In drinking water, high concentrations of fluoride and arsenic can have adverse effects on human health. Waste deriving from the rice industry (rice husk, rice straw, rice bran) can be promising adsorbent materials, because they are (i) produced in large quantities in many parts of the world, (ii) recoverable in a circular economy perspective, (iii) at low cost if compared to expensive conventional activated carbon, and (iv) easily manageable even in developing countries. For the removal of fluoride, rice husk and rice straw allowed to obtain adsorption capacities in the range of 7.9–15.2 mg/g. Using rice husk for arsenic adsorption, excellent results were achieved with adsorption capacities above 19 mg/g. The best results both for fluorides and arsenic (>50 mg/g) were found with metal- or chemical-modified rice straw and rice husk.
1. Rice Industry By-Products
1.1. Rice Industry Value Chain
Agricultural food processing consists of a variety of value chains and generates a significant amount of different types of agricultural waste through their value chain from the way of farm to fork process. According to the previous literature regarding by-products, the usable product ratios of rice straw, rice husk, and rice bran are 0.4–1.4 kg/kg
[1], 0.28 kg/kg
[1], and 0.1 kg/kg
[2] respectively. Rice straw mainly consists of full part of the rice plant, therefore, producing 1 kg of rice may require more rice plants.
A large quantity of valuable wastes is produced during their harvesting and processing stage and hence, those wastes should be studied and analyzed in order to extract their valuable parts in a sustainable way. Paddy straws are produced in large quantities during the paddy harvesting process. These straws are commonly found in fields and are often used as fodder for animals and as bedding for livestock. The remaining part is burned by the farmers when ready for the next cultivation. Additionally, rice husks frequently ended up as a burning material and were discarded in landfills. The destruction of this valuable biomass without proper use will cause irreversible damage to the environment and other living beings on the planet. Bran is also another by-product of the rice processing value chain. After harvesting and processing in mills, residues of rice have a higher potential to convert into another raw material in other products
[3]. Non-harvest rice, half fill grains, dead grains, damaged paddy seeds during the harvesting process, and rotten rice in domestic consumption were not considered for this research.
1.2. Properties of Rice Husk and Rice Husk Ash
Rice husk is generated as a by-product of the rice milling process and it is the outer cover of the rice grain. It is also known as hull and chaff
[4]. This is the most common agricultural by-product in agrarian countries. In general, rice husk is utilized as source biomass fuel in rice mills, silica-rich cementitious material, and poultry farming
[3][5][6]. In addition, a minor amount is used as fertilizer and building material
[7]. Due to its low economic value, the availability in huge quantities, higher chemical stability, mechanical strength, and insolubility in water, rice husk is considered as a sustainable absorbent material
[8][9][10][11]. However, higher quantities of rice husks are frequently terminated in landfills or open burning, creating huge environmental pollution.
Adsorption on Rice Husk
According to the works of literature, rice husk adsorbent has been used in different ways for the removal of various types of pollutants such as inorganic anions, heavy metals, phenol, and other organic compounds, pesticides, and dyes from the water and wastewater. Many investigations have been conducted through different treatment methods, such as untreated rice husk as adsorbents, chemical and biological modification of husk, biochar, activated carbons, and ashes. The use of these different treatments for rice husks shows that many works have reported acceptable results from their research with adsorption capacity more than 10 mg/g. However, comparison among different treatment methods is relatively difficult due to different experimental conditions.
1.3. Properties of Rice Straws
Rice straw is the vegetative part of the rice plant (
Oryza sativa L.) and it is generated after cut at grain harvest. It consists of the stem of the paddy plant, leaves, and spikelet. Typically, rice straw is used for animal feed, composting process, cattle house bedding, the paper industry, to make handmade items, as an energy source in some industries, and covering material for agricultural fields
[12]. In addition, the remaining part of the rice straw is burned in the paddy fields before the next ploughing process to prepare the field.
Adsorption on Rice Straw
Rice straw is rich in cellulose, hemicellulose, silica, and lignin, which provide binding sites for various contaminants like heavy metals and dyes, in water and wastewater
[13][14]. Therefore, straw has a large potential for use as a low-cost adsorbent material in water treatment. Gebrewold et al.
[15] have emphasized that the surface of this type of biomass adsorbent is activated at lower pH values for removal of heavy metals. Furthermore, Cr adsorption process is more favorable in strong acid conditions
[13]. Through the metallic modifications, some more hydroxyl groups can be produced on the rice straw biochar that means more active sites for adsorption
[16]. Moreover, Cao et al.
[17] demonstrated that the adsorption capacity is increased by increasing the number of quaternary ammonium groups. Additionally, the adsorption capacity of the rice straw is decreased by the existence of competitive cation in the solution and chelators
[14].
1.4. Properties of Rice Bran/Rice Polish
Rice bran is one of the most significant wastes generated during the rice processing value chain. Due to its high nutritional value, it is mainly used as an animal feed and is considered as a good fiber source for pet food. Additionally, because of its availability, its price is considerably economical for farmers. Up to 40% of rice bran is added into the dietary plans of cows, dogs, pigs, and poultry due to its higher amount of fat and fiber
[18][19]. Furthermore, rice bran contains 14–18% oil, and therefore, it is a valuable feed for all types of livestock. According to Patsios et al.
[3], defatted rice bran can be used at higher levels of the valorization process than ordinary rice bran. The physical and chemical characteristics of rice bran depend on several factors related to the grain itself and to the milling process. The main factors related to rice grain are rice variety, environmental conditions, size and shape of the grains, distribution, chemical constituents, thickness of outer layers, and resistance to breakage
[20]. Additionally, type of milling machine and conditions of processing are the key factors related to the milling process. Resurrection et al.
[21] have emphasized that different layers of rice kernel at various depths show different chemical compositions.
Adsorption on Rice Bran
Rice bran, due to its granular structure, chemical stability, insolubility in water, and local availability, can be used as an adsorbent material. Its surface contains several active sites
[22] for pollutants’ removal, whose efficiency depends also on the chemical nature of the solution and the presence of other ions than the one/ones to be trapped. The amount of negatively-charged groups on the rice bran matrix increases at lower pH values (around 4.0–6.0). This effect favors the electrostatic interaction between the surface itself and cations leading to metal ions’ binding. Moreover, the adsorption efficiency of rice bran is attributed to some functional groups on the surface such as hydroxyl groups and carbonyl groups
[23].
2. Fluoride Removal
Fluorine accounts for around 0.08% mass of the earth’s crust and is broadly distributed in numerous geological environments. Since fluorine has strong electronegativity, fluorine naturally exists as different forms of compounds such as sellaite, cryolite, fluorite, fluorapatite, etc.
[24][25]. Fluorine is considered as an essential trace element for the human body: the proper amount of fluorine can support maintaining healthy bones and teeth, but an extreme amount of fluorine leads to loss of the toughness of bones and teeth, and makes them brittle
[26]. Fluoride can enter the human body in different ways such as drinking water, foods and beverages, drugs, dermal contact, and air inhalation. However, drinking water is the main source of fluoride to enter the human body
[27], therefore, the concentration of fluoride in this solvent has become a significant parameter for determining water quality. The value of the fluoride concentration determines whether the anion causes beneficial effects or harmful impacts on the human body. Hence, the concentration of fluoride in drinking water is set as 1.5 mg/L by the World Health Organization (WHO)
[28].
The dissolution of fluoride compounds causes the rise of fluoride concentration in groundwater. Literature reports
[24][25][26] have highlighted that there are two main reasons for this phenomenon:
- i.Environmental Factors: weathering, dissolution of fluoride-containing minerals by rainwater, and continuous evaporation of groundwater can affect a gradual increase of fluoride concentration.
- ii.Anthropogenic activities: mining, various industries like glass and coke production, semiconductors, metal smelting, electroplating, photovoltaic activities, etc., release high fluoride concentrated wastewater which is mixed with surface water resulting in fluoride entering groundwater.
2.1. Rice Husk and Rice Husk Ash as Adsorbents for the Removal of Fluoride
Tanvir Arfin
[29] studied the efficiency of fluoride removal by using different adsorbent materials such as rice husk, eggshell, activated carbon, and rice husk ash, showing that the efficiency of fluoride removal was 40% by rice husk while the efficiencies for the other matrices were 61.8, 53.4, and 42.5% respectively. A wide range of treatment methods have been proposed and investigated in literature to improve the adsorption efficiency
[30][31]. Deshmukh et al.
[32] prepared rice husk by chemical impregnation with nitric acid, followed by physical activation to investigate the removal of fluoride in aqueous solution. They reported that the maximum fluoride removal was 75% at pH = 2, with the fluoride uptaking capacity decreasing by increasing pH in the range 2–10. This finding was confirmed by Ahmaruzzaman and Gupta
[33]. Therefore, researchers emphasized that it is preferable to carry out the fluoride adsorption process at lower pH values. Moreover, fluoride adsorption increases with time, reaching equilibrium within 1–1.5 h. Some researchers
[33][34] investigated the removal of fluoride by modifying rice husk ash by coating with aluminum hydroxide. The research showed promising results of fluoride removal capacity, and the adsorption was determined to be 910 mg/g. Other researchers investigated the fluoride removal capacity by using silica nano adsorbent modified by rice husk
[35]. The results illustrated that the fluoride adsorption capacity is 12 mg/g. By way of comparison with rice husk, according to the experiments of Tembhurkar and Dongre
[36], commercially available powdered activated carbon (2.0 g/100 mL) allowed a 94% removal of fluorides under optimal conditions (contact time: 120 min, pH: 2). As reported by Tomar and Kumar
[37], some coal-based adsorbents (lignite, fine coke, and bituminous coal) showed an adsorption capacity in the range of 6.9–7.4 mg/g for the removal of fluoride. A pH between 6 and 12 was more favorable in the case of lignite, while fine coke and bituminous coal showed a greater efficiency at acid pH.
2.2. Rice Straw as Adsorbent for Removal of Fluoride
Rice straw can be utilized as an adsorbent material by subjecting it to different physical and chemical pre-treatments. Daifullah et al.
[38] have investigated the potential of activated rice straw for the fluoride removal process from water. The researchers of the same research concluded that activated rice straw can produce low-density, highly porous materials. Strong oxidants such as hydrogen peroxide, nitric acid, and potassium permanganate were used for treating the activated rice straw carbon. The results showed significant improvement and higher efficiency values compared to similar activated carbon not treated with strong oxidants. Potassium permanganate showed the highest effect, while nitric acid and hydrogen peroxide had comparably less effect.
3. Removal of Arsenic
The rapid increase of arsenic concentration has been discovered in drinking water sources all around the world and it threatens more than 200 million people in both developed and developing countries
[39]. Arsenic has the ability to form compounds in both inorganic and organic phases, therefore, it is distributed in a wide range of environmental sectors
[39][40][41]. In both groundwater and surface waters, arsenic can occur in the form of two inorganic species, arsenate (As(V)) and arsenite (As(III)). The latter is more mobile, toxic, and difficult to remove with the conventionally applied physicochemical treatments than As(V)
[42][43].
Arsenic is grossly available in the human body, earth’s crust, and seawater with the sequential proportion of 12th, 14th, and 20th, respectively, of all the existing elements
[40]. Arsenic is mixed with the natural water system through several leaching processes such as mining waste, weathering of lithosphere deposits, and leaching from geochemically and biologically generated sources. Shih
[44] has reported that the range of arsenic concentration is 3–10 mg/kg and 0.5–2.4 mg/kg in sediment and rocks, respectively, which is reportedly less than soils. Naturally, arsenic consists of more than 300 different mineral forms.
Arsenic is one of the major environmental pollutants and excessive exposure to arsenic can have serious consequences to the human body, especially threatening carcinogenic effects. Therefore, WHO and the United States Environmental Protection Agency (USEPA) have classified arsenic as a Class A carcinogenic pollutant. Furthermore, the WHO and USEPA have defined the limitation of arsenic concentration in safe drinking water and discharge wastewater as 10 μg/L and 0.2 mg/L, respectively
[45][46].
3.1. Rice Husk as Adsorbent for Removal of Arsenic
Many literature papers have shown that rice husk, rice straw, and rice bran (polished rice) can be used as adsorbent material in the arsenic removal process. Babazad et al.
[47] have investigated the efficiency of removal of arsenic from aqueous solution by porous carbon material obtained from the rice husk. The research was carried out in different conditions of pH, adsorbent amount, contact time, and initially adsorbed concentration. According to the adsorption analysis results, the removal of arsenic increased up to 85% under the optimum conditions of pH 6.
To compare, commercial granular activated carbon currently used at drinking water treatment plants allowed As(V) removals between 7 and 61% in batch experiments (carbon dose: 5 g/L, pH: 4, contact time: 24 h). While various types of powdered activated carbon showed a range of 5–84% (carbon dose: 1 g/L, pH: 4, contact time: 24 h)
[48]. The removal of As(III), in a solution with an initial concentration of 10 mg/L and 2 g/L of activated carbon, reached the maximum value of 32.8% in batch experiments carried out at approximately pH 7.0 and at a temperature of 30 °C
[49]. Another experiment with 24 h of contact time reported an arsenic loading with coal-based carbon (surface area: 1125 m
2/g, 5.5% ash content) of 4.09 mg/g of carbon
[50].
3.2. Rice Straw and Rice Bran as Adsorbents for Removal of Arsenic
The efficiency of As(V) removal from aqueous solution was investigated using iron- modified biochar derived from rice straw and synthesized using FeCl
3 as iron source
[46]. The researchers compared the arsenic removal efficiency of raw biochar and iron-modified biochar. The research revealed that the ability of arsenic removal is better for iron-modified biochar with respect to raw biochar at the initial pH of 5.0. Ranjan et al.
[51] carried out an experiment to study the optimal parameters for arsenic removal from water by rice polish using a fixed bed columns system. The investigation was directed to analyze different design parameters like flow rate, bed height, initial concentration and their effect on the adsorption process, and revealed that the As(III) and As(V) uptake capacities are 66.95 and 78.95 μg/g, respectively. Another study was carried out by Mukherjee et al.
[52] in order to remove As(V) from aqueous medium by using rice straw-derived biochar produced from slow pyrolysis process at 600 °C. Batch tests were conducted at room temperature under different conditions, such as initial concentration, contact time, and adsorbent dosages. The study highlighted an adsorption capacity of 25.6 μg/g for As(V) by rice straw biochar.
This entry is adapted from the peer-reviewed paper 10.3390/app12063166