinct peaks of C, N, and O were observed.

Figure 3.

FESEM images of (a) DCBP, (b) chitosan, (c and d) DCBP/Ch, (e) N-DCBP/Ch, and (f) P-DCBP/Ch and EDX of (c) DCBP/Ch, (e) N-DCBP/Ch, and (f) P-DCBP/Ch in the insets.

3.1.5. NMR Analyses of Chitosan, DCBP, and DCBP/Ch

NMR has been used for elucidating structural nuances of natural polymers, biomass, and chemically modified biomass.⁷²⁻⁷⁷ Based on previous reports on the composition of the banana peel, the DCBP is likely to be comprised of pectin, cellulose, hemicellulose, lignin, amino acids, and phenolic compounds. Of these, pectin contains α-d-GalA, β-d-Galp, α-l-Araf, and α-l-Rhap. The ¹H NMR spectrum of DCBP is shown in Figure 4 and described in Table 3. Close scrutiny of the ¹H NMR of DCBP revealed the presence of various constituent sugars of pectin. Notably, H¹, H², H³/H⁴, and H⁵ of α-d-GalA; H¹ and H²/H³/H⁴ of α-l-Araf; H¹ and H²/H³/H⁵ of β-d-Galp; and −CH₃ in α-l-Rhap were observed at 4.45, 3.12, 3.24–3.61, and 3.89; 5.72 and 3.89; 4.52 and 3.24–3.61; and 1.24 ppm, respectively.^32,78,79 Similarly, the presence of cellulose in DCBP could be inferred from the characteristic peaks of H¹, H², H³/H⁴/H⁵, and H⁶ at 4.10, 2.65, 3.07, and 3.61–3.89 ppm, respectively.⁸⁰ Hemicellulose contains β-d-xylp, d-GlcA, and α-l-Araf. The ¹H NMR spectrum of DCBP displayed characteristic hemicellulose-specific chemical shifts, such as H¹, H²/H³/H⁴, and H⁵ of β-d-Xylp; H¹ and O–CH₃ of d-GlcA; and H¹ and H²/H³/H⁴ of α-l-Araf at 4.45, 3.24–3.61, and 4.03; 5.35 and 1.35; and 5.72 and 3.89 ppm, respectively (Table 3).⁸¹⁻⁸³ The presence of lignin in DCBP was confirmed from peaks of oxygenated aliphatic, aromatic, and O–CH₃ protons at 2.18/2.38/3.61–3.89/5.35, 5.52/6.63/6.72/6.91 and 3.89 ppm, respectively.^72,84 The presence of Leu, Val, Phe, and Thr could be inferred from characteristic chemical shifts of each amino acid.⁴⁸ These include α-H, β-H, γ-H, and δ-H of Leu; β-H and γ-H of Val; α-H, β-H, and phenyl ring-H of Phe; α-H, β-H, β-OH, and γ-H of Thr; and N-H of Leu/Val/Phe/Thr observed at 3.61, 1.82, 1.49, and 0.91; 2.36 and 0.91; 4.10, 3.07, and 6.63–6.91; 3.61, 3.89, 3.61, and 1.24; and 6.02 ppm, respectively. Further, the incidence of phenolic compounds such as HCA, F, Fols, and CA was established by characteristic chemical shifts in the ¹H NMR of DCBP.^51,85 These include phenyl ring-H of HCA/F/Fols/CA and vinylic-H of HCA; O–H of the aliphatic and aromatic region in Fols/CA and HCA/CA; −CH^a₂–CH^b(OH)– of Fols; and NH^c₂–CH^d₂–CH^e(OH)– of CA observed at 6.02–6.91; 3.61 and 5.35; 2.65 (a) and 4.45 (b); 6.02 (c), 3.07 (d), and 4.71 (e) ppm, respectively.

Figure 4.

¹H NMR of DCBP.

Table 3. ¹H NMR Analyses.

Chemical Shift (ppm)
Chitosan	DCBP	DCBP/Ch	Explanation
4.61	–	4.63	H1 of chitosan64,86
2.43	–	–	H2 of chitosan64
3.67	–	3.65	H3 of chitosan64,86
3.82	–	3.85	H4 of chitosan64,86
3.54	–	–	H5 of chitosan64,86
3.91	–	3.95	H6 of chitosan64,86
4.92	–	4.91	–NH2 of chitosan64
3.25	–	–	–CH2OH of chitosan64,86
–	4.45	–	H1 of α-d-GalA78,79
–	3.12	3.17	H2 of α-d-GalA78,79
–	3.24–3.61	3.22–3.65	H3/H4 of α-d-GalA32,78,79
–	3.89	3.85	H5 of α-d-GalA78,79
–	5.72	5.64	H1 of α-l-Araf78,79
–	3.89	3.85	H2/H3/H4 of α-α-l-Araf78,79
–	4.52	–	H1 of β-d-Galp78,79
–	3.24–3.61	3.22–3.65	H2/H3/H5 of β-d-Galp78,79
–	1.24	1.23	–CH3 of α-l-Rhap78,79
–	4.10	4.12	H1 of cellulose80
–	2.65	2.61	H2 of cellulose80
–	3.07	–	H3/H4/H5 of cellulose80
–	3.61–3.89	3.65–3.85	H6 of cellulose80
–	4.45	4.46	H1 of β-d-xylp in hemicellulose81
–	3.24–3.61	3.22–3.65	H2/H3/H4 of β-d-xylp in hemicellulose81
–	4.03	–	H5 of β-d-xylp in hemicellulose81
–	5.35	5.31	H1 of d-GlcA in hemicellulose81,82
–	1.35	1.34	O–CH3 of d-GlcA in hemicellulose82
–	5.72	5.64	H1 of α-l-Araf in hemicellulose83
–	3.89	3.85	H2/H3/H4 of α-l-Araf in hemicellulose83
–	2.18/2.38/3.61–3.89/5.35	2.14/2.39/3.65–3.85/5.31	oxygenated aliphatic region in lignin72,84
–	5.52/6.63/6.72/6.91	5.64/6.70/6.88/6.97	aromatic region in lignin72,84
–	3.89	3.85	O–CH3 of lignin
–	3.61	3.65	α-H of leu
–	1.82	1.85	β-H of leu
–	1.49	1.48	γ-H of leu
–	0.91	0.94	δ-H of leu
–	2.36	2.32	β-H of val
–	0.91	0.94	γ-H of val
–	4.10	4.12	α-H of phe
–	3.07	–	β-H of phe
–	6.63–6.91	6.70–6.97	phenyl ring-H of phe
–	3.61	3.65	α-H of thr
–	3.89	3.85	β-H of thr
–	3.61	3.65	β-OH of thr
–	1.24	1.23	γ-H of thr
–	6.02	–	N-H of leu/val/phe/thr
–	6.02–6.91	6.70–6.97	phenyl ring-H of HCA/F/Fols/CA and vinylic-H of HCA
–	3.61	3.65	OH peaks of aliphatic region in Fols/CA
–	5.35	5.31	OH peaks of aromatic region in HCA/CA
–	2.65	2.61	–CH2–CH(OH)– of Fols
–	4.45	4.46	–CH2–CH(OH)– of Fols
–	6.02	–	NH2–CH2–CH(OH)–
–	3.07	–	NH2–CH2–CH(OH)–
–	4.71	4.70	NH2–CH2–CH(OH)–

The ¹H NMR of DCBP/Ch is expected to indicate the presence of both primary components, namely, the DCBP and chitosan. Scrutiny of the ¹H NMR of chitosan led to the attribution of H¹, H², H³, H⁴, H⁵, H⁶, −NH₂, and −CH₂OH peaks of chitosan to the observed chemical shifts at 4.61, 2.43, 3.67, 3.82, 3.54, 3.91, 4.92, and 3.25 ppm (Figure S2 and Table 3), respectively.^64,86 In the case of our chitosan-grafted DCBP, ¹H NMR of the composite showed peaks at 4.63, 3.65, 3.85, 3.95, and 4.91 ppm (Figure 5 and Table 3). Based on comparison with cognate peaks of chitosan, the above peaks could be attributed to H¹, H³, H⁴, H⁶, and −NH₂, respectively, and also indicate the successful incorporation of chitosan in DCBP/Ch. The presence of pectin-, cellulose-, hemicellulose-, lignin-, amino acid-, and phenolic compound-specific ¹H NMR peaks was similarly confirmed in DCBP/Ch. For instance, H², H³/H⁴, and H⁵ of α-d-GalA; −CH₃ of α-l-Rhap; H¹ and H²/H³/H⁴ of α-l-Araf; and H²/H³/H⁵ of β-d-Galp peaks were observed at 3.17, 3.22–3.65, and 3.85; 1.23; 5.64 and 3.85; and 3.22–3.65 ppm, respectively, and indicated the presence of pectin in DCBP/Ch. Further, H¹/H²/H⁶ of cellulose and oxygenated aliphatic, aromatic, and O–CH₃ protons of lignin were inferred from peaks at 4.12/2.61/3.65–3.85 and 2.14/2.39/3.65–3.85/5.31, 5.64/6.70/6.88/6.97, and 3.85 ppm, respectively, thereby confirming the presence of cellulose and lignin in DCBP/Ch. Likewise, the prevalence of hemicellulose in DCBP/Ch was affirmed from H¹ and H²/H³/H⁴ of β-d-Xylp; H¹ and O–CH₃ of d-GlcA; and H¹ and H²/H³/H⁴ of α-l-Araf peaks at 4.46 and 3.22–3.65; 5.31 and 1.34; and 5.64 and 3.85 ppm, respectively. The amino acid residual peaks, such as α-H/β-H/γ-H/δ-H of Leu, β-H/γ-H of Val, α-H/phenyl ring-H of Phe, and α-H/β-H/β-OH/γ-H of Thr peaks at 3.65/1.85/1.48/0.94, 2.32/0.94, 4.12/6.70–6.97, and 3.65/3.85/3.65/1.23 ppm, respectively, corroborated the presence of amino acids of DCBP in DCBP/Ch. Also, the presence of phenolic compounds of DCBP in DCBP/Ch was supported by peaks at 6.70–6.97; 3.65 and 5.31; 2.61 (a) and 4.46 (b); and 4.70 (c) ppm attributable to the phenyl ring-H of HCA/F/Fols/CA and vinylic-H of HCA; O–H of the aliphatic and aromatic region in Fols/CA and HCA/CA; −CH^a₂–CH^b(OH)– of Fols; and −CH^c(OH)– of CA, respectively.

Figure 5.

¹H NMR of DCBP/Ch.

The grafting of chitosan with DCBP through condensation-type association between −CH₂OH of chitosan and >CHOH of α-d-GalA of the pectin moiety of DCBP was inferred from the newly generated >CH^a–O–CH^b₂– linkages in DCBP/Ch at 3.17(a) and 3.95(b) ppm, respectively. The chemical shifts of these protons are consistent with the previously described FTIR peak of >CH–O–CH₂– at 1119 cm^–1 in DCBP/Ch and the disappearance of C–OH def. of chitosan at 1420 cm^–1 in DCBP/Ch. The grafting-aided modification in thermal properties, crystallinity, and surface morphologies were also confirmed from TG, XRD, and FESEM analyses. Moreover, the increase in hydrodynamic radius of DCBP/Ch compared to DCBP (Figure S5a–d) and distinct changes in λ_max values in the corresponding UV–vis spectra also support the bonding of chitosan with DCBP (Figure S5e).

3.2. Characterization to Confirm the Mechanism of Adsorption of NH₄⁺ and PO₄^3– by DCBP/Ch

3.2.1. FTIR Analyses of N-DCBP/Ch and P-DCBP/Ch

The FTIR spectrum of DCBP/Ch (Figure 1b) after NH₄⁺ adsorption at pH_o > pH_PZC suggested participation of specific groups in the chitosan-grafted polymer. The hydrogen-bonded mutual O–H···N–H str. at 3449 cm^–1 became very sharp in N-DCBP/Ch and shifted to 3297 cm^–1. Such a strong bathochromic shift confirmed the increase in the population of N–H functionalities after NH₄⁺ adsorption. Interestingly, the sharp C=O str. of −COOH in DCBP/Ch became very weak in N-DCBP/Ch. This is expected since adsorption was carried out at pH_o > pH_PZC, where most of the constituent −COOH is deprotonated to −COO^–. The higher population of −COO^– could also be inferred from the appearance of new peaks at 1621/1530 and 1453 cm^–1 in N-DCBP/Ch,^70,87 attributed to ν_as(−COO^–) and ν_s(−COO^–), respectively. The association of −COO^– with NH₄⁺ is likely justified by a combination of I and BC modes. In addition, the observed C–N str. at 1047 cm^–1 for DCBP/Ch became very sharp in N-DCBP/Ch, confirming the greater accumulation of amine functionalities.

In contrast to the adsorption of NH₄⁺, the adsorption of PO₄^3– by DCBP/Ch was carried out at a diametrically opposite pH_o, i.e., pH_o < pH_PZC. Hydrogen bonds were purported to become stronger at this pH_o, and free −NH₂ functionalities of chitosan would be protonated to generate NH₄⁺ ions, providing for ideal adsorption conditions for PO₄^3–. The sharpening of hydrogen-bonded mutual O–H···N–H str. at 3449 cm^–1 was observed for DCBP/Ch upon adsorption of PO₄^3–. However, in contrast to N-DCBP/Ch, this hydrogen-bonded mutual O–H···N–H str. exhibited hypsochromic shifting to the center at 3314 cm^–1, thereby confirming the decrease in relative population of N–H functionalities upon binding with PO₄^3–. Also, the appearance of a new sharp peak at 988 cm^–1 indicated the formation of a P–O–H bond in P-DCBP/Ch.^70,88,89

3.2.2. TG Analyses of N-DCBP/Ch and P-DCBP/Ch

In N-DCBP/Ch, the conversion of most of −COOH/–OH into −COO^–/–O^– at pH_o > pH_PZC would likely result in more rapid elimination of water due to the reduced extent of hydrogen bonding compared to DCBP/Ch. However, since PO₄^3– adsorption was carried out at pH_o < pH_PZC, the extent of hydrogen bonding was expected to be retained in P-DCBP/Ch. In the next stage of degradation (243–388 °C) (Figure S1b), elimination of H₂O through anhydride formation was restricted in N-DCBP/Ch because of the participation of −COO^– in ionic/coordinate bonding with NH₄⁺. In contrast, since −COOH is unlikely to be involved with adsorption of PO₄^3–, no gain in thermostability was observed in this region for P-DCBP/Ch. However, the much slower skeletal decomposition of P-DCBP/Ch than those of DCBP/Ch and N-DCBP/Ch and the higher residue in P-DCBP/Ch (i.e., 35.22 wt %) compared to DCBP/Ch and N-DCBP/Ch (24.57 and 18.49 wt %, respectively) suggested the stronger bonding of PO₄^3– with DCBP/Ch than that of NH₄⁺ along with the prevalence of the corresponding metal ion, i.e., Na. In N-DCBP/Ch, skeletal backbone decomposition was maximum because of the evaporation of NH₃. Therefore, from thermal analysis, simultaneous adsorption of NH₄⁺ and PO₄^3– could be envisaged.

3.2.3. XRD Analyses of N-DCBP/Ch and P-DCBP/Ch

XRD of N-DCBP/Ch was signified by the complete disappearance of the hydrogen-bonding-specific XRD peak at 2θ = 11.08° (Figure 2b). This is justified by the weakening of hydrogen bonds because of the deprotonation of −COOH/–OH functionalities in N-DCBP/Ch at pH_o > pH_PZC. Notably, the polymer-specific peak of DCBP/Ch at 2θ = 20.65° was retained at 2θ = 21.67° in N-DCBP/Ch.

The XRD spectrum of P-DCBP/Ch showed major peaks at 2θ = 10.76, 22.58, 32.21, and 33.24°. Since, adsorption of PO₄^3– was carried out at pH_o < pH_PZC, better orderly arrangement of Miller indices was achieved in P-DCBP/Ch, which resulted in the strengthening of hydrogen bonding. The peak at 2θ = 10.76° indicated hydrogen-bonding interactions within hydrophilic functionalities. The peak at 2θ = 22.58° was the characteristic polymer peak, and the last two peaks appeared because of the Na⁺ counterions coming from Na₂HPO₄.

3.2.4. FESEM and EDX Analyses of N-DCBP/Ch and P-DCBP/Ch

Distinct changes in the surface architecture of DCBP/Ch after NH₄⁺ and PO₄^3– adsorption could be visualized via FESEM (Figure 3e,f). Though the heterogeneity of DCBP/Ch persisted after NH₄⁺ and PO₄^3– adsorption, the entire surface was found to be covered by superficial depositions of NH₄⁺ and PO₄^3–. In fact, the fibrous structure of DCBP/Ch could not be visualized upon NH₄⁺ and PO₄^3– adsorption, suggesting thick coverage of adsorbates. This observation was further justified from the higher intensity of the N-specific peak and arrival of the new P-specific peak in EDX plots of N-DCBP/Ch and P-DCBP/Ch (inset of Figure 3e,f), respectively.

In this context, the retention of chitosan after NH₄⁺ and PO₄^3– adsorption could be inferred via chitosan-specific FTIR peaks, such as −CH₂str. of the pyranose ring at 2862 and 2860 cm^–1 in N-DCBP/Ch and P-DCBP/Ch, respectively, as well as grafting-specific −CH₂– str. of >CH–O–CH₂– at 1121 and 1123 cm^–1 in N-DCBP/Ch and P-DCBP/Ch, respectively. Moreover, the polymer-specific peak of DCBP/Ch at 2θ = 20.65° was retained at 2θ = 21.67 and 22.58° in N-DCBP/Ch and P-DCBP/Ch, respectively, suggesting the unaltered polymer structure, i.e., the integrity of chitosan postadsorption. The presence of chitosan after adsorption was evident from the improved thermostability beyond 300 °C compared with DCBP, very much similar to DCBP/Ch. The elevated hydrodynamic radii (R_h) of DCBP/Ch, N-DCBP/Ch, and P-DCBP/Ch, than that of DCBP, also indicated the presence of chitosan after adsorption. The persistence of heterogeneity of DCBP/Ch in N-DCBP/Ch and P-DCBP/Ch indicated no change in the composition of the complex after NH₄⁺ and PO₄^3– adsorption. Finally, the observation of chitosan-specific ¹H NMR peaks at 3.66, 3.81, 3.96, 4.64, and 4.93 ppm (Figure S3) and 3.17, 3.66, 3.83, 3.96, 4.64, and 4.91 ppm (Figure S4) in N-DCBP/Ch and P-DCBP/Ch, respectively, precludes the washing out of chitosan during adsorption.

3.3. Adsorption of NH₄⁺ and PO₄^3– by DCBP/Ch

The equilibrium adsorption data were fitted to linear and nonlinear forms of different isotherm models such as Langmuir, Henry, Freundlich, Sips, and BET (eqs 3–7).^42,78 The Langmuir model provided the best fits for both NH₄⁺ and PO₄^3– adsorption, based on the highest adjusted R²/F and lowest χ² values.⁹⁰

3

4

5

6

7

Here, q_e, C_e, and n/q_BET/γ represent adsorption capacity, residual NH₄⁺ and PO₄^3– concentration at equilibrium, and Freundlich/BET/Sips parameters, respectively. k_F, k₁/k₂, k_S, k_H, and k_L are Freundlich, BET, Sips, Henry, and Langmuir isotherm constants, respectively.

Herein, the adsorption of NH₄⁺ and PO₄^3– has been carried out at pH_o = 7 and 4, which is higher and lower than that of pH_PZC of DCBP/Ch (Figure S6). The maximum AC values, i.e., q_max, of NH₄⁺ adsorption were estimated to be 41.97/42.16, 38.45/39.11, 36.02/36.74, and 34.73/34.65 mg g^–1 from linear/nonlinear fitting at 298, 303, 308, and 313 K, respectively (Figures 6a and S7a and Table 4). Similarly, the q_max values of PO₄^3– adsorption were estimated to be 15.91/12.43, 15.52/10.59, 15.09/10.09, and 14.29/9.95 mg g^–1 from nonlinear/linear fitting at 298, 303, 308, and 313 K, respectively (Figures 6b and S7c and Table 4). In this context, the Langmuir isotherm model was reported as the best fit model during adsorption of NH₄⁺ by various adsorbents, such as chitosan- and zeolite-based compounds, natural turkish (Yıldızeli) zeolite, natural chinese zeolite, transcarpathian clinoptilolite, natural turkish zeolite, and fruit peels.³⁵⁻⁴¹ Similarly, the Langmuir isotherm model was utilized for explaining the adsorption of PO₄^3– by adsorbents, such as modified and virgin tangerine peel biochars, magnetite-incorporated orange peel biochar, and pristine and chemically modified bentonites.⁴³⁻⁴⁶ The superior modeling of observed kinetics with a pseudo-second-order kinetics model compared to pseudo-first-order kinetics models in both linear/nonlinear forms points to the chemisorption of NH₄⁺ (Figures 6d and S7b) and PO₄^3– (Figures 6c and S7d) by DCBP/Ch.⁹¹ The activation energies for NH₄⁺ and PO₄^3– adsorption were estimated to be 70.03 and 35.62 kJ mol^–1, respectively (Figure S8a).⁹¹ Based on the pore-diffusion model of Weber and Morris, three stages of diffusion patterns were inferred for both NH₄⁺ and PO₄^3– adsorption by DCBP/Ch, suggesting the coexistence of bulk, film, as well as pore/intraparticle diffusions (Figure S9).⁹² From the slope of the second phase of diffusion, rate constants of intraparticle diffusion, i.e., k_ip, were calculated. The k_ip values were found to be 0.7207 and 0.4927 mg g^–1 min^–0.5, respectively. The lower k_ip of PO₄^3– indicated the slower intraparticle diffusion rate than that of NH₄⁺, likely due to the larger ionic radius of PO₄^3–. The k₂ values were found to increase with the increase in temperature, confirming faster adsorption at higher temperatures. Transformations of nonlinear Langmuir isotherm and pseudo-second-order kinetics equations to linear forms not only implicitly adjust their error structures but also violate error variance. Therefore, recent investigations have suggested the better applicability of nonlinear fitting over linear fitting for kinetics and isotherm data.⁹³ Thus, we have considered all adsorption isotherm and kinetics parameters of nonlinear fitting. The adsorption thermodynamic parameters for N-DCBP/Ch and P-DCBP/Ch are listed in Table 5. The negative values of ΔH° and ΔG° throughout the entire working temperature range clearly indicate the exothermic and spontaneous character of chemisorption underlying N-DCBP/Ch and P-DCBP/Ch (Figures 6e and S8b).⁹⁴ We have used NH₄⁺ concentrations in this work that are comparable to NH₄⁺ in human urine, as also in our previous work.⁴² As mentioned in Table S1, the total amount of NH₄⁺ was 1.651 g/100 mL. The amount of PO₄^3– used in our current work (Table S1, 0.374 g/100 mL) is higher than the PO₄^3– levels in human urine. This is because our intent in the current work was to simulate a substantive difference in concentration between NH₄⁺ and PO₄^3–, as present in human urine, and develop an adsorbent that is capable of harvesting both. We are currently working on refining the adsorbent material that could work effectively with the precise concentration of PO₄^3–.

Figure 6.

(a/d/e) Nonlinear isotherm, nonlinear kinetics, and thermodynamics fitting of NH₄⁺ adsorption and (b/c) nonlinear isotherm and nonlinear kinetics fitting of PO₄^3– adsorption.

Table 4. Adsorption Isotherm and Kinetics Parameters.

Temperature (K)
Model/Parameters	298	303	308	313
N-DCBP/Ch
Langmuir (Linear)
qmax (mg g–1)/pHi/C0 (mg L–1)	41.97/7/5–30	38.45/7/5–30	36.02/7/5–30	34.73/7/5–30
kL (L mg–1)	1.2876	1.0102	0.8118	0.7603
R2/F	0.9962/3116.83	0.9994/19722.23	0.9965/3647.61	0.9948/2485.61
χ2	0.8365	0.1142	0.5431	0.7437
Langmuir (Nonlinear)
qmax (mg g–1)/pHi/C0 (mg L–1)	42.16/7/5–30	39.11/7/5–30	36.74/7/5–30	34.65/7/5–30
kL (L mg–1)	1.2412	0.9419	0.7551	0.5841
R2/F	0.9975/3710.43	0.9988/8701.93	0.9953/2280.83	0.9914/1259.46
χ2	2.35 × 10–6	1.07 × 10–6	4.37 × 10–6	8.28 × 10–6
Pseudo-second-order (Linear)
qe,cal (mg g–1)/pHi/C0 (mg L–1)	40.01/7/30	36.41/7/30	32.89/7/30	31.51/7/30
qe,exp (mg g–1)	38.51 ± 0.39	35.62 ± 0.35	33.63 ± 0.38	31.10 ± 0.25
k2 (g mg–1 min–1)	0.0016	0.0026	0.0044	0.0056
R2/F	0.9883/5501.01	0.9841/4301.59	0.9789/3258.39	0.9988/61389.96
χ2	1.4849	1.6685	1.8592	0.0937
Pseudo-second-order (Nonlinear)
qe,cal (mg g–1)/pHi/C0 (mg L–1)	39.04/7/30	35.97/7/30	33.56/7/30	31.25/7/30
qe,exp (mg g–1)	38.51 ± 0.39	35.62 ± 0.35	33.63 ± 0.38	31.10 ± 0.25
k2 (g mg–1 min–1)	0.0022	0.0031	0.0030	0.0071
R2/F	0.9998/73026.96	0.9999/182160.61	0.9999/122195.78	0.9999/1290000
χ2	0.0138	0.0065	0.0111	0.0012
P-DCBP/Ch
Langmuir (Linear)
qmax (mg g–1)/pHi/C0 (mg L–1)	12.43/4/1–10	10.59/4/1–10	10.09/4/1–10	9.95/4/1–10
kL (L mg–1)	1.3526	1.8697	2.1592	2.3455
R2/F	0.9903/600.55	0.9789/284.01	0.9757/247.62	0.9804/311.10
χ2	0.0003	0.0008	0.0009	0.0006
Langmuir (Nonlinear)
qmax (mg g–1)/pHi/C0 (mg L–1)	15.91/4/1–10	15.52/4/1–10	15.09/4/1–10	14.29/4/1–10
kL (L mg–1)	0.8597	0.8522	0.9019	1.0272
R2/F	0.9986/9572.18	0.9956/2105.50	0.9944/1663.51	0.9914/1089.85
χ2	0.0309	0.0934	0.1166	0.1745
Pseudo-second-order (Linear)
qe,cal (mg g–1)/pHi/C0 (mg L–1)	12.51/4/10	12.36/4/10	12.09/4/10	12.06/4/10
qe,exp (mg g–1)	12.37 ± 0.23	12.23 ± 0.25	12.03 ± 0.29	11.95 ± 0.21
k2 (g mg–1 min–1)	0.0092	0.0105	0.0222	0.0102
R2/F	0.9998/63302.42	0.9999/113482.23	0.9999/180018.11	0.9999/88714.98
χ2	0.1542	0.0879	0.0572	0.1179
Pseudo-second-order (Nonlinear)
qe,cal (mg g–1)/pHi/C0 (mg L–1)	12.86/4/10	12.52/4/10	12.41/4/10	11.90/4/10
qe,exp (mg g–1)	12.37 ± 0.23	12.23 ± 0.25	12.03 ± 0.29	11.95 ± 0.21
k2 (g mg–1 min–1)	0.0056	0.0074	0.0091	0.0111
R2/F	0.9943/12251.15	0.9930/10352.97	0.9783/3604.18	0.9741/2573.09
χ2	0.0708	0.0817	0.2451	0.3041

Table 5. Adsorption Thermodynamics Parameters for N-DCBP/Ch and P-DCBP/Ch.

Concentration (ppm)/temperature (K)	–ΔG° (kJ mol–1) of N-/P-DCBP/Ch	–ΔH° (kJ mol–1) of N-/P-DCBP/Ch	–ΔS° (J mol–1 K–1) of N-/P-DCBP/Ch
5/298	9.03/–	47.07/–	127.37/–
5/303	8.39/–
5/308	7.88/–
5/313	7.51/–
10/298	8.02/2.87	36.55/4.72	95.34/6.05
10/303	7.66/2.89
10/308	7.25/2.88
10/313	6.97/2.93
15/298	7.34/–	45.39/–	127.96/–
15/303	6.45/–
15/308	5.72/–
15/313	5.81/–
20/298	6.06/–	43.88/–	127.72/–
20/303	4.86/–
20/308	4.17/–
20/313	4.46/–
25/298	4.26/–	32.05/–	93.22/–
25/303	3.72/–
25/308	3.37/–
25/313	3.01/–
30/298	3.11/–	26.27/–	77.64/–
30/303	2.70/–
30/308	2.37/–
30/313	2.06/–

3.4. Binary Adsorption of NH₄⁺ and PO₄^3– from Artificial Urine by DCBP/Ch

Urine is a highly nutrient-dense body fluid containing more than 3000 metabolites.⁹⁵ The direct use of urine in research on nitrogen and phosphorus harvesting is constrained by the risk of pathogenic activities.²⁰ ArUr has been used as a replacement in academic research. We used ArUr to demonstrate the potential of DCBP/Ch in adsorptive recovery of NH₄⁺ and PO₄^3– simultaneously, at pH_o > pH_PZC and pH_o < pH_PZC, respectively.⁹⁶ Similar to our previous work,⁴² three different dilutions of ArUr were used for these experiments, namely, 1266 ppm (undiluted/UArUr), 105.48 ppm (1/12 diluted/112ArUr), and 52.74 ppm (1/24 diluted/124ArUr). Also, similar to the adsorption conditions used for adsorbing NH₄⁺ and PO₄^3– from NH₄Cl and Na₂HPO₄, respectively, 25 mg of DCBP/Ch was immersed in 50 mL each of UArUr, 112ArUr, and 124ArUr solutions. After achieving equilibrium, the supernatant solutions were withdrawn for measuring the residual concentrations of NH₄⁺ and PO₄^3– by ammonia electrode and ICPMS, respectively. In all three cases, almost 80% adsorption was observed. The adsorption of NH₄⁺ and PO₄^3– observed from ArUr was slightly lower than those obtained from NH₄Cl and Na₂HPO₄, respectively. The difference is attributable to the presence of numerous other cations and anions in ArUr, which can compete with NH₄⁺ and PO₄^3– adsorption.

3.5. Comparison of NH₄⁺ and PO₄^3– Adsorption

We performed an elaborate literature study for the adsorption of NH₄⁺ and PO₄^3– by various reported natural polymers, agricultural waste, zeolites, clays, and chemically modified inorganic materials at varying initial concentrations (i.e., 20–1000/1–60 ppm for NH₄⁺/PO₄^3–), temperatures (i.e., 294–323 K), and pH_o (i.e., 3–8). This survey is depicted in Table 6. Based on the comparison, the maximum adsorption capacities of DCBP/Ch are the highest with respect to the previously reported adsorbents.

Table 6. Comparison of Adsorbents.

Adsorbate	Name of adsorbents	Adsorption capacities (mg g–1)/pHo/C0 (ppm)/temperature (K)	refs
NH4+	LMWCa	0.769/7.00/50/303	(35)
	HMWCb	0.331/7.00/50/303	(35)
	NZc	2.162/7.00/50/303	(35)
	ANZd	2.937/7.00/50/303	(35)
	Natural Turkish (Yıldızeli) zeolite	9.64/8.00/20–400/294	(36)
	Natural Chinese zeolite	9.41/8.00/80/300	(37)
	Transcarpathian clinoptilolite	11.6/7.00/1000/298	(38)
	Natural Turkish zeolite	8.12/7.00/25–150/298	(39)
	Orange peel biochar	4.71/9.00/40/298	(40)
	Pineapple peel biochar	5.60/9.00/40/298	(40)
	Pitaya peel biochar	2.65/9.00/40/298	(40)
	Pomegranate peel	6.18/4.00/30/298	(41)
	BNPPe	34.48/7.00/60/293–313	(42)
	DCBP/Chf	42.16/7.00/5–30/298	Present study
PO43–	TBg	1.540/7.00/1/298	(43)
	CTBh	3.608/7.00/1/298	(43)
	FTBi	5.434/7.00/1/298	(43)
	MOP700j	1.24/7.00/2.4/298	(44)
	Zenith/Fek	11.15/7.00/50/298	(45)
	bentonite	4.12/7.00/50/298	(45)
	Phoslock	11.60/7.00/50/298	(45)
	Al-Bentl	12.70/3.00/25–60/303–323	(46)
	Fe-Bentm	11.20/3.00/25–60/303–323	(46)
	Fe–Al-Bentn	10.50/3.00/25–60/303–323	(46)
	DCBP/Chf	15.91/4.00/1–10/298	Present study

^aLow molecular weight chitosan.

^bHigh molecular weight chitosan.

^cNatural zeolite.

^dActivated natural zeolite.

^eBanana peel powder.

^fdried and crushed banana peel/chitosan composite.

^gTangerine peel biochars.

^hTangerine peel biochars chemically activated by CaCl₂.

ⁱTangerine peel biochars chemically activated by FeCl₃.

^jOrange peel and magnetite incorporated magnetic biochar.

^kModified inorganic bentonite.

^lHydroxy-aluminum pillared bentonite.

^mHydroxy-iron pillared bentonite.

ⁿMixed hydroxy-iron–aluminum pillared bentonite.

4. Conclusion

The persistent worldwide use of synthetic fertilizers, mostly containing NH₄⁺ and PO₄^3–, is accompanied by enormous energy burden and environmental disruption. Human urine and industrial effluents contain significant amounts of NH₄⁺ and PO₄^3– ions. Harvesting nitrogen and phosphorus from such sources could lower the reliance on energy-intensive processes while simultaneously addressing environmental pollution. We have previously established the utility of banana waste via dried peels toward effective NH₄⁺ adsorption. In this work, we have developed a pH-responsive biocomposite, DCBP/Ch, by grafting of chitosan with DCBP that is suitable for capturing NH₄⁺ and PO₄^3–. A combination of ¹H NMR, FTIR, XRD, FESEM, EDX, and TGA suggests successful grafting of chitosan onto DCBP, resulting in distinctive structural and surface characteristics and thermal properties. The adsorption of NH₄⁺ and PO₄^3– on DCBP/Ch is suitably modeled by pseudo-second-order kinetics that indicates chemisorption of both ionic species. DCBP/Ch displays excellent adsorption capacities for NH₄⁺ and PO₄^3–, respectively. The biocomposite retains a handsome adsorptive capacity when tested on artificial urine, albeit competing with a host of other ions that are present therein. To the best of our knowledge, this is the first report of a biocomposite adsorbent that is capable of harvesting both NH₄⁺ and PO₄^3–. Considering the ease of preparation, inexpensive components, environmental compatibility, and excellent adsorption capacity, DCBP/Ch can be viewed as an attractive prospect for accessing NH₄⁺ and PO₄^3– from waste channels. The construction and detailed characterization of DCBP/Ch reported in this work also serve as instructive components for further refinement and deployment and are currently being pursued in our laboratory.

Glossary

Abbreviation

AC

adsorption capacity

α-l-Araf

α-l-arabinofuranose

ArUr

artificial urine

asym.

asymmetric

BC

bidentate chelation mode

BNPP

banana peel powder

CA

catecholamines

DCBP

dried and crushed banana peel

DCBP/Ch

biocomposite of DCBP and chitosan

def.

deformation

DLS

dynamic light scattering

EDX

energy dispersive X-ray analysis

F

flavonols

FESEM

field emission scanning electron microscope

Fols

flavan-3-ols

FTIR

Fourier-transform infrared spectroscopy

α-d-GalA

α-d-galacturonic acid

β-d-Galp

β-d-galactopyranose

d-GlcA

4-O-methyl-d-glucuronic acid

HCA

hydroxycinnamic acids

I

ionic mode

ICPMS

inductively coupled plasma mass spectrometry

Leu

leucine

MQW

milli-Q-water

N-DCBP/Ch

NH₄⁺-adsorbed biocomposite

NMR

nuclear magnetic resonance spectroscopy

P-DCBP/Ch

PO₄^3–-adsorbed biocomposite

Phe

phenylalanine

pH_o

operating pH

pH_PZC

pH at point of zero charge

α-l-Rhap

α-l-Rhamnopyranose

str.

stretching

sym.

symmetric

TGA

thermogravimetric analysis

Thr

threonine

UV–vis

ultraviolet–visible spectroscopy

Val

valine

WHO

world health organization

XRD

X-ray diffractometry

β-d-Xylp

β-d-Xylopyranose

λ_max

wavelength of the maximum absorbance

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.3c05229.

• Composition of artificial urine; TGA analyses of DCBP, chitosan, DCBP/Ch, N-DCBP/Ch, and P-DCBP/Ch; ¹H NMR of chitosan; ¹H NMR of N-DCBP/Ch; ¹H NMR of P-DCBP/Ch; DLS study of DCBP, DCBP/Ch, N-DCBP/Ch, and P-DCBP/Ch; UV–vis analyses of DCBP and DCBP/Ch; pH_pzc analyses of DCBP and DCBP/Ch; linear isotherm and linear kinetics fitting of NH₄⁺ and PO₄^3– adsorption; activation energy analyses of NH₄⁺ and PO₄^3– adsorption; adsorption thermodynamics fitting of PO₄^3– adsorption; and diffusion studies of NH₄⁺ and PO₄^3– adsorption (PDF)

The authors declare no competing financial interest.