_Article_  
Study of the Effect of Pyrolysis Temperature on the Cd\({}^{2+}\) Adsorption Characteristics of Biochar  
[PERSON] 1, [PERSON] 1, [PERSON] 1,*, [PERSON] 2, [PERSON] 1, [PERSON] 2  
and [PERSON] 1  
The Beijing Key Laboratory of New Technology in Agricultural Application, Department of Agricultural Resource and Environment, Beijing University of Agriculture, Beinong Road 7, Huilongguan, Changping District, Beijing 102206, China; [EMAIL_ADDRESS] (Y.J. ); [EMAIL_ADDRESS] (S.S.); [EMAIL_ADDRESS] (Q.L. ); [EMAIL_ADDRESS] (T.L.) Institute of Environment and Sustainable Development in Agriculture, Chinese Academy of Agricultural Sciences, No.12 Zhongguancun South St., Haidian District, Beijing 100081, China; [EMAIL_ADDRESS] (S.S.); [EMAIL_ADDRESS] (X.Z.) Correspondence: [EMAIL_ADDRESS]  
Received: 26 May 2018; Accepted: 16 June 2018; Published: 22 June 2018  
**Featured Application: The results showed that there were differences in Cr\({}^{2+}\) removal of biochar under different pyrolysis temperatures and initial pH conditions. Therefore, it is very important to choose suitable material sources, preparation temperature and pH for the application of biochar in remediation of heavy metal polluted soil. **  
**Abstract:** Rice husk and cotton straw were used to produce biochar under 300, 400, 500, 600, and 700 \({}^{\circ}\)C pyrolysis conditions, and the physicochemical properties of the obtained biochar samples were characterised. The effects of various adsorbent amounts, initial pH, and adsorption time on the Cd\({}^{2+}\) adsorption performance were studied. The results showed that, at increasing pyrolysis temperatures, the biochar yield decreased, the ash content increased, the pH transitioned from acidic/neutral to basic/strongly basic, the biochar aromaticity gradually increased, and the biochar structure became more stable. In contrast, the hydrophilicity and polarity decreased, the specific surface area increased, and the number of oxygen-containing functional groups decreased. All these factors resulted in differences in the Cd\({}^{2+}\) adsorption by the biochar samples. With increasing adsorbent content, the rate of Cd\({}^{2+}\) adsorbed on the biochar gradually increased. The adsorption performance was optimal when the initial solution pH of the rice-husk and cotton-straw biochar samples was 5 and 6, respectively. The shortest time to achieve equilibrium was 30 min for rice-husk biochar, and 20 min for cotton-straw biochar. The Cd\({}^{2+}\) adsorption data for both types of biochar were very well fitted with a pseudo-second-order kinetic model. Ion exchange and cation-\(\pi\) interactions may be the main factors influencing the Cd\({}^{2+}\) adsorption by biochar. At the same time, the large specific surface area of biochar also plays a role in the Cd\({}^{2+}\) adsorption. **Keywords:** rice husk; cotton straw; biochar; adsorption; cadmium  
With the current rapid economic development, larger and larger heavy-metal amounts are being discharged into the environment, resulting in frequent heavy-metal pollution events. Therefore, the treatment of such heavy-metal pollution is very urgent. Heavy metals are relatively stable, and are not easily transformed in the natural environment. Ultimately, heavy metals enter the human body through the food chain, threatening human health [1]. According to the nationwide survey of soil contamination announced in April 2014, cadmium pollution exceeded the standard rate of 7.0% in monitoring sites, exceeding the standard limits of all investigated inorganic pollutants [2]. The long-term consumption of drinking water or agricultural crops containing cadmium exceeding the standard limits will result in toxicity to the respiratory tract, liver, and kidneys. Cadmium poisoning particularly results in osteoporosis, osteomalacia, pain, etc. [3]. Therefore, the removal of heavy metals from the environment is a current research hotspot.