Hybrid materials for removal of bisphenol-A from aqueous solution
Hartono, Maria Regina
Date of Issue2017-02-28
School of Materials Science and Engineering
Noxious pollutants such as endocrine disrupting compounds (EDCs) are associated with rapid industrial and urban development. They are difficult to remove using conventional wastewater treatments and thus pose new challenges in environmental remediation. These concerns necessitate more effective and efficient approaches for emerging pollutant removal from wastewater treatment units. (Aim) This study evaluated various strategies for the removal of one of these emerging endocrine disruptor compounds, bisphenol A (BPA). (Scope) In particular, to improve the removal of BPA from aqueous environments, we investigated processes of physical separation of BPA from an aqueous solution through adsorption and then investigated the subsequent integration of adsorption with biological degradation. Among the emerging adsorbent materials, biosorbents predominate due to their low cost and abundance. However, these products have low adsorption capacity. Therefore, to improve these abilities, amphiphilic surfactant treated biosorbent was developed and studied, and the factors affecting its ability to remove BPA were assessed. In the first study, we evaluated pre-treatment of bamboo powder waste, a kind of biosorbents, to remove BPA. Despite the marked increase in removal capacity after treatment with cationic surfactant, its kinetics and adsorption capacity are still considered inferior in comparison to other emerging adsorbent materials. A comparison study was conducted among treated bamboo powder, commercial graphene nanoplatelets, and single- and multi-walled carbon nanotubes (MWCNTs) for the removal of BPA. Our study deduced that carbon nanomaterial (CNM) variants in general possess faster adsorption capacities and kinetics for BPA removal. Indeed, CNM variants have gained tremendous interest as adsorbents due to their high adsorption capacity and improved re-usability through multiple regeneration cycles. However, there are concerns regarding their environmental toxicity, which brings into question about their application as environmentally safe and sustainable adsorbent materials. To overcome this, the second part of our study focused on the development of composite sorbents such as entrapped MWCNTs and TiO2 inside alginate beads to reduce environmental toxicity and to facilitate the recovery of the used sorbent. Because wastewater treatment plants are operated in continuous mode with limited contact time to reach equilibrium, it was necessary to assess the capability of a sorbent during continuous flow mode to assess its practical application. BPA removal from the composite beads in the column reactors was affected by the feed flow rate, bed depth and initial inlet concentrations. The BPA sorption performance was predicted with good correlation using a dose-response model. We showed that photolytic degradation of BPA desorbed from the beads, and thus the regeneration of the spent adsorbent beads can be achieved simultaneously. Results showed that the alginate beads alone, however, were easily abraded and susceptible to destruction by phosphate, rendering their use in their pure form unsuitable for the containment of MWCNTs in water treatment. Therefore, in the next part of our study, we designed a macro-bead platform comprised of a calcium-alginate core and a polysulfone (Psf) layer as an immobilization matrix for MWCNTs. We hypothesized that the additional Psf layer would improve BPA removal and reduce the percentage of MWCNTs that leaked from the beads. We tested these fabricated beads for adsorption of BPA in a batch reactor and tested their re-usability in a column reactor. The physical and chemical properties of the fabricated beads were characterized using electron microscopy, zeta potential, BET surface area, FTIR and Instron microtester. Bioluminescence assay was used as a means to detect the leakage of MWCNTs from the beads. Several accepted kinetic and isotherm models were utilized to characterize the adsorption behavior of the hybrid beads. Our observations show that BPA removal increased with the content of MWCNTs. Also, the addition of the Psf layer improved the amount of adsorbed BPA at lower MWCNT amounts but was less significant with higher counts of MWCNTs. Moreover, the hybrid beads were relatively stable over a wide range of pHs. The BPA adsorption profile of the hybrid beads correlates well with the pseudo-second order kinetic model and the Langmuir isotherm monolayer adsorption model. Addition of a Psf layer on composite beads that contained MWCNTs improved the compressive load capacity of the composite beads up to twelvefold at 40% compression. The improved compressive load capacity in beads layered in Psf could be applicable to both shield and improve the stability of the composite calcium alginate-MWCNT beads in reactors, where the bead sorbents may be subjected to abrasion that could affect bead integrity. Building from our previous studies, in the last part of our work, we attempted to integrate the biodegrading component into the hybrid bead. We used a novel BPA tolerant bacteria strain PBPA2 that was isolated from the environment and embedded it within the calcium-alginate/MWCNT/Psf beads with the purpose of integrating the adsorption performance of MWCNTs and the biodegradation capability of the PBPA2 strain. The effects of enrichment and mineral media composition, bead diameter and cell loading were studied in a batch reactor. It was observed that the addition of 0.05% yeast extract into the mineral medium and the reduction of the bead’s diameter enhanced the removal of BPA. Subsequent experiments in continuous-flow column reactors resulted in enhanced BPA removal, reinforcing the potential of integrating biodegrading bacteria with CNTs. In summary, this work reveals the development of a novel composite sorbent platform for the removal of BPA and elucidates upon its sorption mechanisms and efficiencies.