Waste management was not a priority concern for scientists and engineers for decades. Waste in many industrial and agricultural fields was simply burned or disposed to the land or water with little or no treatment. However, the global warming problem and huge amounts of waste made such options not only harmful to our environment but also to our economy in the long term. The palm industry, including oil palm and date palm industries, still lack effective waste management. Several studies on the applications and products of palm waste were published recently, but the proper industrial usage of such application is still limited. Four manufacturing applications of date palm waste will be discussed in this section from the perspective of engineers, to produce manufacture engineering products, along with discussion of the required treatments, specifications, manufacturing methods, and the final product specifications.
4.1. Composites from Date Palm Fibers
Natural fiber composites (NFC) are composite materials that are mostly polymer based and use natural fiber (or might be called vegetable fiber) as reinforcement (filler). Such fibers have been extracted from sisal, coir, jute, ramie, pineapple leaf (PALF), kenaf, and other natural sources. However, researchers recently started studying the ability to use biomass waste as source of natural fiber. Wastes such as rice husks, wheat-cereal straw, tobacco, sunflower stalks, bagasse, oil palm, bamboo, cotton carpel, and date palm fibers have been mention in the Mehdi et al. review [
37]. Natural fiber has attracted a lot of attention lately due to its superior advantages over conventional reinforcement materials such as fiber glass. Natural fibers are considerably low-cost and are more available compared to conventional fibers with low density; they have good mechanical properties [
38] and very good thermal and acoustic insulation. Natural fibers are also biodegradable and nonabrasive. Unfortunately, natural fibers have adhesion problems with hydrophobic polymer matrix [
38], wide variation in mechanical properties [
38], poor moisture resistance [
38], low degradation temperatures [
39], and processing complexity, which are major problems facing the employment of natural fibers in industrial investments. Researchers in the Middle East, mostly in the Gulf countries, focused their investigation on date palm fibers due to the large number of date palm trees, making its fiber one of the most available natural fibers and the most important one in the Middle East.
To prove the ability of date palm fibers as a great reinforcement, numerous studies on date palm NFC have been reviewed. The following products have been prepared and tested using laboratory equipment, making industrial-level production a problem of time.
- (1)
Polypropylene composites
Haque et al. investigated the mechanical properties of palm fiber as reinforcement for a thermoplastic polymer polypropylene matrix. They used full-length fibers treated chemically with benzene diazonium salt to improve surface adhesion with the matrix. For comparison, untreated raw fibers were used. The result showed a dramatic decrease in tensile strength but, on the other hand, great improvement in the flexural strength and impact strength up to 30%wt fiber. Additional fiber weight in the matrix had no effect in all their tests. Chemical treatment of date palm fibers greatly improved the mechanical properties, compared to untreated fibers, due to the better adhesion with the matrix and the lower wettability [
31].
- (2)
Epoxy composites
Saba et al. investigated the mechanical properties of date palm fiber in an epoxy matrix. They grinded the fiber into 0.8–1 mm powder using a laboratory grinding machine after cleaning and drying the fiber in the oven. The use of date palm fiber as reinforcement greatly enhanced the tensile and impact strength in addition to the plasticity of the composite. They found that the optimum weight percentage for the reinforcement was 50%wt; however, their SEM images (shown in
Figure 6) of the tensile fracture showed the presence of air bubbles in the epoxy matrix, which affects the mechanical properties, unlike the pure epoxy resin where air bubbles were not present [
40]. Finding a better manufacturing process for such a composite is suggested to obtain better mechanical properties.
- (3)
Oriented strand board from date palm
Hegazy et al. [
20] successfully manufactured oriented strand board (OSB), one of the most used engineering wood-based panels; they used date palm fronds collected from Al-Kharj. They made OSB with good mechanical properties and dimensional stability. However, better results could be obtained by improving the adhesion and compression processes [
20]. Such application has great economical potential due to the processing and manufacturing simplicity and the high demand for OSB.
- (4)
Medium-Density Fiberboard (MDF)
Hosseinkhani et al. [
41] successfully manufactured MDF board using date palm fronds that have great mechanical properties. To test the factors that affect the MDF quality, they manufactured the MDF board using 2 resin types, 3 pressing levels, and 10 and 12 %wt resin (total of 15 specimens). They proved that the MDF made of date palm fronds had mechanical properties and wettability resistance that exceeded the requirements of EN standard for MDF [
41].
- (5)
Particle board from date palm
Hegazy et al. [
16] in Saudi Arabia, and Amirou et al. [
26] in Algeria, studied the feasibility of using date palm trunk and rachis as raw materials for particleboard manufacturing. Amirou et al. proved that both of them are superior materials for such applications compared to EN standards for particleboard [
26]. Hegazy et al. [
16] used date palm rachis to study the effect of various factors in particleboard manufacturing, such as date palm cultivar, particle size, hot water extraction, and panel density, on the mechanical properties of the particleboard [
16]. Ferrández-García et al. also used palm rachis to manufacture low-cost particleboard, but they compared between different sources of rachis which were date, canary, and Washingtonian palm [
42]. Researchers who studied the manufacturing of particleboard using date palm waste have reached various results, and the right choice of the manufacturing parameters depends on the intended application of the particleboard and the standards assigned to that application.
- (6)
Gypsum board reinforcement
Gypsum board is usually used as a thermal or acoustic insulator because gypsum board has good insulation. Aymen et al. [
43] studied the thermophysical effect of reinforcing gypsum board with date palm fibers. They proved that date palm fiber reinforcement greatly decreased the thermal conductivity and the density of the gypsum board, leading to better insulation with less weight [
43]. Such improvement happened without sacrificing the mechanical properties. In a real-world setting, according to Al-Rifaie et al. [
44], date palm fiber reinforcement improved the mechanical properties and modified the fracture mode of the gypsum plaster from brittle type to quasi-plastic type [
44].
4.3. Active Carbon
Activated carbon, also known as activated char or solid sponge, is an amorphous, non-graphite, carbonaceous compound and is a product of activating carbonaceous material with a carbon content of 72–90% [
21]. Active carbon can be manufactured by various precursors with no specific chemical composition or formula. However, active carbon can be defined as “a material prepared to exhibit a high degree of porosity and an extended inter-particulate surface area” [
21]. The main properties of active carbon are high surface area, well-developed and tunable pores, very reactive surface characteristics, good absorptive capacity, and dependable physiochemical stability [
18]. Final active carbon properties strongly depend on the precursor properties, pretreatment of the precursor, and the activation process. The production factor determination depends on the application of the active carbon and its required properties. Active carbon can be manufactured almost from any organic material with sufficient carbon content, making date palm waste a suitable precursor due to the versatility of the waste and its chemical properties.
4.3.1. Active Carbon Production from Date Palm Waste
To produce active carbon, the precursor needs to be carbonized (turn into char) then activated. To improve the active carbon quality and avoid property variations, pretreatment processes are required.
Pretreatment
Suggested date palm pretreatment steps are (a) isolation, (b) washing, (c) drying, and (d) sizing.
- (a)
Isolation
Date palm wastes usually are not separated during extraction. Leaflets and rachis need to be separated due to the significant differences in their properties. Date palm fruits are not suitable themselves as a precursor for active carbon; only seeds have been used by researchers. However, date seeds are the main waste of date processing industries.
- (b)
washing:
Date palm waste is usually left on the soil after extraction, causing leaching and absorbing soil minerals and other chemicals, which affects the calorific value of the waste. Washing needs to be done in two phases: washing and soaking with water to remove soil, dust, nylon, ash particles, and any particles that are not chemically pounded to the waste. The second phase is washing with a weak acid that dissolves any chemically bounded pollution and undesirable particles such as ash. Acid washing also helps to dissolve some of the extractive as violate fat. Ash particles significantly affect the active carbon quality, which is why washing is important to ensure efficient active carbon. Abdullah and Sulaiman [
47] studied the effect of washing on ash removal, and their work results are in
Table 8. Washing procedures can be much easier and cheaper by properly harvesting the date palm residue as a raw material, not as waste. This is done by collecting fronds in the warehouse directly instead of leaving them on the soil under climate conditions.
- (c)
Drying:
The aim of the drying process is to remove moisture and extractive content, which can be done by the sun or oven. Puligundla et al. [
23] studied the possibility of using microwaves to dry lignocellulosic biomass, and they recommended oven drying due to its relatively better controllability, easy operation, reliable heating efficiency, and speed [
24].
- (d)
Sizing:
The size of the precursor directly affects the final size of active carbon particles. Sizing is done by first crushing the precursor and then sieving it to avoid particle size variability.
4.3.2. Activation Methods
There are two main activation methods: (1) physical activation and (2) chemical activation. Each method directly affects the properties of active carbon.
- (1)
Physical activation:
Also known as “thermal activation”, this is a double stage pyrolytic process involving carbonization and activation [
18]. The first stage, carbonization, is similar to char production; it aims to convert the precursor into carbon while removing any water content, organic acid, and volatile compounds that were not removed by the pretreatment. In addition, carbonization initiates surface area and pores. The second stage is activation, where the carbonized char is reacted with steam, gas, or a mixture of steam and gas to optimize the initiated surface area in the first stage and create new pores. Most researchers suggest the use of CO
2 as activation gas [
18].
- (2)
chemical activation:
Also known as “wet oxidation”, this is a single-stage activation process that uses a chemical additive to carbonize and activate the precursor at low temperatures, compared to the physical activation. Chemical activation consumes less time and energy compared to physical activation [
18].
4.3.3. Applications of Date Palm Active Carbon
Active carbon has versatile applications. It can be used as gas adsorption, water treatment, gas mask, food decolonization and deodorization, and electrode material of super capacitors. Kyaw et al. [
23] successfully manufactured active carbon from date palm leaflets using chemical activation to produce effective active carbon to be used as multifunctional electrodes in capacitive deionization system to be used in water desalination. Their system was able to remove salt ions and degradation and remove dye molecules [
22]. Haimour and Emeish successfully manufactured active carbon from date seeds by physical activation, but the active carbon could not meet ASTM standards [
21]. Muhammad Vohra was able to treat gas emissions with date pits, chemically activated with active carbon, to treat ammonia-polluted gases [
48].
4.5. Other Potential Applications
The applications discussed previously are not the only applications of palm waste. There are other unconventional applications researchers have studied. Some examples are as follows:
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Porous nano-sheet simultaneous determination of dopamine and uric acid:
Ahammad et al. [
61] were able to detect dopamine and uric acid levels in the human body even in the presence of interfering species. Such detection is important to diagnose some neurological disorders such as Parkinson’s and Alzheimer’s.
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Sound absorption made of date palm fiber:
Due to the hollow structure of date palm fiber, it poses good acoustic insulation ability. Taban et al. [
62] used date palm fiber to manufacture acoustic insulation panels that can be used to insulate buildings. Their approach offers efficient, low-cost, degradable acoustic insulation panels compared to conventional panels made of synthetic fibers.
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Production of fructose sweetener:
Putra et al. [
63] extracted fructose sweetener from date fruit waste using elective fermentation. Due to its high sweetness and solubility, fructose is an important sweetener in beverages and the food industry. Utilizing date fruit waste to extract fructose is an economical approach because it is waste in the first place.
- -
Production of glucose and lactic acid:
Similar to Putra et al., Azam and Ahmad [
64] suggest using date palm waste to produce glucose and lactic acid by transforming cellulose, that date palm waste is full of, to glucose or lactic acid by the help of microorganisms and bacteria.
- -
Biofuel source:
Due to the finite amount of petroleum and the pollution when producing or using fuel extracted from it, biofuel is a solution researchers and engineers have offered as replacement for petrol [
65]. Lattieff [
4] was able to produce biogas that can be used as fuel from date palm fruit waste using anaerobic digestion. He proved the suitability of date fruit waste as a biogas source.
Galiwango et al. [
66] used direct catalytic depolymerization of lignocellulose from date palm waste to produce liquid fuel. Kamil et al. [
67] used date fruit seeds to produce biodiesel. They were able to produce DSO biodiesel using a Model B-811 Extraction System (BUCHI). After extraction, they prepared blends of biodiesel and petrol diesel, and they tested it using a single-cylinder CI diesel engine. Results showed that their blend produced lower levels of CO
2, CO, and HC but higher levels of NO
x emissions. Martis et al. [
68] studied the economic feasibility of date palm waste as biofuel source in UAE by simulating three production methods: pyrolysis, gasification, and fermentation. Their study showed that pyrolysis and gasification are economically more feasible than fermentation.
Table 9 summarizes some applications of date palm waste and their main products.