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[Display omitted] • A novel biochar was prepared by low temperature phosphoric acid activation pyrolysis. • Biochar has large specific surface area and abundant oxygen-containing functional groups. • The obtained biochar has excellent performance in the removal of paclobutrazol from water. • Biochar is rich in N and P nutrients and can promote the growth of mung bean seedlings. • The addition of biochar can reduce the bioavailability and phytotoxicity of paclobutrazol in soil. Using biochar to absorb organic pollutants in water and soil is a simple and efficient method. However, most biochar is typically prepared at high temperatures (>500 ℃), resulting in low yield and high production costs and hence limiting its practical applications. This study aimed to prepare biochar (P-BC) through low-temperature (280 ℃) phosphoric acid assisted pyrolysis of cotton stalks. P-BC possessed a large specific surface area (389.96 m2·g−1) and contained abundant oxygen functional groups as well as phosphorous and nitrogen nutrient elements. This study applied P-BC to remove paclobutrazol (PBZ) from water and remediate PBZ-polluted soil. The water adsorption capacity of P-BC was 192.89 mg·g−1. The adsorption mechanisms of P-BC included pore filling, hydrogen bonding, weak electrostatic effects, and π-π interactions. Adding 1 % P-BC to soil contaminated with 10 mg·kg−1 PBZ could alleviate PBZ induced phytotoxicity in mung bean seedlings, ensuring the normal growth of roots and plants. [ABSTRACT FROM AUTHOR]