Modification and use of biochar from wheat straw (Triticum aestivum L.) for nitrate and phosphate removal from water

Abstract

Surface modification can improve biochar adsorption capacity. Biochars were produced from wheat straw through low-temperature pyrolysis at approximately 450°C, activated with hydrochloric acid (HCl), and coated with iron (FeCl3·6H2O) of different amounts. Results showed that activation with HCl and coating with iron can significantly increase biochar adsorption capacity. The increase after HCl treatment was very small at a high ratio of iron to biochar. The optimal ratio of iron to biochar for iron-coated biochar was 0.70. Adsorption capacity was calculated based on the amount of pure nitrogen or phosphorus. The maximum adsorption capacities were 2.47 and 16.58 mg g−1. The most effective pH values for nitrate and phosphate removal were 3 and 6, respectively. The active substance of the optimal-modified biochar (OMB) was amorphous FeOOH, and the structure [C–Fe(OH2)5](s)3+ was the hydroxylated mineral surface of OMB in a water solution. The equilibrium pH of the adsorption solution was lower than 7 even when the initial pH of the solution was 11 because [C–Fe(OH2)5](s)3+ changed into [C–Fe(OH2)4(OH)](s)2+, [C–Fe(OH2)3(OH)](s)+, and H3O+ with the increase in pH. Adsorption experiments showed that OMB can be utilized to remove nitrate and phosphate from water.