Remediation of soils contaminated with fluoride using seaweed-derived materials: case of slopes of mount Meru

Abstract

While exposure to low fluoride is essential for stronger bones and teeth, exposure to high concentration (> 3 mg/L/day) leads to hyperostosis and osteoporosis. This research evaluated the role of fertilizer application on soil’s fluoride release, and assess the effectiveness of using seaweed (Eucheuma cottonnii) derived materials for remedial purposes. The soil characterization results in the study area, indicated the availability of diverse fluoride fractions and in different quantities in the soil such that; water-soluble (Ws-F) (39.5 ± 0.5 mg/kg), Exchangeable (Ex-F) (3.5 ± 0.5 mg/kg), bound to iron/manganese (Fe/Mn-F) (3.1 ± 1.0 mg/kg), and organic matter bound (Or-F) (9.1 ± 2.1 mg/kg) fluoride whereas the total fluoride (Tot-F) was 422 ± 52.9 mg/kg. The study further reports that the use of three studied fertilizers (diammonium phosphate (DAP), Urea, and farmyard manure) accelerates the bioavailability of fluoride in the soil by increasing Ws-F. These results deliver alerts to the plant health regulators suggesting proper management of the quality of fertilizers used for the enhancement of crop quality particularly those used in fluoride-contaminated agricultural soils. While fertilizer application accelerated the bioavailability of fluoride in the soil, soil amendment with dried seaweed (DSW) led to a decrement of Ws-F from 81.7 ± 3.1 mg/kg up to 28.5 mg/kg whereas the fermented seaweed (FSW) decreased Ws-F from 81.7 ± 3.1 mg/kg to 12 ± 1.3 mg/kg following 5 % (w/w) amendments. But unlike DSW and FSW, seaweed-derived biochar (SB) adsorbed fluoride at specific pH five (5) from 103.1 mg/kg to 91.2 ± 3.2 mg/kg whereas hydroxyapatite activated seaweed-biochar (HSB) exhibited defluoridation capabilities at varies pH (3 – 11) with a maximum Ws-F reduction from 103.1 mg/kg to 21.6 ± 2.1 mg/kg which is close to the recommended limit of 16.4 mg/kg. The DSW and FSW defluoridation was based on complexation reactions, alteration of soil properties, and increasing the soil-specific surface area, but SB and HSB defluoridation was through chemisorption. Therefore, seaweed-derived materials are capable of remediation of fluoride contaminated soils and the study recommends further investigation on fluoride uptake by crops in pot and field experiments post-amendment with seaweed derived materials.