Using a high biomass plant Pennisetum hydridum to phyto-treat fresh municipal sewage sludge

The spatial distribution characteristics of the biomass residual potential in China

Biomass energy as a kind of renewable energy would be one of the industry's future development direction. As a high energy consumption country, China is urgent need of developing the renewable energy. Understanding the distribution and components of biomass could be beneficial to guide the utilizing technologies and investment strategy of biomass residual. The comprehensive statistical methods were applied to calculate the potential biomass residual of each provinces in China. The results show that (1) Nationwide, the residues biomass of agricultural, forest, and urban waste accounted for 64.16%, 10.88%, and 24.96% of total biomass residual, respectively. The intensity of agricultural, forest, and urban waste biomass residual were 1.89, 0.32, and 0.74 PJ per km² year, respectively. (2) The agricultural biomass residual in eastern China was more abundant than that in western China. The straw residues, agricultural processing residues, livestock manure and pruning residues from permanent orchard respectively shared 32.24%, 10.62%, 56.0%, and 1.13%. (3) The stem wood with its intensity 0.29 PJ per km² year was major contributor of forest biomass residual (with its intensity 0.32 PJ per km² year). The forest biomass residual in northern and southern China was larger than that in eastern and western China, but the intensity of forest biomass residual in southern China was larger than that in the other provinces. (4) The intensity of forest biomass was 0.74 PJ per km² year, which was mainly contributed by urban greenery management outside forests (0.736 PJ per km²). The intensity of urban biomass residual in eastern and southern China was usually larger than that in northern and western China.

Phytoextraction and Migration Patterns of Cadmium in Contaminated Soils by Pennisetum hybridum

This study was conducted to identify soil cadmium (Cd) removal pathways and their contribution rates during phytoremediation by Pennisetum hybridum, as well as to comprehensively assess its phytoremediation potential. Multilayered soil column tests and farmland-simulating lysimeter tests were conducted to investigate the Cd phytoextraction and migration patterns in topsoil and subsoil simultaneously. The aboveground annual yield of P. hybridum grown in the lysimeter was 206 ton·ha^-1. The total amount of Cd extracted in P. hybridum shoots was 234 g·ha^-1, which was similar to that of other typical Cd-hyperaccumulating plants such as Sedum alfredii. After the test, the topsoil Cd removal rate was 21.50-35.81%, whereas the extraction efficiency in P. hybridum shoots was only 4.17-8.53%. These findings indicate that extraction by plant shoots is not the most important contributor to the decrease of Cd in the topsoil. The proportion of Cd retained by the root cell wall was approximately 50% of the total Cd in the root. Based on column test results, P. hybridum treatment led to a significant decrease in soil pH and considerably enhanced Cd migration to subsoil and groundwater. P. hybridum decreases Cd in the topsoil through multiple pathways and provides a relatively ideal material for phytoremediation of Cd-contaminated acid soils.

Silicon-mediated regulation of cadmium transport and activation of antioxidant defense system enhances Pennisetum glaucum resistance to cadmium stress

Pennisetum glaucum is an important forage grass for livestock. However, the large accumulation of cadmium (Cd) in plant tissues increases the risk of heavy metals entering the food chain in Cd-contaminated soils. Silicon (Si) can inhibit cadmium (Cd) uptake and enhance tolerance of plant to Cd toxicity, but whether and how Si alleviates Cd toxicity in grass and the underlying mechanisms are unclear. The present study explored the differential mechanisms of silicon-induced Cd transport in apoplast and symplast, Cd distribution in root tissue and antioxidant defense system in P. glaucum under Cd stress through hydroponic and pot experiments. The present results showed that exogenous Si supply significantly reduced Cd concentrations in apoplast and symplast; Si treatment increased monosilicic acid concentration in apoplast and symplast of the roots and shoots under Cd stress. Elemental analysis of root microdomains showed that Si treatment increased the distribution of Cd and Si in the endodermis by 42.6% and 14.0%, respectively. Si alleviated the adverse influences of Cd on plant growth, which were manifested in root morphological traits and root activity. In addition, Si addition significantly increased the activities of catalase and superoxide dismutase by 37.0% and 72.7%, and improved the efficiency of the ascorbate-glutathione cycle in Cd-stress shoots. Furthermore, Si significantly reduced the contents of hydrogen peroxide and superoxide anion in Cd-stressed shoots by 16.6% and 48.7%, respectively. These findings demonstrate that Si enhances the resistance of P. glaucum to Cd stress through regulating Cd transport pathways and activating antioxidant defense systems.

Irrigating digestate to improve cadmium phytoremediation potential of Pennisetum hybridum

The bioavailability of heavy metal and growth of hyperaccumulator are key factors controlling the phytoextraction of heavy metal from soil. In this study, the efficacy and potential microbial mechanisms of digestate application in enhancing Cd extraction from soil by Pennisetum hybridum were investigated. The results showed that digestate application significantly promoted the height, tiller number, and biomass yield of Pennisetum hybridum. The application also increased the activities of urease, sucrase, dehydrogenase, available Cd contents of rhizosphere soils (from 2.21 to 2.46 mg kg^-1), and the transfer factors of Cd from root to shoot and leaf. Assuming three annual harvests, digestate application would substantially reduce time needed for Pennisetum hybridum to completely absorb Cd from soil-from 15-16 yr-10 yr. Furthermore, the results of microbial community diversity analysis showed that digestate irrigation was more facilitated for the growth of the predominant bacteria, which were Actinobacteria and Chloroflexi at phylum level, and Sphingomonas and Nitrospiraat genus level, which mainly have the functions of promoted plant growth and metal resistance. The results suggested that the enhanced phytoextraction of Cd by Pennisetum hybridum with digestate application might mainly attributed to the increased Cd bio-availability and the enhanced plant growth, indicating that an approach combining digestate and Pennisetum hybridum could be a promising strategy for remediating Cd-contaminated soils.

Potential for phytoremediation of nonylphenol from sewage sludge

Nonylphenol (NP) is considered a major contaminant that must be removed to enable safe and environmentally friendly land application of sewage sludge. Phytoremediation is a technology in which plants are used to remove and/or stabilize organic and inorganic contaminants present in the soil, municipal wastewater, and sewage sludge. In this study, a 391-d large pot experiment was conducted to remove NP from sewage sludge by phytoremediation using Zea mays L. 'Yunshi-5', Lolium perenne L., and co-cropping of the two plants. The fate of NP in the soil under the sewage sludge was assessed at the same time. At the end of the experiment, the NP levels in sludge from the various treatments were as follows: control (38.60%) > L. perenne (31.27%) > Z. mays (16.25%) > co-cropping (15.28%). Degradation followed an availability-adjusted first-order kinetics with a decreasing order of half-lives as follows: control (88.2 d) > L. perenne (87.3 d) > co-cropping (66.2 d) > Z. mays (59.1 d). The results indicated that Z. mays and co-cropping could both degrade NP. The concentrations of NP in tissues of different plants differed significantly. The mean bioconcentration factors for Z. mays and L. perenne were 0.16 and 3.69, respectively. Direct removal of NP from sewage sludge by plant uptake was negligible, as was downward movement of NP in the system. Moreover, NP was not detected in soils in any treatments at harvest.

Transformation of heavy metals and dewaterability of waste activated sludge during the conditioning by Fe2+-activated peroxymonosulfate oxidation combined with rice straw biochar as skeleton builder

This work investigated the improvement performances and mechanisms of waste activated sludge (WAS) dewaterability and the transformation behavior of heavy metals (HMs, including Cu, Zn, Pb, Cd and Cr) by jointly conditioning of Fe²⁺-activated peroxymonosulfate (PMS) oxidation and rice straw biochar (RS-BC). Experimental results showed that at original WAS pH of 6.5, the joint conditioning was the most effective when PMS dosage was 0.6 mmol·(g-volatile solids (VS))^-1, Fe²⁺/PMS molar ratio was 0.6 and RS-BC dosage was 120 mg·(g-VS)^-1. Under this condition, the lowest moisture content (MC) was 38.5% and the standardized-capillary suction time (SCST) was as high as 8.74. For the improvement mechanism, Fe²⁺-activated PMS oxidation can significantly disintegrate the extracellular polymeric substances (EPS) composing WAS to release EPS-bound water, and the RS-BC was helpful to form porous structures to improve WAS compressibility, facilitating the subsequent dewatering. In addition, Fe²⁺-activated PMS oxidation can obviously improve the solubilization and reduce the leaching toxicity of Cu, Zn, Pb, Cd and Cr, which was further enhanced by RS-BC. Therefore, the joint application of Fe²⁺-activated PMS oxidation and RS-BC can be a feasible way to improve WAS dewaterability and reduce HMs risk during WAS dewatering.

Stratified chemical and microbial characteristics between anode and cathode after long-term operation of plant microbial fuel cells for remediation of metal contaminated soils

The plant microbial fuel cell (PMFC) is considered as a sustainable technology in which plants, microbes, and electrochemical cells are the major components and have the synergistic effect on electricity generation. Recent study has demonstrated the use of the PMFC system for remediation of hexavalent chromium (Cr(VI)) contaminated soils; however, the electrokinetic effects, fate of Cr and microbial community shift after long-term operation of PMFCs still need to be unveiled. In this study, PMFCs with spiking 50 mg/kg Cr(VI) were operated over 10 months and chemical and microbial characteristics of different locations of PMFC systems were investigated. Distinct chemical and microbial properties for different locations of soil samples were observed within PMFCs. For instance, the pH values of soils around the cathode and anode (cathode and anode soils) in PMFCs with Chinese pennisetum (Chinese pennisetum PMFCs) were 7.03 ± 0.15 and 6.09 ± 0.05 respectively, showing significantly higher pH values of cathode soils than those of anode soils. The electrical conductivity (EC) of cathode and anode soils in Chinese pennisetum PMFCs was 78.00 ± 5.61 and 156.25 ± 7.89 μs/cm respectively, showing significantly lower ECs of cathode soils than those of anode soils. The total Cr of cathode and anode soils in Chinese pennisetum PMFCs was 65.75 ± 3.77 and 84.29 ± 2.87 mg/kg respectively, showing significantly lower total Cr of cathode soils than that of anode soils. The permutational multivariate analysis of variance test of results of 16S rRNA gene high-throughput sequencing revealed that microbial communities in anode and cathode samples had significant difference in compositions. The stratified chemical and microbial characteristics between anode and cathode were primarily driven by the bioelectrochemical processes and electrokinetic effects within PMFCs. The findings in this study help to better understand the underlying effects of operating PMFCs and will be beneficial for future application of PMFCs in the remediation of heavy metal-contaminated soils.

Post - Mining soil as carbon storehouse under polish conditions

The main aim of these studies was to determine the potential for carbon sequestration in brown coal open-cast mine by phytoremediation using scots pine (Pinus sylvestris L.) and giant miscanthus (Miscanthus x giganteus) plants. This paper presents relationships between soil organic carbon (SOC) sequestration and carbon phytosequestration in waste dump associated with open-cast lignite mine in Central Poland. The research is the continuation of previously carried out experiments, but was conducted in field conditions. In reclamation of post-mining landscapes, during field experiment, an effect of sewage sludge, compost and lake chalk amendments and in combination of plants was investigated. The impact of soil amendments on carbon stock, CO₂ emission reduction, plant biomass production and carbon content in shoots and roots was studied. The highest SOC stock was found in soil treated with sewage sludge (33 Mg*ha^-1) and compost (45 Mg*ha^-1) stabilized by lake chalk. These fertilizer combinations also contributed the most in relation to CO₂ emission reduction through SOC stock (83 Mg*ha^-1 and 127 Mg*ha^-1 respectively). In addition, greater amounts (60-100%) of soil organic matter was converted into humic acids fraction. This phenomenon could be the initial stage of the progressive process of organic matter deposition and carbon sequestration in post-mining area. Carbon phytosequestration was determined through carbon bound in plant tissues. The highest carbon content (60%) in both plant species was recorded in treatments with sewage sludge and compost with lake chalk. Stabilization of compost by lake chalk application was good method to improve the efficiency of carbon sequestration in soil and carbon phytosequestration. Improving the efficiency of these two processes, through skillfully selected soil additives and plant species, may be used on a larger scale in the future as an alternative to the storage of carbon dioxide, especially in degraded areas.

Wetland plant microbial fuel cells for remediation of hexavalent chromium contaminated soils and electricity production

The plant microbial fuel cell (PMFC) is a novel technology which integrates plants, microbes, and electrochemical elements together to create renewable energy. However, information regarding using the PMFC system to remediate metal-contaminated soils is still limited. In this study, we evaluate the potential of PMFC systems to remediate soils polluted by Cr(VI). We compare different plants and different electrode materials with regard to their electricity generation and Cr(VI) removals under different soil Cr(VI) concentrations. In PMFC systems, the soil pH was transformed from slightly acidic to neutral, and the electrical conductivity was reduced during operation. The removal efficiency of Cr(VI) in soils could reach 99%, and the total Cr of soils could also be reduced. The closed circuit voltage of PMFC systems of Chinese pennisetum using the graphite carbon felt as the electrodes could reach the daily average value of 469.21 mV. PMFC systems have successfully demonstrated the ability to remove Cr(VI) from soils collected from actual metal-contaminated sites. Our results suggest that using PMFCs to remediate contaminated soils is promising, and the effects of decontamination are mostly contributed by bioelectrochemical processes and plant uptake.

Effects of landscape plant species and concentration of sewage sludge compost on plant growth, nutrient uptake, and heavy metal removal

Landscape plants have great potentials in heavy metals (HMs) removal as sewage sludge compost (SSC) is increasingly used in urban forestry. We hypothesize that woody plants might perform better in HMs phytoremediation because they have greater biomass and deeper roots than herbaceous plants. We tested the differences in growth responses and HMs phytoremediation among several herbaceous and woody species growing under different SSC concentrations through pot experiments. The mixing percentage of SSC with soil at 0%, 15%, 30%, 60, and 100% were used as growth substrate for three woody (Ficus altissima Bl., Neolamarckia cadamba (Roxb.) Bosser, and Bischofia javanica Bl.) and two herbaceous (Alocasia macrorrhiza (L.) G. Don and Dianella ensifolia (L.) DC) plants. Results showed that the biomass, relative growth rate, and nutrient uptake for all plants increased significantly at each SSC concentration compared to the control; woody plants had higher biomass and nutrient use efficiency than herbaceous plants. All plants growing in SSC-amended soils accumulated appreciable amounts of HMs and reduced the contents of HMs present in the substrates. The woody plants were generally more effective than herbaceous plants in potentials of HMs phytoextraction, but A. macrorrhiza showed higher bioconcentration and translocation of Cu and Zn and D. ensifolia had higher bioconcentration and translocation of Cd than woody plants. The optimal application concentrations were 30% or less for woody plants and 15% for herbaceous plants for plant growth and ecological risk control, respectively. Intercropping suitable woody and herbaceous landscape plants in urban forestry might have promising potentials to minimize the ecological risks in the phytoremediation of SSC.

The transformation behaviors of heavy metals and dewaterability of sewage sludge during the dual conditioning with Fe2+-sodium persulfate oxidation and rice husk

This study focused on the behavior of heavy metals (HMs) in sewage sludge after conditioning based on total HMs concentration, fractionation and leaching tests. Fe²⁺-sodium persulfate (SPS) oxidation was applied as chemical conditioner and rice husk (RH) was added as a physical conditioner to improve the dewaterability of sewage sludge. Combined the response surface methodology analysis and our previous research, the capillary suction time (CST) and the water content of sludge cake reduced by 93.8% and 25%, respectively, after conditioned by 125 mg g^-1 dry solid (DS) SPS, 33 mg g^-1 DS Fe²⁺, 333 mg g^-1 DS RH at original pH of sludge. The HMs analysis indicated that the concentrations of Cu, Pb, Cd, Zn and Cr were increased in liquid phase after conditioning process. And after conditioned by Fe²⁺/SPS with RH, the leaching toxicity reduction are 79%, 100%, 93%, 80% and 68% for Cu, Pb, Cd, Zn and Cr, respectively. Results showed that RH combined with Fe²⁺/SPS oxidation has a synergistic effect on risk reduction and immobilization of HMs. The chemical species of HMs were redistributed and the risk of Pb was reduced from medium risk to no risk after sludge conditioning process according to the risk assessment.

Phytoremediation of sewage sludge contaminated by trace elements and organic compounds

Phytoremediation is a green technique being increasingly used worldwide for various purposes including the treatment of municipal sewage sludge (MSS). Most plants proposed for this technique have high nutrient demands, and fertilization is often required to maintain soil fertility and nutrient balance while remediating the substrate. In this context, MSS could be a valuable source of nutrients (especially N and P) and water for plant growth. The aim of this study was to determine the capacity willow (Salix matsudana, cv Levante), poplar (Populus deltoides × Populus nigra, cv Orion), eucalyptus (Eucalyptus camaldulensis) and sunflower (Helianthus annuus) to clean MSS, which is slightly contaminated by trace elements (TEs) and organic pollutants, and to assess their physiological response to this medium. In particular, we aimed to evaluate the TE accumulation by different species as well as the decrease of TEs and organic pollutants in the sludge after one cropping cycle and the effect of MSS on plant growth and physiology. Since MSS did not show any detrimental effect on the biomass yield of any of the species tested, it was found to be a suitable growing medium for these species. TE phytoextraction rates depended on the species, with eucalyptus showing the highest accumulation for Cr, whereas sunflower exhibited the best performance for As, Cu and Zn. At the end of the trial, some TEs (i.e. Cr, Pb and Zn), n-alkanes and PCBs showed a significant concentration decrease in the sludge for all tested species. The highest Cr decrease was observed in pots with eucalyptus (57.4%) and sunflower (53.4%), whereas sunflower showed the highest Cu decrease (44.2%), followed by eucalyptus (41.2%), poplar (16.2%) and willow (14%). A significant decrease (41.1%) of Pb in the eucalyptus was observed. Zn showed a high decrease rate with sunflower (59.5%) and poplar (52%) and to a lesser degree with willow (35.3%) and eucalyptus (25.4%). The highest decrease in n-alkanes concentration in the sludge was found in willow (98.3%) and sunflower (97.3%), whereas eucalyptus has the lowest PCBs concentration (91.8%) in the sludge compared to the beginning of the trial. These results suggest new strategies (e.g. crop rotation and intercropping) to be adopted for a better management of this phytotechnology.