Effects of pretreatments on physical and chemical characteristics of wheat straw used as a maintenance-free compressed green roof substrate material

Abstract

Green roofs can effectively reduce urban runoff and the urban heat island effect. To solve the problems of traditional green roof substrate, such as difficult construction and high maintenance costs, as well as the problem of environmental pollution caused by straw burning, this paper selects wheat straw as a raw material of substrate. Six modification methods (mixed, soaked, hydrothermal, ultrasonic, microwave, and steam explosion) and three liquid media (deionized water, acetic acid, and wood vinegar) were used to treat wheat straw to improve its physicochemical properties and water-absorption expansion performance after compression. The results showed that, in terms of chemical properties, the microwave treatment led to a lower nutrients content (TN = 3.97–5.00 mg·g−1; TP = 0.22–0.29 mg·g−1, and TK = 3.27–4.65 mg·g−1) but effectively reduced the electrical conductivity (= 0.26–0.67 mS cm−1) and ash content (= 7.38–7.51%) of the substrate compared with other groups, effectively reducing the salt toxicity to the plant and improving the utilization rate of organic matter. As a modification medium, wood vinegar could effectively reduce the pH (= 3.33–6.12) and ash content (= 7.38–11.12%) of wheat straw, which was conducive to maintaining the acidity of the substrate over a longer time period. Regarding physical properties, the microwave-assisted acid hydrothermal treatment was the most effective in improving porosity, especially aeration porosity (= 21.73%), which was beneficial to the respiration of plant roots. For the expansion performance of the substrate after compression, the wood vinegar group yielded the highest water-absorption expansion rate (= 151–224%), and the microwave group yielded the highest relaxation ratio (= 6.75–9.64), effectively helping the compressed substrate to restore its porosity and support plant root growth. Overall, the microwave-assisted wood vinegar treatment is a suitable modification method for wheat straw as a green roof substrate.