[Effect of Citric Acid and Phosphorus Coexistence on Cadmium Adsorption by Soil].

In brown-red soil, the effect of phosphorus and citric acid co-existence on the adsorption of cadmium was studied using indoor experiments and isothermal equilibrium adsorption analysis. After treatment with different doses of phosphorus and citric acid, the fractions of cadmium were altered by varying dry and wet conditions. The results showed that:① Soil treated with 10 mg·L-1 of CdCl2 solution showed no notable effect on cadmium adsorption when a low concentration of phosphorus was added (40 mg·L-1); however, higher a concentration of added phosphorus (80 mg·L-1) significantly increased cadmium adsorption (an increase of 78 g·kg-1 and 7.89% compared to the control treatment); ② Using a 40 mg·L-1 phosphorus solution, the addition of citric acid proportionally reduced cadmium adsorption. This inhibition effect was more notable for the soil treated with low-dose phosphorus (40 mg·L-1) than the high-dose treatment with 1 mmol·L-1 and 5 mmol·L-1 citric acid (cadmium adsorption decreased by 30.89% and 40.97%, respectively). The effect of citric acid was not significant, however, at higher concentrations of phosphorus. When the concentration of citric acid reached 5 mmol·L-1, cadmium adsorption was only 1% lower than without citric acid treatment; ③ Periodic dry-wet alternation significantly promoted the transformation of cadmium from a weak acid extractable and reducible state to an oxidizable and residual state in the soil. That is, the availability of cadmium in soil subjected to the combined action of phosphorus and citric acid decreased with an increase in wet and dry alternations.

[Characteristics of Phosphorus Output Through Runoff on a Red Soil Slope Under Natural Rainfall Conditions].

The development of agriculture in the red soil sloping uplands has been increasingly restricted by low water availability, high temperatures, and low fertilizer use efficiency. Subsurface flow has a significant influence on runoff generation, nutrient loss, and soil erosion. The rainfall-runoff process makes it easy for nutrients on the sloping land to enter water bodies through subsurface flow mainly in the liquid phase, which may lead to environmental problems such as eutrophication and groundwater pollution. Phosphorus as one of the common nutrients causing eutrophication is immobile in the soil because it is easily absorbed and fixed by soil particles. Thus, the principal pathway of phosphorus release from the soil is the surface flow. In some regions, sufficient and concentrated rainfall results in the surface-subsurface flow that enhances phosphorus migration. Recently, researchers have studied the migration patterns of red soil phosphorus through surface flow and the impact factors arising from these migrations, as well as the generation of subsurface flow and its influence on phosphorus outputs. However, there are relatively few investigations that have comprehensively considered the influence of both surface flow and subsurface flow on the migration of red soil phosphorus. In order to investigate the characteristics of phosphorus loss through runoff under natural rainfall, a large-scale field lysimeter experiment was conducted with three treatments i.e., grass cover (GC), litter mulch (LM), and bare land (BL) on a red soil slope land in southeast China. Phosphorus loss through surface flow, interflow at different soil layers (30 cm and 60 cm), and groundwater flow (at 105 cm depth) was observed under each natural precipitation event over a one-year period. The results showed that:① The concentrations of total P (TP) and dissoluble P (DP) in surface flow were slightly higher than those in interflow and groundwater flow; the concentrations of TP and DP showed a gradual downward trend with the increase in soil depth. The total amount of TP runoff loss was ordered as BL (1.61 kg·hm-2) > LM (1.33 kg·hm-2) > GC(0.82 kg·hm-2). ② Surface flow, interflow, and groundwater contributed to 57%, 6%, and 37%, respectively, of the phosphorus runoff loss on BL plot; surface runoff was the main pathway of phosphorus loss. Groundwater flow was the crucial route of phosphorus runoff loss once a vegetation cover was in place; groundwater flow contributed to more than 71% of the phosphorus runoff loss while the surface flow contributed less than 14%. ③ Particulate phosphorus was the primary pattern of phosphorus transport which accounted for 64%-97% of the total amount of phosphorus runoff loss. The effect of phosphorus loss through groundwater flow cannot be neglected on the red soil slope land. The loss load of phosphorus through runoff can be controlled by grass cover and litter mulch treatments, whereas the concentrations of phosphorus in runoff do not significantly reduce.

[Effect of phosphate and organic acid addition on passivation of simulated Pb contaminated soil and the stability of the product].

Organic acids can improve the phosphorus availability, influence the immobilization of heavy metals in soil, and has very complicated function in phosphorus activation and heavy metal passivation. This research took simulated Pb contaminated soil as material, phosphate and citric acid as remediation matter, adopted BCR continuous extraction, 0.01 mol · L(-1) CaCl2 and toxicity characteristic leaching procedure (TCLP) to evaluate the remediation effect. Besides, malic acid and NaNO3 were taken as desorption reagents to discuss the stability of the phosphorus-citric acid-Pb system. The results showed that: in the absence of citric acid, the amount of acid extracted Pb decreased along with the increase of P concentration; when the P concentration was 100 and 400 mg · kg(-1), acid extractable Pb increased with the increasing of citric acid concentration. However, residual Pb changed in the opposite direction from acid extractable Pb. The phenomenon showed that P improved the bioavailability of Pb, while citric acid had the opposite effect. With a certain organic acid concentration, extractable Pb contents extracted by 0.01 mol · L(-1) CaCl2 and TCLP both decreased with the increasing P concentration, therefore, P had immobilization effect on Pb in contaminated soil. But at a fixed P concentration, extractable Pb contents by 0.01 mol · L(-1) CaCl2 and TCLP changed in the opposite trend with the increasing citric acid concentration. The desorption rate of Pb in soil increased with the increasing malic acid concentration, the decreasing pH and the increasing ionic strength. The desorption extent of Pb in soil with P only was lower than that with both P and citric acid. But the stability of Pb passivated by the former was higher.