Rice Industry By-Products as Adsorbent Materials for Removing Fluoride and Arsenic from Drinking Water—A Review

Abstract

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. Identifying the next steps of future research to ensure the upscaling of biochar from recovered by-products, it is fundamental to perform: (i) tests on real waters for multicomponent adsorption; (ii) experiments with pilot plants in continuous operation; (iii) cost analysis/real applicability of modification treatments such as metal coupling or chemical synthesis; (iv) more studies on the biochar stability and on its regeneration or recovery after use.

Value Chain Analysis of Rice Industry by Products in a Circular Economy Context: A Review

“…How well these sites work depends on the chemical nature of the solution and whether or not there are other ions in the solution besides the ones to be trapped. Additionally, various functional groups on the surface of rice bran, such as hydroxyl and carbonyl groups, are responsible for its high adsorption effectiveness [39]. The existence of these groups is supported by the ATR-FTIR spectrum shown in Figure 8, which exhibits rice-straw-like peaks.…”

“…Pollards are often a blend of polishings and bran. However, all these by-products are generated during the rice milling process, and their amount is roughly 60% rice husk, 35% rice bran, and 5% polishing from the whole rice mill waste stream [16,[34][35][36][37][38][39]. Total rice consumption worldwide from 2008/2009 to 2021/2022 (in 1000 metric tons) is shown in Figure 5.…”

“…Rice husk is the outer covering of the rice grain and is produced as a by-product of the rice milling process. It is also called hull and chaff [39,76]. In agricultural nations, this is the most prevalent agricultural by-product.…”

“…Rice bran can be used as an adsorbent for polluting substances because it has a granular shape, is chemically stable, does not dissolve in water, and is easy to get. Its surface has several active sites that can remove pollutants [39]. How well these sites work depends on the chemical nature of the solution and whether or not there are other ions in the solution besides the ones to be trapped.…”

Preparation and Modification of Biochar Derived from Agricultural Waste for Metal Adsorption from Urban Wastewater

“…Biochar is a carbon material produced by the thermal decomposition of biomass in an oxygen-free atmosphere [27][28][29][30][31][32][33]. It is a highly aromatic compound, made up of layers of graphene and graphite structures and edge carbon atoms containing functional groups, such as carboxylic, hydroxyl, and carbonyl units [33][34][35].…”