Despite extensive prior work with batch and spectroscopic methods, little information is available about adsorption thermodynamics of phosphate (P) on Al (oxyhydr)oxides. Here we directly quantified for the first time the adsorption heat of P on gibbsite at varying pH (3.0–9.0) and ionic strength (IS, 1–100 mM) with isothermal titration calorimetry (ITC). The obtained thermograms showed a strong effect of pH but a negligible influence of IS on the adsorption thermodynamics. Coupled with results from other methods including batch experiment, in‐situ attenuated total reflectance Fourier‐transform infrared spectroscopy, and nuclear magnetic resonance, the ITC data revealed contrasting mechanisms for P adsorption at pH 9.0 and 3.0. At pH 9.0, P adsorption was probably initiated by a reaction of P with Al(OH)₄⁻ (originated from the solubilization of gibbsite) with high equilibrium constant (k) and small exothermic enthalpy change (ΔH), followed by a ligand exchange between P and the > Al–O⁻ (Al hydroxyl groups on gibbsite surface) with low k and relatively large exothermic ΔH. In contrast, P adsorption at pH 3.0 likely underwent a fast ligand exchange between P and the > Al‐OH₂⁺ on gibbsite surface with high k and exothermic ΔH and then a slow surface precipitation of P with Al³⁺ due to gibbsite solubilization with low k and endothermic ΔH. This study provides complementary information to the prior knowledge on P adsorption on (hydr)oxides and demonstrates how to use ITC in combination with other methods for mechanistic examination of ions adsorption on the surfaces of soils and sediments.

中文翻译：

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等温滴定量热法可用于检测在各种溶液条件下磷酸盐在三水铝石上的吸附机理

尽管在批处理和光谱学方法方面已有大量的先验工作，但关于磷酸盐（P）在Al（羟基氧化物）氧化物上的吸附热力学的信息很少。在这里，我们首次通过等温滴定热法（ITC）首次直接量化了在pH值（3.0–9.0）和离子强度（IS，1–100 mM）下磷在三水铝石上的吸附热。所获得的热分析图显示出强的pH值影响，但IS对吸附热力学的影响可忽略不计。结合批处理实验，原位衰减全反射傅里叶变换红外光谱和核磁共振等其他方法的结果，ITC数据揭示了pH值为9.0和3.0时P吸附的反差机理。在pH 9.0时，P的吸附可能是由P与Al（OH）_4-的反应引发的^。（源自三水铝石的增溶），具有高的平衡常数（k）和较小的放热焓变（ΔH），随后磷与低k的> Al–O ⁻（三水铝石表面的Al羟基）之间的配体交换和相对较大的放热ΔH。相比之下，pH为3.0时P的吸附可能会在P与高k和放热ΔH的三水铝石表面上的> Al-OH ₂ ⁺之间进行快速的配体交换，然后由于三水铝石的增溶作用而使P与Al ^3+的表面缓慢沉淀。k低且吸热ΔH。这项研究提供了有关P在（氢氧根）氧化物上吸附的先验知识的补充信息，并演示了如何结合使用ITC和其他方法对吸附在土壤和沉积物表面的离子进行机械检查。