Agricultural wastes are derived from plants and can be effectively converted into organic fertilizer or burned when favorable conditions exist. These wastes, along with harvested biomass, offer a renewable resource in the form of agricultural biomass. Biomass is abundant, energy-dense and can be transformed into new resources, providing renewable biological chemicals and energy with minimal negative environmental impact
(Pasin et al., 2020). The availability of biomass in nature promotes both environmental sustainability and economic development
(Srivastava et al., 2021; Niphadkar et al., 2018). However, utilizing agricultural waste poses a significant challenge
(Loeppert et al., 1996). The global industrial development and population growth have led to significant heavy metal pollution, disrupting natural ecosystems through their discharge into land and water
(Saravanan et al., 2021) (Fig 1). Converting agricultural waste into an energy resource is an effective way to eliminate it from the environment and create a source of renewable energy
(Xie et al., 2019). Agro-waste products such as sawdust, sugarcane bagasse, rice husk, neem bark, coconut husk and oil palm shell are utilized for removing heavy metals from industrial effluents in various industries that employ adsorption technologies Hala
(Ahmed et al., 2013).
Agro-waste products can also serve as binding agents in concrete cement. Various energy utilization technologies for agricultural waste include gasification, thermal cracking gas, liquefaction (enzymolysis, hydrolysis, water phase catalysis and Fischer–Tropsch synthesis), solidification (steam explosion pretreatment and biomass briquette), power generation (direct straw burning, biogas generation) and straw coal co-firing
(Lam et al., 2015; Chih-Chun et al., 2015). Heavy metals such as cadmium (Cd), copper (Cu), arsenic (As), chromium (Cr), zinc (Zn), lead (Pb), nickel (Ni), manganese (Mn) and mercury (Hg) are major pollutants in freshwater. These metals are toxic, non-biodegradable and persistent in nature
(Babarinde et al., 2006). Developing countries typically employ advanced technologies like vacuum evaporation, ion exchange resins, crystallization, membrane technologies and solvent extraction to remove heavy metal toxicity from wastewater
(Regel et al., 2010). Conventional methods for eliminating heavy metals include chemical precipitation, reverse osmosis, filtration and electrochemical treatments such as ion exchange, redox reactions, evaporation and adsorption. However, these methods are often ineffective and costly to operate
(Giraldo et al., 2008). Researchers are now focusing on developing low-cost and safe alternatives. Table 1 provides an overview of various conventional methods. Concrete, composed of aggregates, cement, reinforced steel bars and water, finds applications in construction, transportation, hydraulics and the military sector
(Yin et al., 2015).
Agricultural waste materials can serve as alternative aggregates in construction but typically do not contribute significantly to supplementary cementitious materials
(Liew et al., 2017). According to
Ismail et al., (1996), numerous studies have demonstrated that agricultural waste possesses a substantial quantity of amorphous silica, making it suitable for use as a partial replacement (10-30%) of cement in concrete. Aromatic compounds like phenols, which are toxic and commonly found in industries such as coal, petroleum refining and plastics, can be effectively removed from aqueous solutions using activated carbons derived from pine bark, apricot stone and rubber seed coat.
Rengaraj et al., (2001) stated that activated carbons derived from agro-wastes present a viable and economical approach for the removal of pollutants.
Conventional methods for removal of heavy metals
Adsorption
Adsorption is a recognized method for the elimination of heavy metals from wastewater. The adsorption process produce high-quality treated effluents. Compared to other wastewater treatment techniques, adsorption technology offers a greater advantage. By adsorbing the ions through the affinity filter, ion exchange processes are selective to the particular ions
(Weber et al., 1972). The adsorption phenomenon takes place between two interface such as solid-liquid, liquid-solid, solid-solid and liquid-liquid. Recent studies showed increased interest in the use of these biomaterials as adsorbents for adsorption of copper by Spent yeast
Apinthanapong et al., (2009), adsorption of Copper ions from aqueous solution onto iron oxide coated egg shell powder
(Areco et al., 2010).
Ion exchange
The ion exchange method is the most promising technology which eliminates important metal ions from the industrial wastewater. In the ion-exchange process, the ion exchange resins are used. This technology uses either synthetic resins or natural resins to exchange the specific cations in the wastewater. Mainly synthetic resins are used in the ion exchange method for the effective elimination of heavy metal ions from the solution
(Alyuz et al., 2009). The ion exchange method is a well-developed process, but to keep it efficient in removing the intended pollutants from wastewater, the resins that are employed need to be periodically renewed.
Chemical precipitation
Chemical precipitation is the most commonly employed technique used for the elimination of heavy metals from wastewater. It mainly involves adding a precipitating reagent to the polluted wastewater which converts the dissolved metals into solid particles. The particles from the wastewater get aggregated by chemical coagulation and are finally removed by filtration or sedimentation. Bivalent metals such as Cu (II), Cd (II), Mn (II) and Zn (II) can be removed by this chemical precipitation method. During this method, the pH is adjusted with alkaline reagents. Chemical precipitation is usually used to treat wastewater containing high concentrations of heavy metal ions and it is ineffective when metal ion concentrations are low or they settle down
(Alvarez-Ayuso et al., 2003).
Membrane filtration
Membrane filtration experiments were conducted to assess the rejection rates of aluminum, nickel and chromium at different operating pressures using various membrane types: ultrafiltration (UF) at 5 and 7.5 bar, nanofiltration (NF) at 10, 15 and 20 bar and reverse osmosis (RO) at 10 and 20 bar. These membrane filtration processes have shown promise in effectively removing metals due to their high removal efficiency, ease of operation and minimal space requirements
(Nuray et al., 2018). In addition to aluminum, nickel and chromium, membrane filtration is widely recognized as a leading method for the removal of other heavy metal ions such as cadmium (Cd
2+), lead (Pb
2+), copper (Cu
2+) and mercury (Hg
2+) from wastewater
Ercarikci et al., (2020).
Reverse osmosis
Reverse osmosis utilizes a semipermeable membrane and operates on the size exclusion and solution diffusion principles. The mechanism is by passing water through the polymer material membrane having netted structure
(Greenlee et al., 2009). Reverse osmosis is the reverse procedure of osmosis, where the solvent travels from a lower concentration to a higher concentration where no external pressure is applied.
Nanofiltration
Nanofiltration is the pressure-driven membrane separation process that is often used for separation of heavy metals (Monovalent and Divalent Ions). Reverse osmosis membranes have pores with a range of 1 to 10 nm, while nanofiltration membranes have slightly larger pores. Nanofiltration is a promising technology for elimination of heavy metals because of its low energy requirements, high efficiency and easy operation
Murthy et al., 2008).
Ultrafiltration
The ultrafiltration method is employed to increase the size of the species in order to separate heavy metal ions from wastewater. It is a low-pressure technique and the membrane’s pore span from 0.003-0.1 m in size
(Vishali et al., 2021). The separation of contaminants from sewage is accomplished by electrostatic and hydrophobic forces. When surfactants are added in excess during the ultrafiltration process, aggregates are formed so that membranes cannot pass through
(Samper et al., 2009).
Coagulation/Flocculation
The coagulation/flocculation comprises of two steps: the first step destabilizes the particles and the second step aggregates (flocculates) the destabilized neutral particles. The wastewater stabilized with coagulants like alum and colloids forms big flocks that can be easily removed by the sedimentation process
(Kurniawan et al., 2006).
Flotation
Flotation is the separation technique of liquid emulsions and suspensions detached from the dispersed phase. Dissolved air flotation, ion flotation and precipitation flotation are the three primary flotation techniques used to eliminate metal ions from wastewater
(Sudilovskiy et al., 2008).
Agricultural adsorbents for industrial effluents
Adsorbents for industrial and agricultural use those are effective at removing heavy metals from wastewater are listed below. Table 2 explains agricultural waste and its suitable extractable materials
(Parisa et al., 2018).
Rice husk
Rice husk is considered to be a major agricultural waste in rice-producing countries as the production is approximately 50 million metric tons. Dry rice husk contains 70-85% of organic matter (lignin, cellulose, sugars,
etc.) and silica, which is present in the cellular membrane of rice husk
(Vempati et al., 1995). The unmodified or modified rice husk acts as the best adsorbent for the removal of pollutants. The study revealed that the removal efficiency of Cd(II) and Zn(II) ions significantly improves with increasing pH beyond 2. Interestingly, at pH levels greater than 8.0, the uptake of these metal ions reaches 100%. These findings demonstrate the strong influence of pH on the sorption process, indicating that higher pH conditions favor the adsorption of Cd(II) and Zn(II) from the solution. The modified rice husk is potentially a useful material for the removal of copper and lead from aqueous solutions
(Wong et al., 2003). Table 3 summarizes the cadmium removal using modified rice husk.
Fly ash
Fly ash is a naturally occurring sedimentation material of coal produced from the combustion of coal in power plants (Fig 2). It is generally extracted by the precipitators in the smokestacks of coal-burning power plants to reduce pollution and fly ash has a capacity as a soil stabilizer and structural concrete admixture
(Marshall et al., 1999).
Sugarcane bagasse
Sugarcane bagasse is an agricultural waste composed of cellulose (50%), polyose (27%) and lignin (23%) (Fig 3)
(Hanafiah et al., 2008). Acinetobacter haemolyticus from sugarcane bagasse is typically employed as a bio-adsorbent to remove the chromium. This bacteria reduces chromium (Cr) to chromium (III), which is less poisonous and soluble than chromium (VI) and removal of more than 90% was achieved
(Ahmad et al., 2013). Using zinc chloride, chromium was extracted from the activated sugarcane bagasse and >87% chromium was removed at an ideal pH of 8.58
(Cronje et al., 2011).
Coconut husk
Coconut waste acts as an adsorbent for chromium removal and its sorption properties are due to the presence of coordinating functional groups such as hydroxyl and carboxyl
(Tan et al., 1993). Coconut coir pith was the best adsorbent for removal of heavy metals (Fig 4). The modified coir pith of coconut waste using the surfactant hexa decyl tri-methyl ammonium bromide was used for chromium removal. The maximum removal obtained with this material was reported as 76.3 mg/g (maximum adsorption capacity). The study showed that the coconut waste coir pith has higher adsorption removal than other agricultural adsorbents
(Sumathi et al., 2005).
Wheat bran
Wheat bran is a by-product of wheat obtained from the shell of flour mill (Fig 5) and is used for the removal of heavy metals
(Kaya et al., 2014). The functional groups such as methoxy, phenolic hydroxyl and carbonyl will help to bind heavy metals and have demonstrated the removal of chromium using wheat bran with a maximum adsorption capacity of 93 mg/g and a maximum removal of 89%
(Ravat et al., 2000).
Coffee residue
Coffee grounds act as an adsorbent to take cadmium out of industrial wastewater. Clay was combined with agricultural waste, such as coffee grounds for heavy metal removal. This creates an efficient adsorbent with a negative charge that encourages cadmium complexation and its removal
(Boonamnuayvitaya et al., 2004). Azouaou et al., (2010) stated that that cafeteria garbage is utilized as a bioadsorbent for the removal of cadmium, with a reported adsorption capacity of 15.65 mg/g, 80% of cadmium was found to be removal at a pH of 7. Fig 6 showsthe elimination of cadmium metal from industrial wastewater
(Arao et al., 2010).
Modified saw dust
Sawdust is a solid waste that is generated in significant amount at sawmills (Fig 7). Saw Dusts are used as an adsorbent material for the removal of heavy metals from wastewater
(Shukla et al., 2002). Saw dust of oak blended with hydrochloric acid is used for elimination of chromium from industrial effluents
(Argun et al., 2007). Pine sawdust as a modified form was used for removal of chromium at an optimum pH of 2 Politi and
(Sidiras et al., 2012).
