Impact of Cellulolytic Fungi on Biodegradation of Hemp Shives and Corn Starch-Based Composites with Different Flame-Retardants

Abstract

Biocomposite boards (BcBs) composed of hemp shives and corn starch are known as thermal insulating or structural building materials. Therefore, they must be stable during exploitation. However, BcBs are exposed to microorganisms present in the environment, and it is of great interest to investigate the biodegradation behaviour of these materials. This work identified microorganisms growing on BcBs that contain either Flovan CGN or expandable graphite as flame retardants and selected fungi such as Rhizopus oryzae and Aspergillus fumigatus to test the way they affect the materials of interest. For this purpose, the enzymatic activity of cellulases and amylases produced by these organisms were determined. In addition, the apparent density as well as compressive strength of the affected boards were evaluated. The results showed that apparent density and compressive strength deteriorated in BcB composition with the Flovan CGN flame retardant. At the same time, the level of deterioration was lower when the expandable graphite was used, suggesting that it also acts as an antimicrobial agent. A scanning electronic microscopy analysis was employed to monitor the growth of microorganisms in the BcBs. Such analysis demonstrated that, regardless of BcB composition, fungi easily penetrate into the middle layers of the material.

Keywords: Aspergillus fumigatus; Rhizopus oryzae; biocomposite; cellulolytic fungi; corn starch; hemp shives; mechanical performance.

Figure 1

The scheme of the test sample for SEM imaging: 1—direction of investigation; 2—microorganism-affected BcB surface; 3—the middle layer of the BcB sample; 4—the entire BcB sample.

Figure 2

Microbial species seen by optical microscopy (magnification 400× for bacteria and 40× for fungi): (a) P. putida, (b) A. fumigatus, (c) Rh. oryzae.

Figure 3

Detection of enzymatic activities from the isolated fungal strain by floating Petri dishes with Congo red for cellulase activity and Gram iodine solution for amylase activity, respectively: (a) Rh. oryzae after 12 days of growth with Congo red, (b) Rh. oryzae after 4 days of growth with Gram iodine, (c) A. fumigatus after 7 days of growth with Congo red, (d) A. fumigatus after 7 days of growth with Gram iodine, (e) P. putida after 14 days of growth with Congo red.

Figure 4

Total enzymatic activities of the microorganisms after growth on BcBs with different flame retardants: (a) cellulase activity; (b) amylase activity. All measurements were in triplicate; the average values and standard deviations are given.

Figure 5

Microstructure of BcB with F composition as determined by SEM: (a) before incubation (magnification ×1000); (b) 6 months after incubation—1 mm depth of BcB (1—A. fumigatus) (magnification ×2000); (c) 6 months after incubation—2–2.5 mm depth of the BcB (1—A. fumigatus, 2—Rh. oryzae) (magnification ×1500); (d) 6 months after incubation—the middle of the BcB (1—A. fumigatus) (magnification ×1500).

Figure 6

Microstructure of BcB with G composition as determined by SEM: (a) before incubation (magnification ×1000); (b) 6 months after incubation—1 mm depth of BcB (1—A. fumigatus, 2—Rh. oryzae) (magnification ×3000); (c) 6 months after incubation—2–2.5 mm depth of the BcB (1—A. fumigatus, 2—Rh. oryzae) (magnification ×3000); (d) 6 months after incubation—the middle of the BcB (1—A. fumigatus, 2—Rh. oryzae) (magnification ×2000).

Figure 7

BcB samples after incubation of P. putida and Rh. oryzae mix at 22 °C temperature and 65% relative air humidity conditions for 6 months: (a) F composition, (b) G composition. All measurements were performed in triplicates, and the average values and standard deviations are shown.

Figure 8

Compressive strength of BcBs before and after incubation for 6 months with mix of P. putida and Rh. oryzae. Measurements for F and G compositions with or without inoculation (control) with microorganisms are demonstrated. Experiments were performed in triplicate, and the average values and standard deviations are shown.