Rice husk and cotton straw were used to produce biochar under 300, 400, 500, 600, and 700 °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 Cd²⁺ 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 Cd²⁺ adsorption by the biochar samples. With increasing adsorbent content, the rate of Cd²⁺ 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 Cd²⁺ adsorption data for both types of biochar were very well fitted with a pseudo-second-order kinetic model. Ion exchange and cation–π interactions may be the main factors influencing the Cd²⁺ adsorption by biochar. At the same time, the large specific surface area of biochar also plays a role in the Cd²⁺ adsorption.