Sewage sludge is promising for the recovery and utilisation of nutrient components, but its complex nature hinders the release of these components. The combination of pH and thermal modifications shows promise for the release of nutrient components from sludge. However, comprehensive studies on the full spectrum of pH levels and corresponding mechanisms of pH-varying thermal modification are lacking. In this study, the main nutrient components, physicochemical properties, molecular structure, and noncovalent interactions of sludge were comprehensively investigated through pH-varying thermal modification (within a pH range of 2.0 to 12.0 under the same thermal condition). The experimental results showed that the release of main organics, particularly nitrogen (N)-containing organics, was well-fitted, with a tick-like function (: 0.74–0.96). The thermal protons exhibited a notable accumulative mutagenic effect on the N-containing organics release, while the thermal hydroxyl ions had a more direct effect, as revealed by the changes in multivalent metals and molecular structures with the protonation–deprotonation of carboxyl groups. The driving force for the release of N-containing organics was identified as the fluctuation of electrostatic interactions at the solid–liquid interface of the sludge. However, the release of phosphorus (P)-containing substances exhibited a contrasting response to that of N-containing substances with varying pH, likely because the reaction sites of thermal protons and thermal hydroxyl ions for P-containing substances were different. Moreover, high concentrations of thermal protons and hydroxyl ions collapsed the Lifshitz–van der Waals interactions of sludge, resulting in a decrease in viscoelasticity and binding strength. These propositions were further confirmed through statistical analyses of the main indicators of the main nutrient components, physicochemical properties, and noncovalent interactions of sludge. These findings can provide a basis for optimising characteristic-specific methods to recovery nutrient components (N/P) from sludge.

中文翻译：

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变pH热改性对污泥的影响：以释放含氮、含磷物质为重点


污水污泥对于营养成分的回收和利用具有广阔的前景，但其复杂的性质阻碍了这些成分的释放。 pH值和热调节的结合显示出从污泥中释放营养成分的希望。然而，缺乏对全谱pH水平和pH变化热改性相应机制的全面研究。本研究通过变pH热改性（相同热条件下pH范围2.0~12.0）对污泥的主要营养成分、理化性质、分子结构和非共价相互作用等进行了综合研究。实验结果表明，主要有机物特别是含氮有机物的释放量拟合良好，具有类蜱函数（0.74-0.96）。热质子对含氮有机物的释放表现出显着的累积诱变作用，而热氢氧离子具有更直接的影响，这一点通过羧基质子化-去质子化引起的多价金属和分子结构的变化所揭示。含氮有机物释放的驱动力被确定为污泥固液界面静电相互作用的波动。然而，随着pH值的变化，含磷（P）物质的释放表现出与含氮物质相反的反应，这可能是因为含磷物质的热质子和热氢氧根离子的反应位点不同。此外，高浓度的热质子和氢氧根离子破坏了污泥的利夫希茨-范德华相互作用，导致粘弹性和结合强度下降。 通过对污泥主要营养成分、理化性质、非共价相互作用等主要指标的统计分析，进一步证实了这些命题。这些发现可以为优化从污泥中回收营养成分（N/P）的特定特征方法提供基础。