**1. Introduction**

Circular economy is the abbreviation of closed-loop flow economy [1], which is an economic development model consisting of 'resources-products-renewable resources' and repeated circulation of materials [2]. Its basic characteristics are low exploitation, high utilization and low emission. The basic code of conduct is the '3R' principle: 'reduce,' 'reuse' and 'recycle.' The three principles of circular economy constitute the basic development idea of the circular economy. The principle of reduction belongs to the input-side approach, which refers to the reduction of resource consumption and waste generation in the process of production, distribution and consumption [3]. Its specific requirements are to fully consider saving resources, improving the utilization rate of resources in the production process of unit products and preventing waste generation on the input-side of the production source [4]; while in the production process, through technological transformation, advanced production processes and clean production to reduce the use of raw materials and emissions of pollutants of unit products in the production process [5]. The principle of reuse belongs to the process approach, which means that the waste is directly used as a product or continues to be used as a product after repair, refurbishment and remanufacturing, or all or part of the waste is used as parts of other products. The principle of reuse requires the establishment of a standardized recycling mechanism for waste materials [6]. Recycling led by producers and operators can encourage and guide consumers to return the items they no longer need to the market system and then safely participate in the new economic cycle [1]. The principle of recycling (also called resource utilization) is the input-side approach, which refers to the direct use of waste as a raw material or the recycling of waste. If it cannot be reused as raw material, it should be heat recycled [4]. There are two ways of resource utilization: firstly, primary resource utilization, which means that the waste abandoned by consumers is recycled to form a new product identical to the original; secondly, secondary resource utilization, which means that the waste is transformed into raw materials of other products and then produced into a different type of product [7]. The order of the three principles of circular economy is reduce−reuse−recycle.

The circular economy not only forms a complete structure within its system but also combines the environment and economy in a close and clever way [8]. A circular economy should adapt to the natural ecosystem cycles and utilize these in economic cycles by respecting their reproduction rates (**Figure 1**). In terms of quality, circular

### **Figure 1.**

*Circular economy for sustainable development [9].*

economy is obviously superior to any single environmental protection measure in both environmental technology and economic technology [10]. At present, the technical means to help the development of a circular economy [6], especially for solid and liquid waste treatment are relatively mature [11, 12]. Many researchers began to focus on the application of laser-induced breakdown spectroscopy [13], as well as the recycling economy method of utilizing the waste after seawater desalination by botanical methods [14]. In the context of carbon neutrality [15], the use of biochar [16, 17] and organic matter [18, 19] is of great concern. Using the idea of circular economy to utilize biochar can not only reduce the waste of resources but also have a beneficial impact on mitigating climate change and improving soil remediation [20, 21].

The circular economy is a major change in the process of economic development, and a new economic form based on the law of energy flow and material circulation of the natural ecosystem, which makes the economic system is integrated into the cycle of the natural ecosystem, and the two promote each other's development [22]. The important idea of a circular economy is to realize sustainable development and establish a production process that realizes comprehensive recycling of resources and waste through clean production [23]. The starting point of the circular economy is to protect the ecological environment [24]. The improvement of scientific and technological innovation is to promote the development of the circular economy while promoting the ecological protection of the environment, and scientific and technological innovation will also bring more green products and create more green consumption methods, which is an important means to protect the environment and promote ecological development [25]. From the principles of circular economy to the methods of using

circular economy ideas to the benefits brought by circular economy ideas, this is a closed-loop industry and also the development direction of the new era.
