Use of Ferrihydrite-Coated Pozzolana and Biogenic Green Rust to Purify Waste Water Containing Phosphate and Nitrate

Using Ca Fe layered double hydroxide transformation to optimise phosphate removal from waste waters

“…for an initial PO4 concentration of 100 mg L -1 , the removal capacity was not determined accurately since the removal rate was close to 100% and the Fe-Ca solid surfaces are certainly under-saturated. However, the data showed that the removal capacity is at least as high as 33 mg P-PO4 g -1 , a value higher than those measured for Ca free iron oxides (Ruby et al, 2016).…”

“…The value of qMax is also significantly higher than the adsorption capacity of ferrihydrite ( 20 mg PO4 g -1 < qMax <  33 mgP-PO4 g -1 in the pH range between 9 and 4, respectively). Note that ferrihydrite with a Surface Specific Area (SSA) of  300 m 2 g -1 was reported to be the best PO4 adsorbent of the iron oxide mineral family (Ruby et al, 2016). This comparison with ferrihydrite suggests that Ca-Fe LDH interacted with PO4 via another mechanism than a simple adsorption process allowing the removal of a much higher quantity of PO4.…”

“…The experiment performed in the column demonstrated that pozzolana (Pz) coated with Ca-Fe LDH was particularly efficient for removing phosphate. Indeed, the experiment may be compared to another set of column experiments performed recently using ferrihydrite (Fh) coated Pz (Ruby et al, 2015;Ruby et al, 2016) where the experimental conditions were quite similar, i.e. identical dimensions of the columns, [PO4]= 100 mg L -1 in the inflow, but with a contact time of  8 h instead of  5.5 h in this study.…”

“…If such materials are intended to be used in horizontal filters, the dimensioning of the reactors depends on numerous factors such as: (i) the number of population equivalent (PE) releasing wastewater, (ii) the quantity of P-PO4 released by 1 PE per day {typically in the range of  1 to 3 g P-PO4 PE -1 (Henze and Comeau, 2008)}, (iii) the PO4 removal capacity (mg P-PO4 g -1 ) of the Filtration Material (FM) under hydrodynamic flow and (iv) the duration of the water treatment until the material is saturated with PO4. As an example, a volume of  0.8 m 3 of FM per PE was estimated to be necessary for a 4 years treatment if ferrihydrite coated pozzolana was used as a FM (Ruby et al, 2016). Such a volume of FM corresponds also to a surface  2 m 2 per PE for a horizontal filter with a depth of 0.5 m.…”

“…The first goal of this study was to determine as precisely as possible the mineralogical transformation of single phase Ca-Fe LDH in the presence of PO4 solutions of varying concentrations. The behavior observed at lower PO4 concentration (<  100 mg L -1 ) will be particularly relevant for application in waste water treatment where the concentration of PO4 is often between  10 and  60 mg L -1 (Ruby et al, 2016). Moreover, we recently developed a new method of synthesis allowing the deposition of a high amount of ferrihydrite onto a pozzolana support (Ruby et al, 2015).…”