Mobilization of Colloid- and Nanoparticle-Bound Arsenic in Contaminated Paddy Soils during Reduction and Reoxidation

Abstract

The association of arsenic (As) with colloidal particles could facilitate its transport to adjacent water systems or alter its availability in soil−rice systems. However, little is known about the size distribution and composition of particle-bound As in paddy soils, particularly under changing redox conditions. Here, we incubated four As-contaminated paddy soils with distinctive geochemical properties to study the mobilization of particle-bound As during soil reduction and subsequent reoxidation. Using transmission electron microscopy−energy dispersive spectroscopy and asymmetric flow field-flow fractionation, we identified organic matter (OM)-stabilized colloidal Fe, most likely in the form of (oxy)hydroxide−clay composite, as the main arsenic carriers. Specifically, colloidal As was mainly associated with two size fractions of 0.3−40 and >130 kDa. Soil reduction facilitated the release of As from both fractions, whereas reoxidation caused their rapid sedimentation, coinciding with solution Fe variations. Further quantitative analysis demonstrated that As concentrations positively correlated with both Fe and OM concentrations at nanometric scales (0.3−40 kDa) in all studied soils during reduction and reoxidation, yet the correlations are pH-dependent. This study provides a quantitative and size-resolved understanding of particlebound As in paddy soils, highlighting the importance of nanometric Fe−OM−As interactions in paddy As geochemical cycling.

Influence of Soil Colloids on the Transport of Cd2+ and Pb2+ under Different pH and Ionic Strength Conditions

“…The second fraction (F2) was identified after 35 min and corresponded to the remaining non-fractionated particles that were larger than 280 kDa but smaller than 450 nm (solutions were 450 nm filtered), and here defined as larger nanoparticles. In this fraction, the signal peaks of OM, Fe, Al, and Mn were found to be neglectable in acidic DBS soil solution; this corresponded with the research by Hu [20], which suggested that acidic conditions would inhibit the formation of colloids in F2. The signal peaks of Fe and Al simultaneously emerged at the same time in HD and SY solutions, indicating a high correlation between Fe and Al.…”

“…Fine nanoparticles were ubiquitous and relatively stable, exhibiting a strong adsorption capacity for Cd and Pb. This fraction was stable and less affected by environmental conditions [20,22], which means that it is an important component of Cd and Pb transport. Larger nanoparticles exhibited high stability in non-acidic solutions, and their adsorption of heavy metals is substantially influenced by their composition.…”

“…Under alkaline conditions of pH 7.8, soil colloids significantly facilitate the transport of arsenic (As) and chromium (Cr), but to a lesser extent facilitate the transport of cuprum (Cu) and cadmium (Cd) [19]. Redox fluctuation results in the dissolution of Fe/Mn-(oxy)hydroxide and sulfide phases and heavy metal release to soil porewater [20]. The colloidal transport of heavy metals in soil is complex, and most conclusions are based on laboratory research, lacking studies under natural conditions.…”

Strong Substance Exchange at Paddy Soil-Water Interface Promotes Nonphotochemical Formation of Reactive Oxygen Species in Overlying Water

“…Furthermore, during the flooding period, Tyndall effects were observed in the three paddy overlying water profiles (Figure S9D), suggesting a colloid existed. This observation may be attributed to the codiffusion of clay-related colloids (Si and Al) with Fe–OM complexes . Redox fluctuations are likely responsible for an enhanced colloidal dispersion and mobilization, particularly in the paddy soil .…”

Multitask Deep Learning Enabling a Synergy for Cadmium and Methane Mitigation with Biochar Amendments in Paddy Soils

“…This model also quantitatively identifies the key factors of the characteristics of biochar that determine the synergy for Cd and CH 4 mitigation, which would be useful for developing guidance and standards for fabricating ideal biochar for paddy soil remediation in practice . Thus, such a multigoal approach through synthesizing two separate data sets by the data-sharing algorithm is likely a promising solution for sustainable agricultural development and can be further extended to other significant goals, such as remediation of Cd-contaminated soils accompanied by other metals (e.g., arsenic and mercury) that has a distinct biogeochemistry and the potential trade-offs between CH 4 and N 2 O mitigation, in future work. Under the context of sustainable development goals, paddy soil remediation needs to balance multiple objectives, such as climate change, carbon footprint, soil quality improvement, and food production safety, while this MTDL model can be further extended to incorporate more independent data sets for achieving the synergy among multiple goals.…”