Purpose

Differences in the composition and properties of soil organic matter (SOM) components with different decomposition stages determine their ability and mechanism to sequester heavy metals in contaminated soils. However, very little research has emphasized the adsorption properties of heavy metals on humic acid (HA) versus particulate organic matter (POM).

Materials and methods

X-ray absorption fine structure (XAFS) spectral analysis, in combination with Fourier transform infrared (FTIR) spectroscopy and energy dispersive X-ray spectroscopy (SEM-EDS), was applied to investigate the structures and compositions of tropical secondary soil-derived POM and HA and their adsorption characteristics for lead (Pb).

Results and discussion

Results revealed that HA was characterized by high aromaticity and cation exchange capacity (CEC) and a smoother structure, while POM mainly consisted of aliphatic components with comparable polarities with high H/C and (N+O)/C ratios was embedded with soil minerals (e.g., gibbsite, silicate, and phosphate) and possessed a larger specific surface area and high content of Si-O-Si and free -OH groups. The equilibrium adsorption data for HA followed fit the Langmuir model with a maximum capacity of 72.57 mg g⁻¹ while those for POM the Freundlich model with a maximum capacity of 22.27 mg g⁻¹. Freundlich-n and K_F values suggested that sorption of Pb on POM displayed a nonlinear isotherm and lower initial affinity relative to that on HA. XAFS results showed that two major Pb species similar to Pb(C₂H₃O₂)₂ and PbO were formed in Pb-loaded HA, while four major Pb species similar to Pb-loaded Al₂O₃, Pb(C₂H₃O₂)₂, Pb₃(PO₄)₂, and PbSiO₃ were formed in Pb-loaded POM. Based on desorption and spectroscopic studies, Pb sorption sites on HA were primarily ascribed to metal exchange and surface inner-sphere complexation with phenolic hydroxyl and carboxyl functional groups, while Pb sorption on POM involved coprecipitation, electrostatic cation exchange, and inner-sphere complexation with complexed organic functional groups and mineral oxides, and these processes accounted for 65% and 48% of the total sorbed Pb on HA and POM, respectively, and led to a series of Ca²⁺, Mg²⁺, K⁺, and Na⁺ releases during the sorption process. In addition, the contribution of hydrogen bonding to Pb sorption on HA and POM was significant, accounting for 34% and 46%, respectively. Thus, differences in sorption mechanisms between HA and POM help us choose effective strategies for enhancing Pb(II) removal and environmental risk assessment in aqueous (e.g., wastewater) and soil environments.

中文翻译：

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腐殖酸对土壤中颗粒状有机质中铅吸附特性的比较

目的

具有不同分解阶段的土壤有机质（SOM）组分的组成和性质的差异决定了它们螯合污染土壤中重金属的能力和机理。但是，很少有研究强调腐殖酸（HA）相对于颗粒有机物（POM）的重金属吸附性能。

材料和方法

X射线吸收精细结构（XAFS）光谱分析，结合傅里叶变换红外光谱（FTIR）和能量色散X射线光谱（SEM-EDS），用于研究热带次生土壤衍生的POM的结构和组成和HA及其对铅（Pb）的吸附特性。

结果和讨论

结果表明，HA具有较高的芳香性和阳离子交换容量（CEC）和较光滑的结构，而POM主要由极性相当的脂族成分组成，具有较高的H / C和（N + O）/ C比值，并嵌入土壤矿物中（例如三水铝石，硅酸盐和磷酸盐），并具有较大的比表面积和高含量的Si-O-Si和游离-OH基团。HA的平衡吸附数据符合Langmuir模型，最大容量为72.57 mg g ^-1，而POM的平衡吸附数据为Freundlich模型，最大容量为22.27 mg g ^-1。Freundlich- n和K _F值表明，与HA相比，POM上Pb的吸附表现出非线性等温线和较低的初始亲和力。XAFS结果表明，在Pb负载的HA中形成了两个与Pb（C ₂ H ₃ O ₂）₂和PbO相似的主要Pb物种，而在四个Pb负载的Al ₂ O ₃，Pb（C ₂ H ₃ O ₂）₂，Pb ₃（PO ₄）₂和PbSiO ₃是在含铅的POM中形成的。根据解吸和光谱研究，HA上的Pb吸附位点主要归因于金属交换和与酚羟基和羧基官能团的表面内球络合，而POM上的Pb吸附涉及共沉淀，静电阳离子交换和内球络合。络合的有机官能团和矿物氧化物，这些过程分别占HA和POM上总吸附Pb的65％和48％，并导致一系列Ca 2 ⁺，Mg 2 ⁺，K ⁺和Na ⁺在吸附过程中释放。此外，氢键对HA和POM上Pb吸附的贡献很大，分别占34％和46％。因此，HA和POM之间吸附机制的差异有助于我们选择有效的策略，以增强在水性（例如废水）和土壤环境中Pb（II）的去除和环境风险评估。