Development and application of ferrihydrite-modified diatomite and gypsum for phosphorus control in lakes and reservoirs

A novel phosphorus (P) adsorbent, ferrihydrite-modified diatomite (FHMD) was developed and characterized in this study.The FHMD was made through surface modification treatments, including NaOH treatment and ferrihydrite deposition on raw diatomite.In the NaOH treatment, surface SiO 2 was partially dissolved in the NaOH solution.The dissolved Si contributed to form stable 2-line ferrihydrite, which deposited into the larger mesopores and macropores of the diatomite.The 2-line ferrihydrite not only deposited into the pores of the diatomite but also aggregated on the surface.Filling the larger mesopores and macropores of the diatomite and aggregation on the diatomite surface with 0.24 g Fe/g of 2-line ferrihydrite resulted in a specific surface area of 211.1 m 2 /g for the FHMD, which is an 8.5-fold increase over the raw diatomite (24.77 m 2 /g).The surface modification also increased the point of zero charge (pH PZC ) values to 10 for the FHMD from 5.8 for the raw diatomite.Effects of the formation process parameters such as concentrations of FeCl 2 , NaOH, and drying temperature on the formation mechanism and crystalline characteristics of FHMD were studied by using X-ray absorption near-edge structure (XANES) spectroscopy.The spectra were recorded in both the total electron yield (TEY) and the fluorescence yield (FY) modes to investigate the chemical nature of Fe and Si on the surface and in the bulk of ferrihydrite-modified diatomite, respectively.It was found that only the surface SiO 2 was partially dissolved in the NaOH solution with stirring and heating, whereas the bulk of diatomite seemed to be preserved.The dissolved Si was incorporated into the structure of ferrihydrite to form the 2-line Si-containing ferrihydrite.The crystalline degree of ferrihydrite increased with the increasing FeCl 2 concentration and the Brunauer-Emmett-Teller (BET) specific surface area of FHMD decreased with the increasing FeCl 2 concentration.The NaOH solution of higher concentration partially dissolved more surface SiO 2 and the crystalline degree of ferrihydrite decreased with the increase in NaOH concentration.The dehydroxylation on the surface of FHMD occurred in the high temperature calcination, causing an iii energy shift in the Si L-edge spectra to the high energy side and an increase in the crystalline degree of ferrihydrite.In this study, the optimal synthesis conditions for the FHMD with the least crystalline degree and the highest surface area were found to be as the follows: 100 mL of 0.5M FeCl 2 solution, 6M NaOH solution and the drying temperature of 50 C.Phosphorus adsorption behavior and adsorption mechanism of FHMD were investigated in the research.The Langmuir model best described the P adsorption data for FHMD.Because of increased surface area and surface charge, the maximum adsorption capacity of FHMD at pH 4 and pH 8.5 was increased from 10.2 mg P/g and 1.7 mg P/g of raw diatomite to 37.3 mg P/g and 13.6 mg P/g, respectively.Phosphorus showed the best affinity of adsorption onto FHMD among common anions.K-edge P XANES spectra demonstrate that P is not precipitated with Fe (III) of FHMD, but adsorbed on the surface layer of FHMD.Phosphorus removal from lake water and limiting phosphorus release from sediment by FHMD was examined.Phosphorus removal from lake water proceeded primarily through P adsorption onto the surface of FHMD.When a dose of FHMD of 250 mg/L was applied to lake water, a total phosphorus (TP) removal efficiency of 88% was achieved and a residual TP concentration was 17.0 g/L which falls within the oligotrophic TP range (3.0-17.7 g/L).FHMD settled down to the bottom of the 43 cm high cylinder within 6 hours, which suggested that retention time of FHMD in the 5.5 m of Jackfish lake water column was close to the equilibrium time of P adsorption onto FHMD (72 hours).During the 30-day anoxic incubation period, TP concentrations in lake water treated by 400, 500 and 600 mg/L of FHMD showed a slight decrease and maximum TP concentrations remained at levels lower than 15 g/L.The addition of FHMD resulted in a marked increase in Fe-P fraction, a pronounced decrease in labile-P and organic-P fractions, and stable Al-P, Ca-P and residual-P fractions.The effect of FHMD on limiting P release was comparable with those of the combination of FHMD and alum solutions with logarithmic ratios of Al to mobile P of 0.5 and 0.8.FHMD not

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