Date of Award
Doctor of Philosophy (PhD)
The millions of tons of phosphate rock ore mined every year for use as phosphorus fertilizer have created two challenges. First, phosphate rock is a mined, non-renewable resource that is also heavily geographically concentrated, raising concerns about the long-term sustainability and availability of phosphorus. Second, after it is applied as a fertilizer, phosphorus will also be released to the broader environment. There, it contributes to harmful algal blooms and eutrophication, which threaten surface water quality and can harm aquatic ecosystems and the communities and industries that rely on them. These considerations motivate the coupled development of techniques for removing and recovering phosphorus from aqueous waste streams and environments, where it is largely present as phosphate (symbolized in this work as P). To remove and recover P from wastewater, this dissertation developed and investigated a novel method using mineral-hydrogel composite systems that consist of a hydrogel matrix (calcium alginate) and two mineral seeds (calcium silicate and calcium phosphate (CaP)). Previously, we developed a mineral-hydrogel composite containing only CaP for both P removal and recovery. The CaP mineral seeds in hydrogel composites provided a favorable surface for removing P through heterogeneous CaP nucleation and growth, while the hydrogel held the nanoscale mineral seeds in a large, easily separable matrix. To improve the mineral-hydrogel composite’s P removal and recovery performance further, this dissertation considers the addition of calcium silicate hydrate (CSH) (Task 1). In Task 1, CSH was added to the CaP mineral hydrogel composites. The newly synthesized mineral-hydrogel composites were characterized with X-ray scattering techniques and a combination of scanning electron microscopy and energy dispersive X-ray spectrometry to determine the mineral seed’s size, phase, and elemental composition. After successfully synthesizing mineral-hydrogel composites containing CSH and CaP, we tested their P removal and recovery performance. The addition of CSH enabled fast P removal kinetics, reduced the materials required for synthesis, and provided more robust P removal/recovery. When the CSH mineral seeds dissolve, they create favorable local aqueous conditions (high [Ca2+] and increased pH) for heterogeneous CaP nucleation and growth on the CaP mineral seeds within the mineral-hydrogel composite. Through this synergy, the CaP + CSH mineral hydrogel composites demonstrated promise for large-scale P removal and P recovery from P-enriched aqueous environments. Next, the calcium silicate and CaP mineral-hydrogel composites were further refined to meet the specific challenges and requirements of P-recovery (related to phosphorus supply) and P-removal (related to phosphorus pollution). For P-recovery, we need to recover large amounts of P from highly concentrated aqueous P waste streams (>50 mg-P/L) quickly and efficiently. To have effective P-removal, on the other hand, we aim to achieve very low aqueous [P] levels (> 0.1 mg-P/L) through P uptake into the mineral-hydrogel composites. To engineer the mineral-hydrogel composites for P recovery (Task 2), we hypothesized that the quickly dissolving CSH mineral seeds would achieve quick P uptake into the mineral-hydrogel composites. Interestingly, when tested, the CSH mineral seeds alone proved the dominant driver for P recovery, and the importance of the CaP mineral seeds in the composite was diminished in these conditions because of the presence of high saturation with respect to hydroxyapatite. Using quickly dissolving CSH mineral seeds, we recovered 61% of the initial dissolved P (50 mg-P/L), achieving a final loading of 108.8 mg-P/g-dry mineral seed at a final concentration of 16 mg-P/L. Furthermore, we developed a conditioning process to improve P recovery performance over multiple cycles of P recovery. We also evaluated the final P content and solubility of the recovered P to assess their reusability. Overall, the CSH mineral-hydrogel composites demonstrated promising P recovery performance, showing their ability to support a more sustainable P cycle. For effective P removal, in Task 3, a slowly dissolving calcium silicate mineral, wollastonite (CaSiO3), was substituted for the CSH mineral to remove P through heterogeneous nucleation and growth of CaP within the mineral-hydrogel composites. This substitution provided excellent P removal performance, from an initial concentration of 6.2 mg-P/L (characteristic of municipal water influent) to 0.067 mg-P/L, a 98.9% removal rate. Furthermore, at this removal level, we achieved a high final P loading of 9.3 mg-P/g dry mineral seed, exhibiting the mineral seed’s high affinity for P at this concentration. Here, we identified the role of the hydrogels in P removal and developed methods to increase the reswelling ability of the hydrogel matrix for improved applicability. To test the composites’ ability to affect the growth of cyanobacteria for alleviating harmful algal blooms, experiments were conducted using a model cyanobacteria, Synechococcus elongatus 2973 (Task 4). When applied as a pretreatment, the CaP + wollastonite mineral-hydrogel composites restricted the growth of S. elongatus, demonstrating its ability to starve a harmful algal bloom. While harmful algal blooms (undesirable algal growth) are a key challenge resulting from P release to the environment, algal-based biorefinery (desirable algal growth), a fast-developing field for bioproduct synthesis, demands sustainable P supply. As the mineral-hydrogel composites only restrict the growth of cyanobacteria through P-restriction, one exciting potential reuse option for P-recovered composites is its use as a substrate for P-supply in biorefinery applications. When P-recovered composites were applied in the beneficial algal growth culture medium, they provided sufficient P and supported desirable cyanobacterial growth. In this dissertation, mineral-hydrogel composites utilizing calcium phosphate mineral seeds and calcium silicate-based mineral seeds were engineered for the important problems of P recovery and P removal. We elucidated the important roles of calcium silicate dissolution in providing calcium precursor ions and of a favorable localized aqueous environment for CaP heterogeneous nucleation and growth. Thus, this dissertation establishes the promise of these novel mineral-hydrogel composites for supporting a more sustainable P cycle.
Zhen (Jason) He, Srikanth Singamaneni, Kimberly Parker, Jianjun Guan,
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