A systematic review of biochar aging and the potential eco-environmental risk in heavy metal contaminated soil.

Biochar is widely accepted as a green and effective amendment for remediating heavy metals (HMs) contaminated soil, but its long-term efficiency and safety changes with biochar aging in fields. Currently, some reviews have qualitatively summarized biochar aging methods and mechanisms, aging-induced changes in biochar properties, and often ignored the potential eco-environmental risk during biochar aging process. Therefore, this review systematically summarizes the study methods of biochar aging, quantitatively compares the effects of different biochar aging process on its properties, and discusses the potential eco-environmental risk due to biochar aging in HMs contaminated soil. At present, various artificial aging methods (physical aging, chemical aging and biological aging) rather than natural field aging have been applied to study the changes of biochar's properties. Generally, biochar aging increases specific surface area (SSA), pore volume (PV), surface oxygen-containing functional group (OFGs) and O content, while decreases pH, ash, H, C and N content. Chemical aging method has a greater effect on the properties of biochar than other aging methods. In addition, biochar aging may lead to HMs remobilization and produce new types of pollutants, such as polycyclic aromatic hydrocarbons (PAHs), environmentally persistent free radicals (EPFRs) and colloidal/nano biochar particles, which consequently bring secondary eco-environmental risk. Finally, future research directions are suggested to establish a more accurate assessment method and model on biochar aging behavior and evaluate the environmental safety of aged biochar, in order to promote its wider application for remediating HMs contaminated soil.

[Distribution Characteristics and Risk Assessment of Tetracycline Antibiotics (TCs) in Soil-Vegetable System with Soil Fertilized with Animal Manure].

The effects of antibiotic contamination on vegetable safety and the ecological risks of soil after returning livestock and poultry manure to the land require sufficient future attention. Tetracycline antibiotics (TCs) are often detected at high concentrations in livestock manure and vegetable production soils. Recently, pot experiments and field investigation methods have often been used to understand the effects of TCs contamination on the vegetable safety and ecological risks of soil, whereas field experiments are employed less frequently. This study investigated the distribution characteristics of TCs in the soil-vegetable system following manure application using a combination of pot and field experiments. The human health risks of the edible parts of Chinese flowing cabbage were assessed using the health risk quotient method based on the acceptable daily intake (ADI-HQ), and the ecological risks of TCs-contaminated soils were evaluated using the risk quotient method associated with the species sensitivity distribution model (SSD-RQ). The results showed that oxytetracycline (OTC) was the major type of TCs in Chinese flowering cabbage based on both the pot and field experiments. The maximum contents (dry weight) of OTC in the aboveground parts of the Chinese flowering cabbage for the pot and field experiments were 29.25 µg·kg-1 and 45.03 µg·kg-1, respectively, whereas those of their underground parts were 87.32 µg·kg-1 and 135.44 µg·kg-1, respectively. Meanwhile, higher contents of TCs were detected in Chinese flowering cabbage collected from the field experiment than those from the pot experiment. OTC was also the major type of TCs in soil from both the pot and field experiments, with their contents up to 604.30 µg·kg-1 and 1013.68 µg·kg-1, respectively. Higher residual contents of three TCs were detected in soils collected from the field experiment than those from the pot experiment. Under the experimental conditions, with the except that OTC in Chinese flowering cabbage from the field experiment would pose medium health risks (HQ>0.1) to children, the contents of three TCs in other treated Chinese flowering cabbage would pose low health risks (HQ ≤ 0.1) to adults and children. In the pot experiments, three TCs present in Chinese flowering cabbage would pose low health risks (HQ ≤ 0.1) to adults and children. Additionally, the TCs in soils with manure application from the pot and field experiments may have posed both moderate or high levels of ecological risks (HQ>0.1 or HQ>1). Therefore, the effects of antibiotic contamination on vegetable safety and their potential ecological risks on soil following manure fertilization need to be given special attention.

Cadmium adsorption behavior of porous and reduced graphene oxide and its potential for promoting cadmium migration during soil electrokinetic remediation.

In this study, a porous reduced graphene oxide (PRGO) carbon nanomaterial was successfully obtained by activation of natural graphite with KOH at high temperature and was applied as an auxiliary electrode in soil electrokinetic remediation to investigate the promoting effect on Cd migration. We found that PRGO contained a large amount of oxygen-containing groups (hydroxyl and carboxyl groups) and exhibited high Cd2+ adsorption efficiency at pH values above 4, achieving a maximum adsorption capacity of 434.78 mg/g for Cd. In addition, PRGO could selectively adsorb Cd, Pb, Cu, and Zn but not K, Na, or Mg from soil solution. The electrokinetic remediation experiment showed that the PRGO auxiliary electrode promoted the migration of Cd and effectively controlled the increase in soil pH near the cathode, possibly due to ion exchange between the surface functional groups on the auxiliary electrode and Cd2+. In addition, the location of the PRGO auxiliary electrode strongly influenced the migration of Cd. For instance, the soil Cd concentration of treatment H-5 was 57.86% lower than that of H-0 at a distance of 5-10 cm from the electrode; however, the soil Cd concentration measured at 0-5 cm for treatment H-5 was 34.84% higher than that of treatment H-0. Our study demonstrated that PRGO could be applied as an auxiliary electrode to promote Cd migration during electrokinetic remediation of Cd-contaminated soil.

Effective lead immobilization by phosphate rock solubilization mediated by phosphate rock amendment and phosphate solubilizing bacteria.

Lead can be immobilized in contaminated soils by phosphate rock (PR) amendment, but its efficiency is generally limited by low solubility of PR. Our study aimed to elucidate whether phosphate solubilizing bacteria (PSB) can promote Pb immobilization through PR solubilization. Results showed that P. ananatis HCR2 and B. thuringiensis GL-1 could effectively solubilize PR by producing citric, glucose, and α-Ketoglutaric acids. In broth assay, phosphate solubilized from PR by PSB rapidly reacted with Pb2+ and formed insoluble lead compounds, as confirmed by scanning electron microscope, energy dispersive X-ray, and X-ray photoelectron spectroscopy. Pot experiment using lettuce (Lactuca sativa L.) and diffusive gradients in thin films (DGT) verified the effectiveness of soil remediation using PR amendment and PSB inoculation, as plant shoot biomass and net photosynthetic rate as well as soil bioavailable phosphate concentration have significantly increased, while the phytoavailability of Pb, Cd, and Zn greatly reduced. This study suggested that PR amendment combined with PSB inoculation could be applied for remediation of agricultural fields contaminated with multiple heavy metals.

[Continuous remediation of heavy metal contaminated soil by co-cropping system enhanced with chelator].

In order to elucidate the continuous effectiveness of co-cropping system coupling with chelator enhancement in remediating heavy metal contaminated soils and its environmental risk towards underground water, soil lysimeter (0.9 m x 0.9 m x 0.9 m) experiments were conducted using a paddy soil affected by Pb and Zn mining in Lechang district of Guangdong Province, 7 successive crops were conducted for about 2.5 years. The treatments included mono-crop of Sedum alfredii Hance (Zn and Cd hyperaccumulator), mono-crop of corn (Zea mays, cv. Yunshi-5, a low-accumulating cultivar), co-crop of S. alfredii and corn, and co-crop + MC (Mixture of Chelators, comprised of citric acid, monosodium glutamate waste liquid, EDTA and KCI with molar ratio of 10: 1:2:3 at the concentration of 5 mmol x kg(-1) soil). The changes of heavy metal concentrations in plants, soil and underground water were monitored. Results showed that the co-cropping system was suitable only in spring-summer seasons and significantly increased Zn and Cd phytoextraction. In autumn-winter seasons, the growth of S. alfredii and its phytoextraction of Zn and Cd were reduced by co-cropping and MC application. In total, the mono-crops of S. alfredii recorded a highest phytoextraction of Zn and Cd. However, the greatest reduction of soil Zn, Cd and Pb was observed with the co-crop + MC treatment, the reduction rates were 28%, 50%, and 22%, respectively, relative to the initial soil metal content. The reduction of this treatment was mainly attributed to the downwards leaching of metals to the subsoil caused by MC application. The continuous monitoring of leachates during 2. 5 year's experiment also revealed that the addition of MC increased heavy metal concentrations in the leaching water, but they did not significantly exceed the III grade limits of the underground water standard of China.

Zinc, cadmium and lead accumulation and characteristics of rhizosphere microbial population associated with hyperaccumulator Sedum alfredii Hance under natural conditions.

A field survey was conducted to study the characteristics of zinc, cadmium, and lead accumulation and rhizosphere microbial population associated with hyperaccumulator Sedum alfredii Hance growing natively on an old lead/zinc mining site. We found significant hyperaccumulation of zinc and cadmium in field samples of S. alfredii, with maximal shoot concentrations of 9.10-19.61 g kg(-1) zinc and 0.12-1.23 g kg(-1) cadmium, shoot/root ratios ranging from 1.75 to 3.19 (average 2.54) for zinc, 3.36 to 4.43 (average 3.85) for cadmium, shoot bioaccumulation factors of zinc and cadmium being 1.46-4.84 and 7.35-17.41, respectively. While most of lead was retained in roots, thus indicating exclusion as a tolerance strategy for lead. Compared to the non-rhizosphere soil, organic matter and total nitrogen and phosphorus content, CEC and water extractable zinc, cadmium, and lead concentration were significantly higher, but pH was smaller in rhizosphere soil. The rhizosphere soil of S. alfredii harbored a wide variety of microorganism. In general, significantly higher numbers of culturable bacteria, actinomycetes, and fungi were found in the rhizosphere compared to bulk soil, confirming the stimulatory effect of the S. alfredii rhizosphere on microbial growth and proliferation. Analyses of BIOLOG data also showed that the growth of S. alfredii resulted in observable changes in BIOLOG metabolic profiles, utilization ability of different carbon substrates of microbial communities in the rhizosphere soil were also higher than the non-rhizosphere, confirming a functional effect of the rhizosphere of S. alfredii on bacterial population.

[Effect of co-planting of Sedum alfredii and Zea mays on Zn-contaminated sewage sludge].

The sewage sludge produced in Guangzhou and other cities contains heavy metals such as Zn which exceeds the national standard for agricultural use and should be taken into consideration. A phyto-treatment system consisted of metal hyperaccumulator Sedum afredii Hance and low-accumulating corn was exploited to phytoextract metals from sludge, in order to reduce heavy metals in sludge and meanwhile to stabilize the sludge and gain innocuous agricultural products. The two plants were co-cropped directly on the sludge plots, the plant biomass and metal uptake were determined as well as the changes of the treated sludge. A pot experiment was conducted to study the interaction mechanisms between the two plant roots. The results of the experiment in plots showed that the efficiency of the phyto-extraction of Zn/Cd by S. alfredii was significantly improved by co-planting and Zn content in S. alfredii reached 9 910 mgx kg(-1), 1.5 folds of that in the mono-crop. Meanwhile the produced corn grain was conformed to the national standards for foods or feeds concerning heavy metals and the treated sludge was biologically stabilized. The results from the pot experiment studying the interaction mechanisms showed that corn roots, separated from S. alfredii with a mesh barrier, decreased pH in the sludge solution, increased DOC and resulted in higher Zn/Cd concentration than that of S. alfredii mono-crop, which caused more Zn/Cd transported to the S. alfredii side and then enhanced the uptake of the heavy metals by the hyperaccumulator.

Selection of appropriate organic additives for enhancing Zn and Cd phytoextraction by hyperaccumulators.

Chelant-enhanced phytoextraction is one of the most promising technologies to remove heavy metals from soil. The key of the technology is to choose suitable additives in combination with a suitable plant. In the present study, laboratory batch experiment of metal solubilization, cress seeds germination were undertaken to investigate the metal-mobilizing capability and the phytotoxicity of organic additives, including ethylene diamine triacetic acid (EDTA), citric acid, acetic acid, oxalic acid, glutamine and monosodium glutamate waste liquid (MGWL) from food industry. Experiments in pots were carried out to study the effects of the additives on Zn and Cd phytoextraction. Furthermore, a leaching experiment with lysimeter was performed to evaluate the environmental risks of additive-induced leaching to underground water. The results showed that EDTA had a strong mobilizing ability for Zn and Cd, followed by mixed reagent (MR) and MGWL. MGWL and acetic acid at 5 mmol equivalent per liter resulted in seed germination index less than 2%. Experiments in pots verified the phytotoxicity of acetic acid and MGWL. Addition of the mixed reagent at 6-10 mmol/kg significantly increased Zn phytoextraction by Thlaspi caerulescens. The same for EDTA and the mixed reagent at 10 mmol/kg by Sedum alfredii. But only mixed reagents could significantly increase Cd phytoextraction by the studied hyperaccumulators. This suggested that the strong chelant was not always the good agent to enhance phytoextraction. S. alfredii combined with 2-10 mmol/kg soil MR was preferred for phytoremediation of Cd/Zn contaminated soils in southern China, this could result in high phytoextraction of Cd/Zn and reduce the leaching risk to underground water than EDTA assisted phytoextration.

Assessing copper thresholds for phytotoxicity and potential dietary toxicity in selected vegetable crops.

Copper pollution in soils is widespread, and its accumulation in crop products could pose a risk on human health. In this paper, bioavailability of added copper (Cu) and critical Cu concentrations in a vegetable garden soil was evaluated for Chinese cabbage (Brassica chinensis L.), pakchoi (Brassica chinensis L.), and celery (Apiumg graveolens L. var. dulce DC) based on human dietary toxicity. The availability of added Cu in the soil decreased with incubation time, and had minimal change after 10-12 weeks. After incubated for 12 weeks, about 60% of added Cu was not extractable by DTPA. The same crops were also grown in sand culture to determine their responses to solution Cu. Shoot growth was significantly inhibited at Cu concentrations above 10 mg kg(-1) in the solution or above 150 mg kg(-1) (DTPA-Cu) in the soil. The sensitivity of the crops to Cu toxicity differed among the three vegetable crops. Copper concentration in shoots and edible parts varied with Cu supply levels and type of the vegetables. Negative correlations (r=-0.90-0.99**) were noted between Cu concentration in shoots and fresh matter yields, but Cu concentrations in the edible parts were positively correlated with available and total Cu in the soil (r=0.91-0.99**). The critical tissue Cu concentrations at 10% shoot DM reduction were 19.4, 5.5, 30.9 mg kg(-1) for Chinese cabbage, pakchoi, and celery, respectively. Based on the threshold of human dietary toxicity for Cu (10 mg kg(-1)), the critical concentrations of total and available Cu in the soil were 430 and 269 mg kg(-1) for pakchoi, 608 and 313 mg kg(-1) for celery, and 835 and 339 mg kg(-1) for Chinese cabbage, respectively.

Differences of cadmium absorption and accumulation in selected vegetable crops.

A pot experiment and a sandy culture experiment grown with three vegetable crops of Chinese cabbage (B. chinensis L., cv. Zao-Shu 5), winter greens (B. var. rosularis Tsen et Lee, cv. Shang-Hai-Qing) and celery (A. graveolens L. var. dulce DC., cv. Qing-Qin) were conducted, respectively. The initial soil and four incubated soils with different extractable Cd (0.15, 0.89, 1.38, 1.84 and 2.30 mg Cd/kg soil) were used for the pot experiment. Five treatments were designed (0, 0.0625, 0.125, 0.250 and 0.500 mg Cd/L) in nutrient solution in the sandy culture experiment. Each treatment in pot and sandy culture experiments was trireplicated. The objectives of the study were to examine Cd accumulation in edible parts of selected vegetable crops, its correlation with Cd concentrations in vegetable garden soil or in nutrient solution, and evaluate the criteria of Cd pollution in vegetable garden soil and in nutrient solution based on the hygienic limit of Cd in vegetables. Cadmium concentrations in edible parts of the three selected vegetable crops were as follows: 0.01-0.15 mg/kg fresh weight for Chinese cabbage, 0.02-0.17 mg/kg fresh weight for winter greens, and 0.02-0.24 mg/kg fresh weight for celery in the pot experiment, and 0.1-0.4 mg/kg fresh weight for Chinese cabbage, 0.1-1.4 mg/kg fresh weight for winter greens, and 0.05-0.5 mg/kg fresh weight for celery in the pot experiment (except no-Cd treatment). The order of the three test vegetable crops for cadmium accumulation in the edible parts was celery > winter greens > Chinese cabbage in both the pot experiment and the sandy culture experiment. Cadmium accumulation in edible parts or roots of the vegetable crops increased with increasing of cadmium concentration in the medium (soil or nutrient solution). And cadmium concentrations in edible parts of the test vegetable crops were significantly linearly related to the Cd levels in the growth media (soil and nutrient solution). Based on the regression equations established and the limit of cadmium concentration in vegetable products, the thresholds of Cd concentration in the growth medium evaluated was as follows: 0.5 mg/kg soil of extractable Cd for soil and 0.02 mg/L for nutrient solution. The high capacity for cadmium accumulation in the edible parts of different vegetable crops together with the absence of visual symptoms implies a potential danger for humans.