Hydrothermal synthesis of a novel ecological revetment material by sediment mixed with biochar

Highlights

• A revetment can be synthesized hydrothermally from the local sediment and biochar.

• The novel revetment materials possess the merits of both soft and hard revetments.

• The revetment materials possess lower pH and good adsorption of heavy metals.

• The revetment has been used to improve the ecology of the Beiheng river (China).

Abstract

Ecological revetments, as a sustainable way of ecological construction of water conservancy projects, have attracted much attention. Herein, we synthesized a novel revetment material hydrothermally with the local sediment enriched with biochar made from waste biomass. The effect of Ca(OH)₂ content, biochar content, curing temperature and curing time on the strength and porous structure of the material was studied, and with 25 mass% Ca(OH)₂ and 2 mass% biochar cured at 200 °C for 12 h, the synthesized materials possess a lower pH (≤9.32), higher flexural strength (20.8 MPa) and superior removal performance of ammonia nitrogen (45%), phosphorus (100%) and heavy metals (≥90%). The formation of C–S–H and tobermorite in the matrix were found to enhance the strength of the materials, and the porous structure was attributed to the original porous structure of sediment, further the formed tobermorite and added biochar. The Sorption kinetics and equilibria of the material for heavy metals followed pseudo-second order and Langmuir isotherm equations respectively, which indicates that the mechanism of adsorption for heavy metals mainly is attributed to the chemical adsorption. In addition, the synthesized materials were used as revetments in Beiheng River (Shanghai, China), which provides a site for demonstration of the flood control and ecological restoration.

Introduction

The riparian zone, regarded as the ecological ecotone between terrestrial and aquatic ecosystems, has diversified functions of regulating runoff, conserving water sources and controlling water pollution. It also provides habitats and breeding places for aquatic and terrestrial organisms (Griffiths, 1998).

The earliest natural revetments were composed of sediment and woods (soft revetments), which have good hydrophilic effect and strong self-purification ability (Vagnetti et al., 2003), but poor resistance to damage (Xie et al., 2021). It will take longer time for the natural recovery when it is seriously damaged in natural disasters (Tang et al., 2019) and human activities (Bilkovic and Roggero, 2008). With the progress of flood control and regulating projects, the natural revetments were gradually replaced by hard revetments made of stone or concrete (Kiss et al., 2019). The compact structure of alkaline concrete revetments hinders the mass and energy exchanges between river and land (Browne and Chapman, 2011), which inevitably destroys the original ecosystem of riparian zone (Nakamura et al., 2002).

In the 1990s, the concept of ecological revetments, the combination of plants and civil engineering materials, has been put forward by some scholars (Schiechtl and Stern, 1996). Subsequently, the construction of ecological revetments has been used as a method of ecological restoration of rivers at home and abroad (Wu et al., 2017). The substrates are one of important components of ecological revetments (Wang et al., 2020b), which should be structurally stable and biocompatible. Common ecological substrates are ecological stone cages (Beikircher et al., 2010), ecological bags (Yu et al., 2017), turf reinforcement mats (Yuan et al., 2015) and ecological concrete (Yuan et al., 2017). However, the wires and the cotton mesh in ecological stone cages, ecological bags and turf reinforcement mats are easily broken due to corrosion and abrasion by flood, resulting in unstable structure (Thompson et al., 2016). And ecological stone cages and ecological concrete are mainly made of hard materials, such as stone (Beikircher et al., 2010) and concrete (Yuan et al., 2017). Due to the lack of stone and the high alkalinity of concrete, it is impractical to construct large-scale ecological revetment projects. In addition, the water conservancy projects will dig out a large amount of sediment with inorganic resource, which has good ecological self-purification performance (Huang et al., 2020). By replacing stone and concrete with the sediment, it will not only save large amounts of resource, but also achieve a real and comprehensive ecological bank protection project.

Hydrothermal solidification technology, which has also been applied widely to inorganic synthesis (Wang et al., 2020a, Zheng et al., 2021) and organic material conversion (Yang et al., 2019; Zhong et al., 2019a, 2019b), is regarded as a short-term laboratory reproduction of the long process of formation of sedimentary rock. Low-temperature (≤200 °C) hydrothermal solidification can not only retain the microscopic porosity and affinity of the raw materials (Lan et al., 2021; Miao et al., 2021), but also synthesize hardening phases of cement from the inorganic waste, i.e. calcium hydrate silicate (C–H–S) and tobermorite (Jing et al., 2007). The previous studies have shown that the formation of tobermorite can improve the mechanical strength and porosity of the material (Jing et al., 2008). Furthermore, it will also adsorb heavy metals (Sajidu et al., 2006) and organic pollutants (Al-Wakeel et al., 2001) effectively.

It is well known that Si, Al, and Ca are the main elemental components of the sediment, which are consistent with the elemental compositions of the C–S–H and tobermorite. Therefore, it has a high potential to reconstitute Si, Al, Ca of the sediment into C–S–H or tobermorite to reach high strength for revetment materials like cement and concrete. In addition, the biochar produced by the pyrolysis technology has porous structure (Amonette and Joseph, 2009), which enables the adsorption of heavy metals (Wang et al., 2019) and nutrients such as N and P (Xiao et al., 2018). Therefore, it is prospective to improve ecological performance by adding the biochar into revetment materials.

Moreover, the revetment materials were produced hydrothermally from the local sediment mixed with biochar made from the local waste reeds, which had been used as revetments in the Beiheng River on the east coast of Chongming Island to detect the flood control and ecological restoration capacity.

The purpose of this study is (1) to investigate how to solidify a high-strength composite from the sediment and biochar; (2) to study the biocompatibility and adsorption of the composite; (3) to explore the adsorption mechanism for heavy metals; (4) to demonstrate this material as the revetments with the flood control and ecological restoration capacity.