Distribution and Bioavailability of Heavy Metals in Soil Aggregates from the Fenhe River Basin, China

Plant litter as a heavy metal migration strategy following application of sewage sludge to subtropical forest soils

The environmental risks of migration of heavy metals (HMs) following applications of sewage sludge (SS) to forest soils are garnering increased attention. Plant litter at the forest floor may modify HM migration pathways through impacts on soil aggregates and water/soil erosion; however, HM migration responses to plant litter are poorly understood. The aim of this study was to determine the effects of plant litter cover on HMs migration, and water and soil erosion following the application of SS to subtropical forest soils. Effects of addition of SS along and SS plus plant litter at 0.75 or 1.5 kg m-2 on the migration of cadmium, chromium, copper, nickel, lead, and zinc in surface runoff, soil interflow, and sediments were quantified across nine simulated rainfall events in a laboratory experiment and following natural intense rain events in a field experiment. Addition of SS elevated HM concentrations in surface runoff by 38.7 to 98.5 %, in soil interflow by 48.3 to 312.5 %, and in sediment by 28.5 to 149.4 %, and increased the production of sediment aggregates <0.05 mm that led to greater cumulative migrations of HMs in surface runoff and sediment; sediment accounted for 89.5 % of HM migrations. Addition of plant litter reduced cumulative migration of HMs by 87.1-97.27 %; however, the higher rate of plant litter led to a decrease in surface runoff and sediment yield, and an increase in soil interflow. Addition of plant litter shifted the main pathway of HM migration from sediment to surface runoff and soil interflow. The potential ecological HM risk index was "low" for each treatment. We found consistency in HM concentrations and migrations via surface runoff between the field and laboratory experiments. Overall, the addition of plant litter with SS mitigated soil erosion and reduced total migration of HMs, resulting in a 88.7-97.3 % decrease in the ecological risk index of the six HMs. We conclude that the addition of plant litter may provide a management strategy for the mitigation of HM risks to environmental safety for the disposal of SS in subtropical forest systems.

An insight on the plausible biological and non-biological detoxification of heavy metals in tannery waste: A comprehensive review

A key challenge for the tannery industries is the volume of tannery waste water (TWW) generated during the processing of leather, releasing various forms of toxic heavy metals resulting in uncontrolled discharge of tannery waste (TW) into the environment leading to pollution. The pollutants in TW includes heavy metals such as chromium (Cr), cadmium (Cd), lead (Pb) etc, when discharged above the permissible limit causes ill effects on humans. Therefore, several researchers have reported the application of biological and non-biological methods for the removal of pollutants in TW. This review provides insights on the global scenario of tannery industries and the harmful effects of heavy metal generated by tannery industry on micro and macroorganisms of the various ecological niches. It also provides information on the process, advantages and disadvantages of non-biological methods such as electrochemical oxidation, advanced oxidation processes, photon assisted catalytic remediation, adsorption and membrane technology. The various biological methods emphasised includes strategies such as constructed wetland, vermitechnology, phytoremediation, bioaugmentation, quorum sensing and biofilm in the remediation of heavy metals from tannery wastewater (TWW) with special emphasize on chromium.

How different are the arsenic fractions inhibit alkaline phosphatases on aggregates scale?

Arsenate [As(V)], in general, is associated with various aggregates and exists as different species in soil, which in turn influences its toxicity and potential contamination. Previous studies have demonstrated the usefulness of alkaline phosphatases (ALP) to evaluate As(V) pollution. However, the effect of different arsenic fractions on ALP among soil aggregates is still unclear. Thus, the distribution of As fractions and ALP kinetics was determined in four-month As-aged paddy soil aggregates. Results revealed the two major fractions of As in aggregates were humic-bound and Fe and Mn oxides-bound [both around 30% under 800 mg kg^-1 of As(V)]. Besides, it was observed that available soil phosphorus could positively affect the relative content of water-soluble, exchangeable and carbonate-bound arsenic. In the kinetics experiment, both the Michaelis-Menten constant (K_m) and maximum reaction velocity (V_max) of ALP increased with increasing As(V) concentration under four months ageing for each size aggregate. Multiple linear stepwise regression analysis between k_cat and the relative content of arsenic fraction indicated that carbonate-bound arsenic is the main fraction that inhibited the k_cat for macroaggregates (> 0.25 mm size). For soil aggregates of 0.1-0.25 mm size, k_cat increased with an increase in arsenic residual fraction. As for aggregates <0.1 mm size, Fe and Mn oxide-bound fraction is the main fraction that inhibited the k_cat. Overall, this study suggests carbonate-bound and Fe and Mn oxide-bound arsenic fractions could decrease the ALP activities via a decrease in the catalytic efficiency in macroaggregates and <0.1 mm size aggregates, respectively. Besides, available phosphorus should be considered as the main factor when assessing As biotoxicity and mobility.

Biochar Application Does Not Improve the Biochemical Properties of Ni Contaminated Soil

Waste disposal, metal plating, refineries, and mining operations frequently contaminate soils with nickel (Ni). We explored the effects of artificial Ni contamination (0, 56, and 180 mg Ni kg^-1) on the soil biochemical indices. The lab experiment also investigated the possible use of kunai grass (Imperata cylindrica) biochar at a 0.75% dry weight basis to alleviate contamination effects. The biochemical indices such as dehydrogenase enzyme activity, acid phosphatase enzyme activity, and soil respiration rates were monitored in three replications. High level of Ni (180 mg kg^-1) suppressed soil respiration rate by 37% and dehydrogenase activity by 62% up to 15 days. The acid phosphatase activity was not affected by Ni levels and was insensitive to Ni contamination. Biochar application to the Ni contaminated soil did not improve the soil's key biological properties. The beneficial effects of biochar could be limited to improvements in soil chemical properties and not on index biological properties.