Biochar is the carbonized material produced from biomass and is used in several environmental applications. The biochar characteristics depend on the carbonization conditions and feedstock. The suitability of a given biochar for soil improvement depends on the biochar characteristics, soil properties, and target plants. Biochar has been applied at 1-20% (w/w) in the soil, but currently there is a lack of information on what type and concentration of biochar are most suitable for a specific plant and soil quality. Too much biochar will reduce plant growth because of the high alkalinity of biochar, which will cause long-term soil alkalinity. In contrast, too little biochar might be insufficient to enhance plant productivity. In this study, a suitable concentration of cassava stem (an abundant agricultural waste in Thailand) biochar produced at 350 °C was evaluated for green bean (Vigna radiata L.) growth from germination to seed production in pots over 8 weeks. The soil fertility was increased with increasing biochar concentration. At 5% (w/w) biochar, the soil fertility and plant growth were significantly enhanced, while 10% (w/w) biochar significantly enhanced bean growth and bean pod production. The increased biochar concentration in the soil significantly increased the soil total nitrogen and extractable potassium (K) levels but did not affect the amount of available phosphorous. Biochar at 10% (w/w) significantly induced the accumulation of K in the stems, leaves, nut shells, and roots but not in nut seeds. Moreover, biochar not only increased the K concentration in soil but also increased the plant nutrient use efficiency of K, which is important for plant growth. Graphical abstract ᅟ.

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The biochar was prepared from rice husk by pyrolysis at slow pyrolysis at 300, 400, and 500ºC. The obtained biochars were activated by combined physico-chemical methods. An alkaline solution of 1.63 M of KOH was used for chemical pre-activation and made second pyrolysis at the same condition as first pyrolysis. Adsorption efficiency was studied using the batch static method under laboratory conditions. Cd(II) ion removal efficiency of activated biochars was increased from 13-20% to 95-97% over non-activated biochar. The activated biochar at a pyrolysis temperature of 300°C showed the highest Cd(II) ion removal efficiency of 97% at 180 min of contact time. Cd(II) ion adsorption isotherms of activated biochar were fitted well both in Langmuir and Freundlich model. The maximum Cd(II) ion adsorption capacity of activated biochar pyrolysis at 300ºC was 45.87 mg g -1 , about six times higher than that of the non-activated biochar (7.76 mg g -1 ). The activated biochar increased the BET surface area from 7.71 to 11.57 m 2 g -1 . It has been suggested that the simplicity of the physico-chemical activation technique, technically feasible, eco-friendly, and cost effective, can stimulate high Cd(II) ion removal efficiency of rice husk biochar.

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