Layered double hydroxides for phosphorus recovery from acidified and non-acidified dewatered sludge

Abstract

Phosphate, which contains the essential element phosphorous (P), is a necessary fertilizer for agriculture, but the current phosphate deposits are running out and alternative sources are needed. Sludge obtained from wastewater treatment plants contains high concentrations of phosphorus and represents an alternative, sustainable source. In this study, sludge obtained from a wastewater treatment plant with biological and chemical phosphorus removal was acidified (pH = 3, 4, 5 and 6) to release orthophosphate followed by sequestration of the orthophosphate by a zinc aluminum layered double hydroxide (Zn2Al-LDH). Sulfuric acid (H2SO4), nitric acid (HNO3), and hydrochloric acid (HCl) was tested, which showed that only sulfate anions compete with phosphate and results in reduced phosphate recovery (25-35%). The orthophosphate concentration in the liquid phase increased from 20 % (raw sludge) to 75 % of the total phosphorus concentration at a pH of 3, which enhanced the phosphate uptake by the ZnAl-LDH from 1.7±0.2% to 60.3±0.6%. During acidification, the competing anion carbonate is degassed as CO2, which further improved the phosphate uptake. PXRD showed the intercalation of carbonate in the LDH in the raw sludge at pH = 8, whereas orthophosphate was intercalated at lower pH values. 27 Al MAS NMR spectroscopy and powder Xray diffraction (PXRD) proved preservation of the LDH at all pH values. Furthermore, about a third of the Al is present as an amorphous aluminum phosphate (AlPO4) upon exposure to phosphate at low pH (pH = 3 and 2 5) based on 27 Al MAS NMR spectroscopy. At a pH of 6 about a third of the P is present as brushite (CaHPO4•2H20).

Atomic Level Understanding of Orthophosphate Adsorption by Magnesium Aluminum-Layered Double Hydroxides—A Multitechnique Study

“…50 Guo et al assigned the two dominating resonances at δ iso ( 31 P) = -14.5 and -7.5 ppm in the 31 P MAS NMR spectrum to phosphate associated with the LDH. 49 However, δ iso ( 31 P) = -14.5 ppm are in the same range as aluminum phosphate synthesized under acid condition 36,39,60 and δ iso ( 31 P) = -7.5 ppm match newberyite. 27,61,69 Both minerals are also clearly identified by PXRD.…”

“…33,35 Such AOH has clearly been identified by 27 Al SSNMR for MgAl-and ZnAl-LDH phases used for phosphate adsorption. 28,[36][37][38] Moreover, the presence of amorphous aluminum hydroxides is critical as these have a high affinity for phosphate [39][40][41] and will lower the actual Al content in the LDH, 33,[35][36]42 i.e., result in a lower TAC. Thus, experimental techniques with atomic level resolution, which do not require long-range order such as solid-state NMR spectroscopy (SSNMR), X-ray extended absorption spectroscopy (EXAFS), vibrational spectroscopy (FT-IR and Raman) is needed to probe surface complexation and amorphous phases.…”

“…The formation of phosphate minerals, e.g. newbeyrite, 27,49 bobierite, 27,67 aluminum phosphate, 36 calcium phosphates 29,68 and iron phosphates, 26 have been detected by 31 P SSNMR and PXRD after exposure of LDH to phosphate. Magnesium phosphate minerals were the predominant phase identified from adsorption experiment with a starting pH of 7 and a final pH of 12 except for a broad resonance in the region 0-5 ppm, tentatively assigned to phosphate affiliated with the MgAl-LDH.…”

“…were assigned to phosphate in association with the LDH. 36 The different δ iso ( 31 P) for the resonances is ascribed to the presence of Zn instead of Mg. In the current study, which was performed under alkaline condition, three sites (A, B, and C) have been assigned to phosphate associated with the LDH.…”

Mechanistic Understanding of Beneficial Aspects of Iron Salts and Waterworks Sludge Uses in Urban Wastewater System

“…This implies the possible formation of inner-sphere surface complexes between PO4 3ions and aluminium oxides (as evidenced by the XRD pattern of Al-sludge) ( Figure 68). (Lookman et al, 1997;Lundehøj et al, 2019), is due to octahedral Al-coordination with a single PO4 3or four PO4 3and two hydroxyl/water ligands. This is further evidenced by the chemical shift (iso, Al) ppm results from 27 Al spectra deconvolution presented in Table 33.…”