Phosphorus recovery and recycling from model animal wastewaters using materials prepared from rice straw and corn cobs

Abstract

Anthropogenic loss of phosphorus to surface waters not only causes environmental problems but depletes valuable phosphorus reserves. In this study, magnesium amended biochars and magnesium silicate, synthesized from corn cobs and rice straw, respectively, were evaluated for phosphorus uptake including the effects of pH and alkalinity. The overall goal was to close the phosphorus loop by recovering phosphorus from animal waste and reusing it as fertilizer. After phosphorus uptake, spent materials were tested for phosphorus release using modified soil tests representing different soil pH and alkalinity conditions. In experiments using model animal wastewaters containing both ammonia and bicarbonate alkalinity, dissolved phosphorus was removed by struvite (MgNH4PO4·6H2O) formation, whereas in deionized water, dissolved phosphorus was removed by adsorption. Alkalinity in the model animal wastewaters competed with phosphate for dissolved or solid-associated magnesium thereby reducing phosphorus uptake. Spent materials released significant phosphorus in waters with bicarbonate alkalinity. This work shows that abundant agricultural wastes can be used to synthesize solids for phosphorus uptake, with the spent materials having potential application as fertilizers.

Phosphate Uptake by Precipitation in Model Animal Wastewaters: Adjusting Ionic Strength and Ionic Composition to Maximize Phosphorus Removal

“…First, rice straw ash was prepared by pyrolysis of ground rice straw (a mixture of Oryza sativa and Tropical japonica) that was donated by the Dale Bumpers National Rice Research Center, Stuttgart, Arkansas. Details of the procedure were reported previously [13]. Then, one gram of rice straw ash (containing 53% (w/w) silicon [13]) was mixed with 1.03 g Mg(OH) 2 , 0.4 g NaOH, and 2.3 L water and stirred in a capped bottle for three days.…”

“…Details of the procedure were reported previously [13]. Then, one gram of rice straw ash (containing 53% (w/w) silicon [13]) was mixed with 1.03 g Mg(OH) 2 , 0.4 g NaOH, and 2.3 L water and stirred in a capped bottle for three days. The pH was 12, which served to dissolve silicon from rice straw ash [24].…”

“…The pH was 12, which served to dissolve silicon from rice straw ash [24]. The slurry was then settled overnight, filtered in a conical funnel using Grade 1 filter paper (GE Healthcare Whatman, Fisher Scientific, Pittsburgh, PA, USA), rinsed with a small volume (50 mL) of DI water, and the solids oven dried at 60 • C. The solid yield was 0.895 g. Previous XRD analysis showed that this material was mostly amorphous with broad peaks that were consistent with magnesium silicate minerals such as antigorite and enstatite, as well as SiO 2 (quartz) [13]. This material is referred to hereafter as Mg silicate, although its composition is likely heterogeneous.…”

“…Equilibrium phosphorus removal was measured in a model wastewater containing 0.00221 M NaH 2 PO 4 (68.5 mg/L as P), 0.0355 M NaHCO 3 (alkalinity: 0.0355 eq/L, or 1775 mg/L as CaCO 3 ) [13], and a concentration of NH 4 Cl equal to one of the following values: 0.0236 M, 0.059 M, 0.118 M, 0.177 M, or 0.236 M. The concentrations of total orthophosphate, ammonia, and alkalinity in the model wastewater are similar to reported values from actual animal wastewaters [9,18,23]. The concentrations of added NH 4 Cl were chosen to yield values of 10.7 to 107 for the molar ratios of total ammonia (i.e., NH 4 + and NH 3 and any other complexes or solid phases containing the MINEQL+ component NH 4 + (hereafter called [NH 4 ] T ) to total orthophosphate (hereafter called [PO 4 ] T ).)…”

“…Our research focuses on the utilization of agricultural wastes to prepare water treatment materials for phosphorus recovery. Sources of magnesium for phosphorus recovery as struvite include MgO(s) precipitated on high surface area chars prepared from corn cobs and wheat straw, and magnesium silicates prepared using magnesium salts and dissolved silica extracted from rice straw [13]. Other researchers have also utilized Mg-enriched plant chars for phosphorus removal (e.g., [14,15]).…”