## 7 Conclusions  
The global problem of removing excess fluoride and arsenic from drinking water was the subject of this review, in which particular emphasis was placed on low-cost solutions applicable in both developed and developing countries. The removal of fluorides and arsenic by adsorption appears to be one of the potentially most suitable technologies, especially by virtue of the possibility of exploiting recovered and readily available adsorbent material, such as by-products deriving from the rice industry (e.g., rice husk, rice straw, rice bran).  
From the studies of the scientific literature, it emerged that the adsorption capacity, with the same adsorbent material, is strongly dependent on the operating conditions (e.g., pH, temperature, contact time, dose of adsorbent material, initial concentration of pollutant) applied in the experiments. For the removal of fluoride, excluding an experiment performed with rice straw biochar loaded with La/Fe/Al oxides (adsorption capacity of about 111 mg/g), comparable adsorption capacities (in the range 10-16 mg/g) were obtained with both the use of (i) unmodified rice straw biochar and (ii) alumina/manganese impregnated on activated carbon derived from rice straw. Moreover, (i) the unmodified rice husk, (ii) rice husk treated with NaOH or Al(OH)\({}_{3}\), and (iii) silica nano adsorbent modified from rice husk allowed to obtain adsorption capacities generally between 8 and 15 mg/g. The highest adsorption capacity, equal to 55 mg/g, was obtained from the application of an adsorbent material synthesized by silica nano derived from rice husk doped on calcium peroxide.  
Generally, for the removal of arsenic, the lowest adsorption capacities were obtained with rice straw and rice bran (in the range 25.6-79.0 \(\upmu\)g/g), while the highest was with iron oxide amended with rice husk nanoparticles (up to 82 mg/g), both cases with synthetic water. On the other hand, using unmodified rice husk biochar, excellent results were obtained (adsorption capacity up to 19.3 mg/g).  
Although the rice industry by-products represent a promising adsorbent material, they should be studied in more detail as several aspects need to be better clarified, such as (i) the application to contaminated groundwater to study the complexity of multiple coexisting pollutants, (ii) experimental test at pilot scale in continuous conditions for a significant time, (iii) the improvement of some characteristics of the biochar such as surface area and porous structure with specific treatments (among others, metal coupling and chemical modifications), (iv) cost analysis on the effective full-scale applicability of biochar modifications treatments, and (v) the stability of the biochar during water treatment and its possible regeneration or recovery after use. Greater clarity on these aspects can lead to the identification of positive environmental/economic trade-offs, which can make real-scale implementation of these by-products as adsorbents in the drinking water industry feasible.  
Conceptualization, M.C.C., S.S., W.A.M.A.N.I and F.M.C.; methodology, M.C.C., S.S., W.A.M.A.N.I and C.M.; validation, M.C.C., S.S. and C.M.; formal analysis, C.M., W.A.M.A.N.I, F.M.C. and S.C.; investigation, W.A.M.A.N.I and F.M.C.; data curation, C.M., W.A.M.A.N.I and F.M.C.; writing--original draft preparation, C.M., W.A.M.A.N.I. and F.M.C.; writing--review and editing, C.M., W.A.M.A.N.I. F.M.C. and S.C.; visualization, W.A.M.A.N.I. and F.M.C.; supervision, M.C.C., S.S. and C.M. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
The authors thanks Azienda Agricola Cascina Alberona (Mortara, Pavia, Italy), Azienda Agricola Cassi Cugini (Sant'Angelo Lomellina, Pavia, Italy), and Azienda Agricola DICRISTIANA (Robbio, Pavia, Italy) for supplying rice wastes for the IR analyses and for further investigations linked to circular economy and environmental topics.  
The authors declare no conflict of interest.