###### 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. low-cost defluorination; groundwater adsorption; geogenic pollutants; rice waste; rice husk; rice straw +
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than 25 countries, around 200 million people are exposed to drinking water with fluoride concentrations above the World Health Organization (WHO) guideline value of 1500 \(\upmu\)g/L. This situation is experienced in India, China, Korea, Thailand, Sri Lanka, Indonesia, Yemen, Pakistan, Iraq, Turkey, Syria, Jordan, Palestine, Bangladesh, Iran, Saudi Arabia, and in Mexico and Argentina [6; 7; 8; 9; 10]. A recent study used an algorithm to model more than 12,000 groundwater fluoride concentration data from across India. The results predicted that the total number of people at risk of fluorosis from fluoride in groundwater is around 120 million, or 9% of the population [8]. Moreover, not all chemical elements perform essential functions; on the contrary, some of them (including arsenic, cadmium, lead, mercury) do not seem to provide any beneficial contribution, or can lead even to serious and irreversible damage [2]. Arsenic, in particular, is a ubiquitous toxic element that can cause different cancerous diseases, skin pigmentation problems and cardiovascular disorders [5; 11; 12]. More than 300 million people in 105 countries have sources of drinking water with arsenic concentrations above the WHO limit of 10 \(\upmu\)g/L available. The highest exposures were found in India (West Bengal), Bangladesh, Nepal, Pakistan, China, Taiwan (China), Japan, Cambodia, Vietnam, Australia, Hungary, and Romania, and in the arid and semi-arid areas of fourteen Latin American countries [3; 5; 12; 13]. For the first time in Taiwan (China), As(V) and As(III) species have been studied in Taiwanese spring waters, which have contaminated drinking and irrigation water resources. The highest concentration of As(III), more than 1400 mg/L, was found in the geothermal springs of Beitou [14]. These pollutants, due to their natural presence in the environment, can accumulate in an uncontrolled way in groundwater. However, there are also some human activities, such as mining and the use of phosphate pesticides, which can contribute to the contamination of ground and surface waters by arsenic and fluorine respectively [6; 7; 13]. Therefore, it is significantly important to either decrease or remove some amount of these toxic compounds from drinking water. Conventional treatment techniques such as chemical precipitation, biological treatments, oxidation, and reduction using chemicals, electro dialysis, ion exchangers, reverse osmosis, ultra-filtration, membrane separation, and adsorption processes have been studied and are currently utilized for the treatment of different environmental pollutants [15; 16; 17; 18].