Solids retention time dependent, tunable diatom hierarchical micro/nanostructures and their effect on nutrient removal

Yan Li; Chiqian Zhang; Xiaoqing He; Zhiqiang Hu

doi:10.1016/j.watres.2022.118346

Solids retention time dependent, tunable diatom hierarchical micro/nanostructures and their effect on nutrient removal

Water Res. 2022 Jun 1:216:118346. doi: 10.1016/j.watres.2022.118346. Epub 2022 Mar 22.

Authors

Yan Li¹, Chiqian Zhang², Xiaoqing He³, Zhiqiang Hu⁴

Affiliations

¹ NingboTech University, Ningbo 315000, China; Department of Civil & Environmental Engineering, University of Missouri, Columbia, Missouri, 65211, USA.
² Department of Civil & Environmental Engineering, University of Missouri, Columbia, Missouri, 65211, USA.
³ Electron Microscopy Core Facility, University of Missouri, Columbia, Missouri, 65211, USA; Department of Mechanical and Aerospace Engineering, University of Missouri, Columbia, Missouri, 65211, USA.
⁴ Department of Civil & Environmental Engineering, University of Missouri, Columbia, Missouri, 65211, USA. Electronic address: HuZh@missouri.edu.

PMID: 35358880
DOI: 10.1016/j.watres.2022.118346

Abstract

The hierarchical three-dimensional (3D) micro/nanostructures of diatoms make them a promising biomaterial for fabricating nanomaterials, producing bioactive pharmaceuticals or nutraceuticals, and removing micropollutants. For diatom production in a continuous flow system, little is known how bioreactor operating parameters, especially solids retention time (SRT), affect the 3D structures of diatoms. This study demonstrated that tunable diatom micro/nanostructures could be produced by varying the SRT of membrane bioreactors (MBRs). A diatom strain (Stephanodiscus hantzschii) was cultivated in two identical MBRs with a fixed hydraulic retention time (HRT) of 24 h and staged SRTs from 5, to 10, and to 20 d. As SRTs increased from 5 to 20 d, important characteristics of diatom micro/nanostructures showed linear decreases: the diameters of foramina on the areola layer decreased from 170 ± 10 to 130 ± 12 nm, the numbers of nanopores per cribrum layer decreased from 20 ± 3 to 12 ± 2, and the specific surface areas of the diatoms decreased from 36.01 ± 1.27 to 12.67 ± 2.45 m²·g^-1. However, the average diatom heights increased from 2.9 ± 0.3 to 3.9 ± 0.4 µm, while diatom cell diameter (5 µm) and nanopore size (20 nm) remained unchanged. The silicon content of diatoms also linearly increased with SRT. The decrease in diatom porosity and increase in silicon content were probably due to the reduced diatom growth rates (likely resulting in less pores) at increasing SRTs, which also facilitated silica deposition as the overall diatom population stayed longer in the MBRs. As the SRTs increased from 5 to 10, and to 20 d, the nitrate (NO₃^-) removal efficiency decreased from 75% to 70%, and to 60%, respectively, whereas phosphorus (P) removal efficiency increased from 74% to 80%, and to 90%, respectively. The opposite trends in efficiencies were because NO₃^--N was removed by cellular uptake and biomass waste whereas P was mainly removed through diatom-assisted chemical precipitation.

Keywords: Membrane bioreactors; Nanostructure; Nutrient removal; Porosity; Silicon; Stephanodiscus hantzschii.

MeSH terms

Bioreactors
Diatoms*
Nanostructures*
Nutrients
Silicon
Waste Disposal, Fluid / methods

Substances

Silicon