Comparison of characteristics of twenty-one types of biochar and their ability to remove multi-heavy metals and methylene blue in solution

- The oxygen content, molar O/C and H/C ratio decreased with the increased temperature.
- The removal of MB presented the similar trend with multi-metals removal by biochars.
- Biochars produced at 350 °C showed stronger adsorption of multi-heavy metals.
- Cotton straw biochar from slow pyrolysis showed a higher adsorption of metals.
- Dissolved phosphorus, Mg2 +, Ca2 + and K+ played important roles in metal removal.

The characteristics of twenty-one types of biochars as well as their capacities to remove multi-heavy metals (Cu2 +, Pb2 +, Cd2 +, and Zn2 +) and methylene blue were compared in order to screen biochars applied for pollutants removal. The adsorption mechanisms of heavy metals and methylene blue by biochars were analyzed. Results showed that the oxygen and hydrogen contents, molar O/C and H/C ratios of biochars decreased while the carbon and ash contents, the pH value increased with pyrolysis temperature increased from 350 °C to 550 °C. The removal efficiency of Pb2 + (1.90-30.42%) was higher than Cu2 + (0.34-12.83%) and other metals in 100 mg L− 1 multi-heavy metals solutions. Cotton straw biochar from slow pyrolysis showed the highest removal capacity for Pb2 + and Cu2 + in multi-heavy metals solutions, while wheat straw biochar from gasification showed the highest removal efficiency of methylene blue (96.28%). Dissolved phosphorus and magnesium participated in the precipitate forming with heavy metals and K played an important role in ion exchange with heavy metals.

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