Preparation of calcium-based phosphate adsorbent and mineral-rich humic acid fertilizer from biomass ash and bamboo by hydrothermal-pyrolysis: Performance and mechanism

Biomass ash (BA) contains alkaline cations such as K, Ca, and Mg. Due to its high pH, direct application to the soil may result in soil salinization. Composting of BA with organic matter is an effective strategy, but the composting cycle is long and there is a large amount of insoluble residue in the product. Therefore, this research proposed for the first time using the hydrothermal method to rapidly convert BA and bamboo powder (BP) into water - soluble fertilizer (WSF) within 4 h. The insoluble hydrothermal residue was further converted into calcium - rich biochar phosphorus adsorption material by a simple pyrolysis process. WSF was neutral and contained humic acid and elements like K, Ca, Mg, and Si. A 14 - day wheat hydroponic experiment showed that the addition of 0.0125% WSF increased the fresh weight of wheat by 18.77% compared with deionized water. The calcium - based biochar adsorbent produced by pyrolysis had an ideal adsorption capacity of up to 113.6 mg P g⁻¹ for phosphate in water, higher than many existing reports. The adsorption mechanisms mainly included surface precipitation, ion exchange, and electrostatic attraction. Moreover, the calcium - rich biochar sample slowly released phosphorus into water after adsorbing phosphate. When the pH was 3 or 4, the removal rate of Pb²⁺, Cd²⁺, and Cu²⁺ at 15 – 20 mg L⁻¹ was as high as 99%. This indicated its potential as a slow - release fertilizer and heavy metal remediation agent. This research provided a new way of thinking for the treatment and disposal of BA.