Phosphorous Retention and Release by Sludge-Derived Hydrochar for Potential Use as a Soil Amendment

To solve both the problems of P deficiency in arable soil and excessive waste sludge disposal, we evaluated hydrothermal carbonization of sludge with the aim of recycling sludge P resources for soil amendment. In contrast with pyrochars obtained through pyrolysis, hydrochars generated from hydrothermal carbonization often feature variable properties and therefore require detailed characterization. In this study, sludge-derived hydrochars were evaluated to determine their P content and fractionation, release and availability of P, and P adsorption and desorption behavior. We also assessed changes in P availability after soil was amended with the hydrochars. Our results showed that the chars were rich in total P (up to 25,175 mg kg −1 ), but most of the free fractions were transformed to bound fractions, thereby reducing the available P concentrations. However, available P content was >417 mg kg −1 , which was far higher than soil demand. The hydrochars shifted from releasing to adsorbing P adsorbent when the environmental P concentration increased above 20 mg L −1 . The hydrochars showed a high phosphate adsorption capacity (up to 23,815 mg kg −1 ) and the adsorbed P could be readily released. The addition of 1% P-laden hydrochar significantly enhanced the soil available P content by 8.93 mg kg −1 . These findings have important implications for further development of hydrochar-based P carriers as a slow-releasing fertilizer. Core Ideas Charring transformed P from readily available fractions to moderately available ones. Sludge-derived hydrochars showed the best performance in long-term P release. Sludge-derived hydrochars had better P retention capacity than their feedstock. Applying the sludge-derived hydrochar improved the available P content of soil.