Capturing effects of filamentous fungi Aspergillus flavus ZJ-1 on microalgae Chlorella vulgaris WZ-1 and the application of their co-integrated fungi-algae pellets for Cu(II) adsorption

Highlights

• FAPs was synthesized by Aspergillus flavus ZJ-1 capturing Chlorella vulgaris WZ-1.

• ZJ-1 pellets captured 97.85% of WZ-1 cells under optimal conditions.

• FAPs can further be used as a biosorbent to remove Cu(II) from aqueous solution.

• Langmuir maximum adsorption capacity of Cu(II) on FAPs_2 h was 123.61 mg·g⁻¹.

Abstract

Using filamentous fungi to capture unicellular microalgae is an effective way for microalgae recovery in water treatment. Here, fungi Aspergillus flavus ZJ-1 and microalgae Chlorella vulgaris WZ-1 isolated from a copper tailings pond were used to study the capture effect of ZJ-1 on WZ-1. The highest capture efficiency (97.85%) was obtained within 6 h under the optimized conditions of 30 °C, 150 rpm, fungi-algae biomass ratio of 2.24:1, and initial pH of 9.24 in microalgae medium. The formed fungi-algae pellets (FAPs) were further used to remove Cu(II) from aqueous solution. Results showed that the FAPs formed at different capture times all adsorbed Cu(II) well, and the PAFs formed within 2 h (PAFs_2 h) exhibited the highest Cu(II) adsorption capacity (80.42 mg·g⁻¹). SEM images showed that Cu(II) caused a change in the internal structure of PAFs_2 h from loose to compact, the mycelium shrunk, and the microalgal cells were concave. Cu(II) adsorption by PAFs_2 h was well conformed to the pseudo-second-order kinetics and the Langmuir isotherm (123.61 mg·g⁻¹ of theoretically maximum adsorption capacity). This work opens a way for applying FAPs in the remediation of heavy metal-contaminated wastewater, and the metal adsorption effect was determined by the capture amount of microalgae.

Introduction

The development of mineral resources, metal processing, and smelting, produce a large amount of various heavy metal (HM) wastewater (Li et al., 2021). Unlike organic contaminants, HMs cannot be environmentally degraded. If HMs are directly discharged into the environment without necessary pre-treatment, they will persist for a long time and accumulate in organisms through the food chains, thereby endangering the health of humans and other organisms, and seriously threatening the ecological balance (Li et al., 2013, Zhang et al., 2019a, Zhang et al., 2020c). Many physical, chemical, and biological treatment methods have been successfully applied to HM wastewater treatment (Chai et al., 2021).

Microbial treatment technology has always played an important role in removing HMs in wastewater due to its advantages of being green, environmentally friendly, and low cost (George et al., 2012, Zhang et al., 2019b, Yu et al., 2020). Abundant microbial materials, such as bacteria, fungi, and microalgae, have shown considerable metal sequestering capacity, as reported by numerous studies. Hassan et al. (2018) discovered that the maximum adsorption capacity of Cd(II) and Zn(II) by Neopestalotiopsis clavispora has reached 185.3 mg·g⁻¹ and 153.8 mg·g⁻¹, respectively. The live cells of some microalgae, such as Chaetoceros calcitrans, Chlorella spp., and Spirulina sp., have adsorption capacity of hundreds or even thousands of mg·g⁻¹ for Cd(II), Cu(II), Ni(II), Cr(III), Pb(II), etc. (Suresh Kumar et al., 2015). However, some unicellular microalgae are challenging to recover after adsorbing metals from wastewater due to their dispersibility and suspension (Singh and Patidar, 2018). And if the recycling is not thorough, which may cause potential secondary contamination.

In recent years, studies have found that filamentous fungi can efficiently capture unicellular microalgae and immobilize them on fungal mycelia to form co-integrated fungi-algae pellets (FAPs), which can be easily removed by simple filtration from the water phase (Gultom et al., 2014, Chu et al., 2021b). Li et al. (2019) found that these Aspergillus oryzae-Chlorella vulgaris pellets can be used well for the As remediation from contaminated wastewater during pellet formation, and can be easily recovered after remediation. And the study of Cd removal from wastewater with Aspergillus niger-Chlorella vulgaris pellets by Bodin et al. (2017) also confirmed similar results. Besides, in recent reports, the FAPs are mainly used for microalgae harvesting, nutrient removal, and bio-energy production (Cao et al., 2017, Chen et al., 2018, Yang et al., 2019, Lal et al., 2021, Lin et al., 2022, Pathak and Pandey, 2022). There are limited studies on the further application of residual or newly formed FAPs for heavy metal removal from wastewater after these processes are over. Therefore, FAPs with high metal tolerance and metal accumulation properties can provide new insight in the bioremediation of heavy metal-contaminated wastewater.

With this in mind, this study aims to evaluate the capture effect of fungi on microalgae and the potential of formed FAPs to remediate Cu(II)-contaminated wastewater. Well known, Cu(II) is one of the most widespread heavy metals from electroplating, textile, battery manufacture, painting, and electrical wiring, etc. (Chen et al., 2019). And the excessive release of Cu(II) will endanger the ecological environment and the health of organisms. The fungi ZJ-1 and microalgae WZ-1 strains used in this study were screened from a copper tailings pond in Dongchuan district, Yunnan province (103°06′10.4″E, 26°11′25.4″N), and they showed good copper tolerance based on the maximum tolerable concentration (MTC) test. FAPs were prepared through ZJ-1 capturing WZ-1 and characterized by SEM analysis, and used for Cu(II) removal from aqueous solution by batch experiments. The possible mechanisms of Cu(II) removal by FAPs was also investigated.