1 Introduction

Walnuts are one of the four most consumed dry fruits worldwide and are thus considered an important oil tree, economic and ecological tree species. Walnut fruit comprises three major parts, i.e., walnut kernel, walnut shell (WS), and walnut husk/hull (WH). Even though WSs and husks/hulls are considered processing waste, recent studies have shown many benefits of these by-products showing their potential in the medical, nutrition, agriculture, and industrial field. Mates et al. [1] studied the antioxidant capacity of the walnut septum and reported that the septal hydro-alcohol extracts showed a high antioxidant capacity compared to butylated hydroxyl toluene. The overall composition and application of WS and WH (Fig. 1), while the pictorial representation of WS composition and its application is given in Fig. 2.

Fig. 1
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The overall composition and application of WS and WH

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Fig. 2
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WS Composition and its application

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The WS is made up of lignin (52.3%), cellulose (25.5%), and hemicellulose (22.2%). The tough, non-toxic, recyclable, and renewable nature of the shell is made possible by its structural makeup. The shell and tissues that have stopped growing are supported by parenchymatous cells. The lignified cells that make up the secondary wall of shells are dense, robust, and hard. WSs can be used to create biological oil rich in organic compounds like phenol, ketone, and ester through pyrolysis. They may be used to develop biological carbon using modified manufacturing techniques which is a key component in creating active carbons with specific surface areas and high porosity [2].

WSs have unique application capabilities in producing supercapacitor electrodes and industrial wastewater treatment. Since WSs are tough and fragile, they can polish delicate equipment and passivate ultra-hard blades after superfine grinding. Additionally, after processing and employing specialized technologies, this shell may be utilized as a composite filler, industrial reductant, material for extracting biomass compounds, and a tool for separating oil from water [3].

The greenish color covering of the walnut surrounding the fruit is known as WH. It is a by-product of the walnut processing business that, if used correctly, can bring many benefits and find its uses in various fields of life [4]. WH not only contains many important polyphenols and phytochemicals but the chemical juglone present in the WH finds its use as a sustainable dyeing agent. Flavonoids are polyphenolic compounds with antioxidant, anti-inflammatory, and anti-carcinogenic properties [5]. Nutraceuticals have a range of benefits, including growth promoters, antibiotics, immunostimulants, antioxidants, flavorings, colorants, and natural substitutes for artificial compounds [6]. Li et al. [7] identified 75 phenolic compounds, including flavonoids, ellagic acid, and gallotanins, in the methanolic extract of the walnut septum. They reported that walnut septum contained 122.78 mg GAE/g dimers and trimers of procyanidins. Walnut septum extracted on human A172 glioblastoma cells. A 70 µg/ml dose significantly reduced the A172 cell viability without altering HFF-1 cell viability [8]. The pictorial representation of WH composition and its application is given in Fig. 3.

Fig. 3
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WH Composition and its application

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This review aims to explore different by-products of walnut produced during processing and identify various biochemicals and components present in the WS and WH and their potential uses. Through the comprehensive research exploration, this review highlights the potential applications of these two by-products in different industries as eco-friendly materials and energy sources for pharmaceutical, nutraceutical, and agriculture applications.

2 Search study

We thoroughly searched the literature in PubMed, Google Scholar, Science Direct, and Scopus for publication addressing the chemical composition and potential application of walnut by-products from 2017- 2023 (January). The keywords used for searching literature were ''walnut'', ''walnut husk'', ''walnut hull'', ''walnut shell'', ''walnut by-products'', ''industrial application of walnut by-products'', "agriculture application of walnut husk", "agriculture application of walnut hull", "agriculture application of walnut shell", "agriculture application of walnut by-product", "medicinal application of walnut husk", "medicinal application of walnut hull", "medicinal application of walnut shell", "medicinal application of walnut by-product", "food application of walnut husk", "food application of walnut hull", "food application of walnut shell", "food application of walnut by-product", ''nutraceutical application of walnut husk", ''nutraceutical application of walnut hull", ''nutraceutical application of walnut shell", ''nutraceutical application of walnut by-product'', "antioxidant profile of walnut by-products'', "antioxidant profile of walnut husk'', "antioxidant profile of walnut hull'', "antioxidant profile of walnut shell", ''nutritional profile of walnut by-products'', ''nutritional profile of walnut husk'', ''nutritional profile of walnut hull", ''nutritional profile of walnut shell" and ''applications of Juglan regia by-products.

3 Important compounds present in the WS

3.1 Investigation of essential oils and fatty acids of WS

Several studies demonstrated that extracts from WSs could be rich in essential oils and fatty acids (FA). The proximate composition of WS is shown in Table 1.

Table 1 Proximate and ultimate /elemental composition of the WS (%)
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For example, Andrade et al. [12] studied the physicochemical and mechanical attributes of oil and macadamia WSs. The physicochemical and FA profile was examined using NMX-F-083–1986 gas chromatography and mass spectrometry (GC–MS) standards. The FA content in the oil is stearic acid (3.39%), oleic acid (31.74%), palmitic acid (13.61%), linoleic acid (1.21%), and linolenic acid (0.08%). It is shown that the oil has a higher content of oleic acid (31.74%), which is monounsaturated and called omega-9, which involves lipids' metabolism. The Fourier transfer infrared spectroscopy (FT-IR) spectra were used to analyze the sample, and spectra were obtained ranging from 400–4250 cm−1. Two major absorption areas could be seen in the examined sample: the first was between 700–1750 cm−1, and the second was between 2700–3600 cm−1; Hydroxyl (OH) group typical of cellulose could be the source of the signal at 3330 cm−1. The stretching vibration of the carbon-hydrogen bond is what the signal is at 2923 cm−1. A signal associated with the O–H bond of water could be seen between 1630 cm−1 and 1650 cm−1. Another research by Yuan et al. [13] checked the bioactive compounds and FA profile in walnut using GC–MS. The highest to the lowest concentration of the major substances identified in the WS were oleic acid (0.30 mg/g), palmitic acid (0.21 mg/g), sitosterol (0.41 mg/g), and stearic acid (0.06 mg/g).

In another research, the nutritional attributes of the oil from WSs were determined using GC–MS [14]. 26 FA were discovered in the WS, including saturated (13) and unsaturated FAs (13). Saturated FAs included decyclic acid (0.53 mg/kg dry weight (DW), palmitic acid (1778.28 mg/kg DW), arachidic acid (22.7 mg/kg DW), stearic acid (483.7 mg/kg DW). In contrast, unsaturated FA included linoleic acid (7041.5 m/kg DW), oleic acid (2001.71 mg/kg DW), arachidonic acid (1.29 mg/kg DW), and cis-11-eicosenoic acid (15.5 mg/kg DW). Oleic, as well as linoleic acid, was dominant. Linoleic acid is a member of the omega-6 series and is recognized as a necessary compound with good oxidation ability, which lowers the amount of low-density lipoprotein cholesterol and the maintenance of high-density lipoproteins in the blood. These FAs were also called "safety fatty acids." As found by GC and GC–MS, major FAs and oils are present in the WS (Table 2).

Table 2 Groups of components present in the WS (mg/g) are determined by GC and GC–MS
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4 Investigation of polysaccharide content of WS

Walnut oligopeptides which have a dosage ranging from 110–440 mg/kg, have the potential to greatly enhance innate as well as adaptive immunity by the activation of natural killer cells along with improving in vivo immunoglobulin synthesis. Polysaccharides content of the WS includes cellulose, hemicellulose, and lignin, which are 25, 19, and 56%, respectively [16].

Another study was conducted by Domingos et al. [17] to check the chemical composition of the WS by FT-IR. The results represented that WSs contained lignin (35%), cellulose (30.4%), and 24.9% polysaccharides and hemicellulose. The high content of lignin makes WSs suitable for producing adhesive agents.

Another research checked the impact of WS and salinized WS on the thermal and mechanical attributes of polyurethane (PUR) foams. PUR foams' compressive, flexural, and impact strengths made using polyol (WS-based) range 255–310 kPa, 420–458 kPa, and 340–368 kPa, respectively. The PUR foams' thermal conductivity varied from 0.026 W/mK to 0.032 W/mK, increasing with the amount of polyol. WS has hemicellulose (24.4%), lignin (50.3%) as well as alpha-cellulose (23.9%) [18]. Major polysaccharides present in WSs are given in Table 3.

Table 3 Chemical composition (% of total dry mass), polysaccharides composition (% of total dry mass), and mineral composition (mg/kg dry mass) of WS
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5 Investigation of phenolics of WS

Another research was performed by Kizatova et al. [20] to check the chemical content of walnut and their application in the food industry. Catechin tannins, quercetin, and P-vitamins are flavonoid compounds found in WSs that are particularly beneficial for the heart and safeguard cognitive processes. Having an antibacterial (antimicrobial) function, they promote connective tissue and enhance blood circulation. Quercetin concentrations in WSs ranged from 0.945–1.51 mg, whereas catechin concentrations ranged from 2.46–12.07 mg, and tannin concentrations ranged from 611.32–805.62 mg.

Another study was performed by Simsek & Sufer [21] to check the antioxidant profile of WS-based tea by employing liquid chromatography coupled with MS. Eight different types of compounds were identified, which were salicylic acid, hydroxybenzoic acid, hydroxycinnamic acid, vanillic acid, trans-resveratrol, catechin hydrate, gallic acid and fumaric acid with concentrations of 5.35, 10.76, 184.79, 371.79, 38.13, 46.21, 56.30 and 235.92 µg/L, respectively.

Han et al. [22] checked the impact of ultrasound-assisted extraction on the characteristics of WS phenolic compounds. The extraction yield from an ultrasonic probe was 51.2 mg GAE/g DW, two times more than that of the ultrasonic bath method (20.6 mg GAE/g DW) and the shaking method (25.8 mg GAE/g DW), respectively. The size of the WSs was reduced, which further increased phenolic extraction. The best extraction yield for the particle size (45–100 mesh) was observed, 52.8 mg GAE/g DW.

6 Applications of WS across various fields

6.1 Industrial applications of WS

Many value-added products extracted from WSs and hulls have found uses in the food industry and other areas. For example, Santigo et al. [23] used two distinct extraction sequences for lignin from WSs. Autohydrolysis, delignification, solid cleansing, and precipitation are the processes involved in lignin formation. The soda procedure used sodium hydroxide solution (7.5%, w/v) as a reagent at 121 °C in the same solid/liquid ratio (1:6) as the organosolv technique (solubilize lignin without affecting its original structure), which used water and ethanol (30:70, v/v) at 200 °C in solid to liquid ratio of 1:6. Lignin was used extensively in lignin-based adhesive and WS-derived activated carbon [24].

A crucial step toward creating light, eco-efficient bricks with increased thermal insulating properties is the creation of agriculture waste-doped burned bricks. Research conducted by Barnabas et al. [25] utilized WS waste as an additive in developing fire ceramics bricks in fired clay. The findings demonstrated higher water absorption and specific heat capacity. Higher WS content was associated with increased porosity and water absorption properties. When walnut content increased, linear shrinkage and thermal conductivity decreased at higher temperatures, i.e., 1100˚C and 950˚C, respectively. Fire temperature enhanced the resistance of bricks against salt crystallization. Another study was conducted by [26] to examine the elimination of metolachlor (MET) by biochar (BC) generated from WS (WSBC) as opposed to BC made using corn cobs (CBC) as well as cow dung (DBC). The removal rates of MET were 87.89%, 10.91%, and 52.91% for WSBC, CBC, and DBC, respectively. Sorption capacities of BCs employing the Langmuir isotherm model were 37.88, 96.15, and 11.98 mg/g for DBC, WSBC, and CBC, respectively. Another research conducted by Mohammed et al. [27] used WS as a sand replacer from cement mortar subjects. The outcomes showed that taking grounded walnut lowered every tested property. Initial experiments with a water-to-cement ratio (w/c) of 0.5 and an optimal grounded walnut utilization ratio of 20% produced a lightweight cement mortar appropriate for structural uses before and after subjection to 400 °C.

Yu et al. [28] investigated the impact of WS extract on Haematococcus pulvialis’s concurrent synthesis of astaxanthin and lipids during abiotic extract. When 15% WS extract was added, it increased the number of lipids and astaxanthin by 77.57% and 23.39%, respectively, versus the control. The result showed increased light-oxygen-voltage histidine kinase (LOV-HK), reactive oxygen species (ROS), and glutathione levels, but chlorophyll, protein, and carbohydrate levels decreased. Zhou et al. [29] investigated the impact of WS on pore structural properties when Zhoundong (ZD) coal is sintered with 1200˚C as the working temperature. According to X-ray computed microtomography (XCT) findings, during the whole sintering process, the porosity of samples ranged from 5.46–40.92%. Small pores with a diameter of less than 0.3 mm or the corresponding spherical diameter comprised most of the pore samples' internal structure. The porosity increased from 42.50% to 50.30% when adding 10% Walnut shell ash (WSA). Other studies regarding the industrial application of WS have been mentioned in Table 4.

Table 4 Various applications of WS and its constituents
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7 Environmental applications of WS

To eliminate Cu (II) found in groundwater samples, the study was carried out by [30] to create a novel magnetic nanocomposite using WSA, starch, and Fe3O4. Using WSA and WSA/starch/Fe3O4, maximum Cu (II) sorption capacities of 29.0 mg/g and 45.4 mg/g were observed. Depending on the water quality, the composite adsorbent (AC) could be reused thrice with adequate efficiency based on its capacity to extract copper from three groundwater samples. WSBC was also used by [31] to remove Cr (VI) from wastewater. With a pH of 5.5, 2 h of spinning, and 1.1 g/L of WSBC, it was possible to effectively remediate Cr (VI) by removing up to 93% of the maximum Cr (VI) concentration from an aqueous phase. Palaniyappan et al. [32] prepared a walnut/PLA biopolymeric composite, including a hexagonal lattice structure. According to the experiment results, the optimal printing conditions of a 210˚C nozzle temperature, height (0.1 mm), infill density (100%), and printing speed (20 mm/sec) produced a maximum tensile strength of 2.684 MPa. [33] used WS powder to remove lead from an aqueous medium. At optimum values of the parameters mentioned earlier, for starting 100 mg/L content of Pb2+ and using 1 g/100 mL natural WS dose, more than 90% yield was reached in less than 2 min. From the Langmuir equation, the adsorption capacity was calculated as 9.912 mg/g at room temperature. Dovi et al. [34] used a cationic surfactant modified (CTAB) based functionalized WS surfactant for the removal of bisphenol A (BPA) along with Congo red (CR) dye from the solution. For BPA and CR dye, the maximal monolayer adsorption quantities of WS-CTAB have been determined as 38.5 mg/g and 104.4 mg/g, respectively. The adsorption capability of BPA decreased by 66% in the binary system, whereas CR only slightly reduced by 8.0%. This showed that compared to BPA, WS-CTAB exhibited a greater binding affinity for CR with greater selectivity. Dovi et al. [35] used amino-functionalized WS to remove CR from the solution. At 303 K, the Langmuir model determined that WSAC (walnut shell-based adsorbent) had an adsorption capacity of 224.4 mg/g. Due to a stronger driving force for CR adsorption onto WSAC, column masses transfer capacity parameter (β) rose as concentration increased (0.461, 1.59, and 2.47 for 50, 100, and 150 mg/L, respectively), but adsorbate-adsorbent compatibility (β) showed a declining trend. Similar studies have been displayed in Table 4.

8 Agricultural application of WS

WS or WSBC is extensively used to remove pesticide residue from the soil. For example, another research by Liu et al. [36] applied WSBC to remove MET by adsorption method from the soil. It modified it with montmorillonite, kaolinite, illite, and the original BC. The results demonstrated that montmorillonite BC (MBC) had the most stability, with illite BC (IBC) and kaolinite BC (KBC) having superior thermal and chemical stability than OBC (original BC). OBC, MBC, IBC, and KBC had Met adsorption rates of 62.15, 92.47, 87.97, and 83.31%, respectively. Adsorption mechanisms and the results of the kinetic fitting demonstrated that adding minerals to BC improved MET's physical adsorption. OBC, MBC, IBC, and KBC each had a maximum MET adsorption capacity of 39.68, 68.49, 65.79, and 65.36 mg/g, respectively.

8.1 Use of WS as a Catalyst of WS

There is a great trend in developing eco-friendly catalysts because metal-based catalysts harm the environment. For example, research performed by Wu et al. [37] used WS as the raw material in their experiment to create nitrogen-doped BC, which is then used as a green catalyst for the chlorination of acetylene using the condensation product of dicyandiamide or dicyandiamide-formaldehyde as a nitrogen source. A large proportion of quaternary nitrogen as the active center is responsible for the 2.5 DF/BC-850 catalyst's unusual catalytic activity, which results in acetylene conversion (94.5%) at 45 h−1 and 220 °C. The metal-free nitrogen-doped BC catalyst's thermal stability is increased by the addition of formaldehyde, which successfully prevents carbon from depositing throughout the catalytic reaction.

Jovičić et al. [38] studied the liquefaction characteristic of distinct WS particle sizes (five) employing liquefaction agent (glycerol) under different reaction parameters. Results showed that WS biomass might be successfully converted into glycerol utilizing sulfuric acid as a catalyst, with liquefaction efficiencies ranging from 89.21–90.98%.

8.2 Food applications of WS

To preserve product quality, Zhang et al. [39] used a procedure involving the pasteurization of in-shell walnuts (15.01% moisture) employing a radio frequency oven (27.12 MHz; 6 kW). During variations in kernel color among radio frequency (RF) samples and control after accelerated storage were insignificant, this RF treatment yielded more than the decrease in Staphylococcus aureus ATCC (4-log). FA levels of both samples were under 0.6% throughout storage. In contrast, the peroxide content of RF treatments rose by more than 1 mEq/kg during storage but did not significantly vary from controls. During accelerated storage, moisture and water activities of WS and kernels of both treatments first declined and then stabilized. S. aureus ATCC 25923's population in RF treatments steadily declined throughout storage to levels below the detection threshold. Other food applications of WSs have been summarized in Table 4.

8.3 Medicinal applications of WS

Copper nanoparticles (CuNPs) attached to cellulose WS material were studied for their antibacterial, antioxidant, and anticancer properties. By raising the concentration to 10%, CuNPs' antioxidant capabilities were markedly enhanced. On K562 cells, CuNPs have a dose-dependent cytotoxic impact. The IC50 of materials against PBMCs (peripheral blood mononuclear cells) was substantially higher than the IC50 of the produced NPs against K562 cancer cells (25.24 5 g/mL) [40].

Moxibustion with WS glasses has been examined to determine its effectiveness and acceptability in treating patients with dry eye disease (DED) and to offer therapy choices. Moxibustion combined with WS eyewear may help DED patients stabilize their clinical symptoms and tear film. However, it is not significantly more effective than sodium hyaluronate eye drops in reducing corneal damage and tear production. Compared to sodium hyaluronate eye drops, moxibustion with WS spectacles might have a superior impact on ocular pain (self-assessment) [41]. Similar medical applications of WSs have been shown in Table 4.

9 Walnut husk/hull

9.1 Investigation of essential oils and fatty acids in WH

Supercritical carbon dioxide extraction of WH shows that WH is more unsaturated than saturated FAs. The most abundant saturated FA present in WH extract was found to be palmitic acid which was 27.16%. Linoleic acid and eicosenoic acid were major unsaturated FAs present in WH, with 33.83% and 21.90%, respectively. Even though supercritical fluid extraction of WH showed an abundance of unsaturated FAs, docosahexaenoic acid (0.31%) was found to present in a minor amount [61].

9.2 Investigation of polysaccharides in WH

Polysaccharides present in walnut green husk/hull (GWH) are found to be beneficial in terms of their role in preventing inflammation, oxidative stress, and obesity, as well as damage to the colon or liver due to a high-fat diet among rats. These polysaccharides positively affect gut microbiota and enhance the number of short-chain FAs [62]. Another study by He et al. [63] focused on isolating and characterizing polysaccharides (water soluble) from the WH. Glucose, Galactose, along with Arabinose containing heteropolysaccharides were observed. Other monosaccharides present in WH included rhamnose, ribose, mannose, and fucose.

9.3 Investigation of polyphenols and phytochemicals in WH

Supercritical carbon dioxide extraction of WH showed the high antioxidant capacity of polyphenols present in it, as shown by 2,2-diphenyl-1-picryl-hydrazine-hydrate (DPPH) and 2,2'-casino-bis (3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) analyses. 10,750 mg GAE/100 g total polyphenols were measured in the extract using supercritical carbon dioxide extraction, while the water extract of WH contained total polyphenolic content of 3038.32 mg GAE/100 g. The hydroxybenzoic acid, chlorogenic acid, syringic acid, vanillic acid, caffeic acid, o-coumaric acid, ferulic acid as well as juglone was identified in WH extract in large amounts of 103.98 mg/100 g, 27.88, 6.16, 15.34, 631.78, 986.96, 77.03 and 1192.04 mg/100 g, respectively. The chlorogenic (23.37 mg/100 g) and caffeic acid (121.77 mg/100 g) were found to be in greater quantity in methanolic extracts of WH [61].

Ultra-high-performance liquid chromatography coupled to Orbitrap high-resolution mass spectrometry (UPLC-Q_Orbitrap HRMS) and high-performance liquid chromatography (HPLC) test of the WH and a pellicle, was carried out to identify 110 different compounds, including tannins, phenolics, flavonoids, and quinones. WH's total phenolic and flavonoid content ranged from 0.54–1.33 mg gallic acid and 0.34–1.01 mg rutin equivalent/g, respectively. Gallotannins were found in large quantities, while ( +)-catechin was found to be the major falavan-3-ol. Naphthoquinones and flavonoids were majorly identified in the husk compared to the pellicle. WH contained compounds like quercetin pentoxide isomer, prorcatechualdehyde 7-hydroxy-methyl coumarin, and taxifolin [64].

10 Applications of WH across various fields

10.1 6.1 Industrial Applications of WH

GWH and compounds like sodium lignosulfonate work synergistically to inhibit the corrosion of materials like cold-rolled steel showing an increased inhibition efficiency (97.2%) [65]. Similarly, the inhibitory impact of WH on the corrosion of mild steel samples in 1 M hydrochloric acid (HCl) electrolyte has also been observed [66], as mentioned in Table 5. Wu et al. [67] also utilized WH extract to inhibit the corrosion of magnesium alloys submerged in sodium chloride solution. The extract was observed to increase inhibition efficiency from 44.7% to 92.5%.

Table 5 Various Applications of walnut husk/hull and its constituents
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WH may act as a mordant in the dyeing industry. Hosseinnezhad et al. [68] used WH combined with myrobalan as a mordant to dye wool yarn up to 10%. A 7.5:2.5 ratio of WH and myrobalan produced the best results. In another study, color strength and fastness were improved when alginate and WH were used as a mordant to dye viscose rayon fabric using an anthocyanin solution obtained from Hibiscus sabdariffa L. calyces [69]. Wool yarn dyed with WH possessed antimicrobial activity of up to 35%. When the dyed wool yarns were treated with sodium salt NPs, their excellent antimicrobial property was observed, maintaining themselves (91%) after ten repeated washes. A good wash and light fastness properties were also marked [70, 71], and they also attempted to print alpaca wool using WH and iron sulfate. WH-extracted pectin using ultrasound-assisted extraction has been found to contain good emulsifying attributes [72]. A 2.15 times increase in versatile peroxidase production has also been observed by Pleurotus eryngii through solid-state fermentation [73].

10.2 Environmental applications of WH

Methanolic WH extract has the potential to act as a sustainable and environment-friendly replacement of propyl gallate as the antioxidant to improve the oxidation stability index of biodiesels like wasted cooking oil [74]. The comparison of WH extract and propyl gallate through life cycle assessment also proves the ability of WH as an eco-friendly antioxidant for biodiesel fuels.

WH extracts have also been found as environment-friendly absorbents of toxic dyes and metals from the environment. Titan yellow dye also has been removed using WHs up to more than 85.5% when used in a dose of 0.06 g with a dye concentration of 15 ppm [75]. WHs as composites with other compounds have also been administered for the same purposes. Polyaniline and WH nanocomposite have been studied and found cheap, efficient AC of E102 dye [76]. Similarly, nanocomposites of WH extract and silver (Ag) have efficiently removed heavy metals like lead, chromium, and cadmium in wastewater produced during petroleum treatment. 0.75 g of these NPs can remove lead, chromium, and cadmium up to 72.6, 81.3, and 88.1% during 5 h [77]. Other environmental applications of WH have been shown in Table 5.

10.3 Agriculture applications of WH

WH extracts have been found to contain insecticide and nematicide properties showing their potential in agriculture to replace pesticides detrimental to humans and the environment, as shown in Table 5. The nematicidal effect of WH has also been observed against Pratylenchus thornei [78] and Meloidogyne javanica [79]. Insecticide activity against Pieris rapae larvae and Helicoverpa armigera larvae has also been followed by the walnut extract showing mortality in the shape of stomach toxicity up to 50% [80].

10.4 Food applications of WH

WH extract is being utilized in many industries to preserve food items and improve shelf life, as shown in Table 5. Wichita pecans were preserved using a juglone coating from the WH [81]. At the same time, freshly cut apples [82] and rainbow trout [83] also improved shelf life and sensory attributes when WH extract was employed. Dehghani et al. [84] attempted to add WH extract to ketchup to improve its antimicrobial attributes and found an improved quantity of flavonoids in the prepared ketchup. WH extract was also added to sunflower extract to prevent oxidation.

10.5 Nutraceutical/Medicinal Applications of WH

Medicinal properties of WH have also been studied in recent years, and WHs are an effective remedy for many diseases. WH has been found beneficial against cancerous cells. SCG7901 (human gastric cell line) was observed to show inhibition of cancer cell proliferation when treated with WH extract [85]. Twenty-seven naphthoquinones and their derivatives were extracted from GWH and studied for their cytotoxic effect against the human cancer cell line HepG-2 [86] also isolated 13 compounds from husk through recrystallization and column chromatography. They studied their effect on hepatoma cells (HepG-2). Oleanolic acid extracted from WH was found to show an inhibitory effect against human cancer cell lines.

WH is efficient in patients with liver injury. Juglanin has been found to reduce fibrosis and inflammation among mice that suffer from acute liver injury [87]. Similarly, WH extracts protected mice from hepatic steatosis and vascular endothelial dysfunction among fructose-induced mice [88]. WH has been found to improve lipid levels and the gut microbiome of obese rats due to high-fat diets. It improves insulin resistance and glucose metabolism and reduces fat accumulation and adipose tissue hypertrophy among obese rats. The gut microbiota is also changed when WH extract was given to the rats as it reversed the disorders of the gut microbiome by reducing Fuso bacteria, Firmicutes and Peptostreptococcaceae along with increasing Bacteroidetes at the phylum level [89]. Wang et al. [62] also observed inhibition of chronic inflammatory responses and colonic tissue damage among rats fed high-fat diets due to WH extract.

WH extract possesses antimicrobial and antioxidant properties. Hence, it has been found effective against acne vulgaris [90]. In infected rats, candidiasis can also be efficiently treated with WH-based cream [91]. WH has been found to inhibit α-glucosidase, which is a key factor of hyperglycemia, and controlling it can be an effective way of controlling postprandial blood glucose levels [92]. WH extract has been observed to prevent platelet aggregation. Acetone extract from WH has demonstrated protein secretion by 50% along with prevention of aggregation of platelets. The extract activates caspase and suppresses the generation of ROS [93]. WH has also demonstrated wound-healing properties among rats with incisions. [94, 95] studied the potential of WH-extracted juglone to load scaffolds and be used as a potential wound dressing. Other medicinal studies regarding WH are shown in Table 5.

10.6 Cosmetic industry applications of WH

WH found its use in the cosmetic industry and natural and non-irritant hair dye. Beiki et al. [96] developed hair dye using WH as a dye, aloe vera as second, ferrous sulfate as the primary mordant, and ascorbic acid (developer). The hair dye resulted in dark brown color when applied to hair samples. Good color strength, along with hair surface morphology, was observed. Good resistance to daylight fastness and washing was also demonstrated. Irritation tests on rat models were also done, which showed no irritation and maximum antimicrobial attributes when compared with commercial and semi-synthetic dyes. Hair dye developed from an iron mordant and WH extract incorporated in hydrogel hair dyes were also used for dyeing grey unbleached human hairs, which showed good color strength. The irritation potential of plant dyes was also assessed using a reconstructed human epidermis which showed no potential for the developed dye to cause skin irritation, as mentioned in Table 5.

10.7 Laboratory applications of WH

WH extract is employed in some studies as an alternative staining gel, as demonstrated in Table 5. Aqueous extract of the WH can stain proteins in polyacrylamide gels in about ten minutes, with its sensitivity comparable to Coomassie R-250. The bands produced by hull extract are more intense than those produced by Ponceau S and Coomassie. Similarly, WH extract has been used to stain Platyhelminthes [97]. Apart from its use as a staining dye, WH acts as a great catalyst for the synthesis of NPs like phenolic capped silver nanoparticles (AgNPs) that act as colorimetric sensors [98] and benzylpyrazolyl coumarin derivatives [99].

11 Conclusion and future recommendations

WS and WH are the by-products of walnut processing and consumption and comprise many beneficial bioactive compounds. WS and husk/hull are extensively used in agriculture, industrial, medicinal, cosmetics, and environmental fields. These by-products can act as sustainable, environment-friendly, low-cost alternatives to many expensive toxic chemicals. The thorough review of the literature also reveals the gaps in the current understanding and points to the need for additional research to (I) predict the performance of the adsorption process for the removal of different adsorbates by WS from real effluents under a wide range of operating conditions, (II) look into the removal of adsorbates from mixed wastewater, (III) better understand the adsorption mechanism of various hazardous materials on WS, (IV) confirm the viability of various chemically modified WS types on an industrial scale. Moreover, there is a need for more research regarding the potential health effect of the bioactive compounds present in these waste products on humans and studies regarding their safe and effective dosage and the side effects of their overdose so that they can be employed as nutraceuticals. Even though the positive uses of these by-products are many, there is a need for the systematic implication of these by-products at a large scale, which needs proper communication between researchers and respective stakeholders.