Effect of hydroxypropyl-β-cyclodextrin on diuron desorption and mineralisation in soils

Purpose

The herbicide diuron has the unfortunate property of being strongly adsorbed onto soil organic matter particles, and hence, is slowly degraded in the environment because of its reduced bioavailability. The aim of this work was to gain insight into the fate and behaviour of diuron in the soil–water system, and develop and test an environmentally friendly soil decontamination technique that could give rise to an enhancement of diuron mineralisation by sensitive soil endogenous microorganisms, by means of increasing the bioavailability of the pollutant employing cyclodextrin (CD) solutions what would represent an improvement from both economic and environmental standpoints.

Materials and methods

Selected soil colloidal components: montmorillonite, a synthetic humic acid and a synthetic acicular goethite, and two different soils were employed in this study to perform batch adsorption–desorption experiments. Desorption experiments were performed using a 0.01 M Ca(NO₃)₂ solution with and without hydroxypropyl-β-cyclodextrin (HPBCD) 50 mM. Assays to study the mineralisation of ¹⁴C-labelled diuron were performed in respirometers, into which 10 g of soil and 50 mL of mineral salts medium (MMK) were placed, obtaining a final concentration of 50 mg?kg^?1 and a radioactivity of approximately 900 Bq per flask.

Results and discussion

Humic acid could be clearly discerned as the major colloidal component responsible for adsorption. HPBCD was used in diuron desorption experiments from soil, showing a strong extracting power on its removal. The mineralisation of diuron in the presence and absence of HPBCD was tested in a soil managed with diuron for several years, involving therefore the presence of microorganisms that have some specificity for diuron. Natural soil attenuation for diuron was improved when a HPBCD solution was used in the presence of micronutrients as a bioavailability enhancer, obtaining 66 % of mineralisation in comparison to that obtained with only micronutrients addition (44 %).

Conclusions

The use of HPBCD solution at a very low concentration of only 10 times the diuron equimolar concentration in soil, acts as a bioavailability enhancer, accelerating the passage of the diuron-desorbing fraction from the soil particle surface to the soil solution, and hence, improving the accessibility of the microorganisms to the herbicide. Diuron mineralisation rate and the extent of its mineralisation were improved when the HPBCD solution was employed in the presence of micronutrients.     

Treatment additives reduced arsenic and cadmium bioavailability and increased 1,2-dichloroethane biodegradation and microbial enzyme activities in co-contaminated soil

Purpose

Bioremediation of co-contaminated environments is difficult because of the mixed nature of the contaminants and the fact that the two components often must be treated differently. This study investigated the use of inorganic treatment additives, namely calcium carbonate (CaCO₃), gypsum (CaSO₄·2H₂O), and disodium phosphate (Na₂HPO₄) to improve remediation of soil co-contaminated with 1,2-dichloroethane (1,2-DCA) and arsenic or cadmium.

Materials and methods

The soil samples were collected from a specific site in the Westville area in Durban, KwaZulu-Natal, South Africa. Microcosms were set up by artificially co-contaminating the soil sample (100 g mixed with 75 ml of synthetic groundwater in sterile screw-capped 250-ml serum bottles) with 1,2-DCA + risk elements; As³⁺ (150 mg/kg); or Cd²⁺ (170 mg/kg). Thereafter, each microcosm was amended with either 5 g CaCO₃, 2 g CaSO₄·2H₂O, or 1.12 g Na₂HPO₄ + 0.046 g NaCl, separately. The samples were analyzed for the degradation of 1,2-DCA using GC–MS, while total 1,2-DCA degrading bacterial populations were determined at different sampling times using a standard spread plate technique. Soil dehydrogenase and urease activities were also monitored during the experimental period using standard enzyme assays.

Results and discussion

Addition of CaCO₃ resulted in an approximately 2-fold increase in 1,2-DCA degradation in both the As³⁺ and the Cd²⁺ co-contaminated soil as compared to the co-contaminated soil without CaCO₃. All the treatment additives were more effective in the As³⁺ co-contaminated soil resulting in 11.19, 9.25, and 5.63% increase in 1,2-DCA degradation in the presence of CaCO₃, Na₂HPO₄ + NaCl, and CaSO₄·2H₂O, respectively, compared to the Cd²⁺ co-contaminated soil. The total 1,2-DCA degrading bacterial population increased in treated soils over time. Overall, soil dehydrogenase and urease activities were lower in the heavy metal co-contaminated samples compared to the treated soil. The inhibitory effect of heavy metal was less in As³⁺ co-contaminated soil for both CaCO₃- and Na₂HPO₄ + NaCl-treated soil, with up to 7.92% increase in dehydrogenase activity obtained compared to soil co-contaminated with Cd²⁺.

Conclusions

Results from this study indicate that treatment additives can be used to reduce bioavailable fractions of risk elements in the soil matrices, thereby limiting the toxicity of these risk elements to 1,2-DCA degrading microorganisms. Thus, this approach can be applied to enhance organic compound degradation in co-contaminated soil environments.

     

Influence of mixtures of acenaphthylene and benzo[a]anthracene on their degradation by Pleurotus ostreatus in sandy soil

Purpose

Polycyclic aromatic hydrocarbons (PAHs) are a class of organic compounds commonly found as soil contaminants. Fungal degradation is considered as an environmentally friendly and cost-effective approach to remove PAHs from soil. Acenaphthylene (Ace) and Benzo[a]anthracene (BaA) are two PAHs that can coexist in soils; however, the influence of the presence of each other on their biodegradation has not been studied. The biodegradation of Ace and BaA, alone and in mixtures, by the white rot fungus Pleurotus ostreatus was studied in a sandy soil.

Materials and methods

Experimental microcosms containing soil spiked with different concentrations of Ace and BaA were inoculated with P. ostreatus. Initial (t ₀) and final (after 15 days of incubation) soil concentrations of Ace and BaA were determined after extraction of the PAHs.

Results and discussion

P. ostreatus was able to degrade 57.7% of the Ace in soil spiked at 30 mg kg^?1 dry soil and 65.8% of Ace in soil spiked at 60 mg kg^?1 dry soil. The degradation efficiency of BaA by P. ostreatus was 86.7 and 77.4% in soil spiked with Ace at 30 and 60 mg kg^?1 dry soil, respectively. After 15 days of incubation, there were no significant differences in Ace concentration between soil spiked with Ace and soil spiked with Ace + BaA, irrespective of the initial soil concentration of both PAHs. There were also no differences in BaA concentration between soil spiked with BaA and soil spiked with BaA + Ace.

Conclusions

The results indicate that the fungal degradation of Ace and BaA was not influenced by the presence of each other’s PAH in sandy soil. Bioremediation of soils contaminated with Ace and BaA using P. ostreatus is a promising approach to eliminate these PAHs from the environment.     

Pig manure vermicompost (PMVC) can improve phytoremediation of Cd and PAHs co-contaminated soil by Sedum alfredii

Purpose

A major challenge to phytoremediation of co-contaminated soils is developing strategies for efficient and simultaneous removal of multiple pollutants. A pot experiment was conducted to investigate the potential for enhanced phytoextraction of cadmium (Cd) by Sedum alfredii and dissipation of polycyclic aromatic hydrocarbons (PAHs) in co-contaminated soil by application of pig manure vermicompost (PMVC).

Materials and methods

Soil contaminated by Cd (5.53?mg?kg^?1 DW) was spiked with phenanthrene, anthracene, and pyrene together (250?mg?kg^?1 DW for each PAH). A pot experiment was conducted in a greenhouse with four treatments: (1) soil without plants and PMVC (Control), (2) soil planted with S. alfredii (Plant), (3) soil amended with PMVC at 5?% (w/w) (PMVC), and (4) treatment 2?+?3 (Plant?+?PMVC). After 90?days, shoot and root biomass of plants, Cd concentrations in plant and soil, and PAH concentrations in soil were determined. Abundance of PAH degraders in soil, soil bacterial community structure and diversity, and soil enzyme activities and microbial biomass carbon were measured.

Results and discussion

Application of PMVC to co-contaminated soil increased the shoot and root dry biomass of S. alfredii by 2.27- and 3.93-fold, respectively, and simultaneously increased Cd phytoextraction without inhibiting soil microbial population and enzyme activities. The highest dissipation rate of PAHs was observed in Plant?+?PMVC treatment. However, neither S. alfredii nor PMVC enhanced PAH dissipation when applied separately. Abundance of PAH degraders in soil was not significantly related to PAH dissipation rate. Plant?+?PMVC treatment significantly influenced the bacterial community structure. Enhanced PAH dissipation in the Plant?+?PMVC treatment could be due to the improvement of plant root growth, which may result in increased root exudates, and subsequently change bacterial community structure to be favorable for PAH dissipation.

Conclusions

This study demonstrated that remediation of Cd and PAHs co-contaminated soil by S. alfredii can be enhanced by simultaneous application of PMVC. Long-term evaluation of this strategy in co-contaminated field sites is needed.     

pH,nitrogen mineralization,and KCl-extractable aluminum as affected by initial soil pH and rate of vetch residue application: results from a laboratory study

Purpose

Initial soil pH determines the direction and magnitude of pH change after residue addition. This study aimed to evaluate the relative importance of initial soil pH and rate of residue application in determining subsequent pH change, nitrogen (N) mineralization, and soil-exchangeable aluminum (Al).

Materials and methods

An incubation experiment was conducted for 102 days on a Plinthudult soil and a Paleudalf soil, where pH gradients were produced after application of direct current (DC). Rates of vetch applications were 0, 5, 15, 30, and 50 g kg^?1 soil.

Results and discussion

Increasing rates of vetch application caused greater increases in soil pH, but no consistent increase in soil pH at higher initial pH range (4.40～6.74), because of nitrification. There was a positive correlation between alkalinity production and the initial soil pH at day 14, while correlations became negative at days 56 and 102. Mineral N accumulated as NH₄ ⁺–N in low pH soils, due to limited nitrification, while NO₃ ^?–N dominated in higher pH soils. Application of vetch decreased KCl-extractable Al, probably because of complexation of Al by organic matter and precipitation of Al as a result of increased pH, reductions in Al concentration increased with increasing rates of vetch application. However, this amelioration effect on Al concentration weakened with time in higher pH soils.

Conclusions

Application of vetch residue can significantly increase soil pH and concentrations of mineral N and reduce exchangeable Al. These amelioration effects are enhanced with increased rate of vetch addition and vary with time depending on the initial pH of the soil.     

Response surface methodology to understand the anaerobic biodegradation of organochlorine pesticides (OCPs) in contaminated soil—significance of nitrate concentration and bioaccessibility

Purpose

Problems associated with Organochlorine pesticide (OCP)-contaminated soils have received wide attention. To understand the anaerobic biodegradation process constraints, innovative mathematical analysis methods are effective.

Materials and methods

Response surface methodology (RSM) and Tenax TA extraction method combined with the first-three-compartment model were employed to systematically investigate the role of nitrate concentration and bioaccessibility enhancer (methyl-β-cyclodextrin, MCD) in the anaerobic biodegradation of OCPs in contaminated soil.

Results and discussion

The sole addition of either KNO₃ or MCD could facilitate the anaerobic biodegradation of OCPs. The highest biodegradation for total OCPs, hexachlorocyclohexanes, endosulfans, and chlordanes were 71.6, 82.1, 68.3, and 55.6 %, respectively, when 20 mM KNO₃ and 3.0 % (w/w) MCD were applied simultaneously. As predicted by RSM, the theoretical maximum biodegradation for total OCPs ranged from 60 to 80 % when 20 to 25 mM KNO₃ and >2.5 % (w/w) MCD were applied simultaneously. Tenax TA extraction method demonstrated the enhancement of OCP bioaccessibility caused by MCD addition. Changes in the soil microbial activities also suggested the positive effects of adding suitable amounts of KNO₃ as a cosubstrate to facilitate the anaerobic biodegradation of OCPs.

Conclusions

The amount of KNO₃ and MCD are crucial in influencing OCP biodegradation. RSM was demonstrated to be a powerful tool to estimate and predicting the optimal OCP biodegradation under KNO₃ and MCD application simultaneously.     

Speciation of 90Sr and other metal cations in artificially contaminated soils: the influence of bone sorbent addition

Purpose

The influence of bone sorbent addition onto distribution of ⁹⁰Sr in artificially contaminated soil was preliminary studied to assess the possibility of biogenic apatite utilization for reduction of ⁹⁰Sr mobility and availability. Simultaneously, the disruption of soil micro- (Cd, Zn, Co, Cu, Cr, and Ni,) and macroelements (Al, Fe, Mn, K, Mg, and Ca) upon Sr contamination and sorbent addition was monitored.

Materials and methods

The model soil was contaminated by inactive Sr, in the form of Sr(NO₃)₂ solution. As a soil additive, sorbent obtained by annealing bovine bones at 400 °C (B400) was applied. Both the uncontaminated and Sr-contaminated soils were mixed with 1, 3, 5, and 10 % of sorbent, suspended in distilled water (initial pH?5; solid/solution ratio, 1:2), and equilibrated for 15 days on a rotary shaker. Solid residues were subjected to modified Tessier five-step sequential extraction analysis, and the amounts of chosen metals in each fraction were determined by inductively coupled plasma–optical emission spectroscopy.

Results and discussion

In the original soil, Sr was mainly found in exchangeable (61 %) and carbonate phase (16 %), whereas after contamination, the content of Sr in exchangeable phase raised to 94 %. With the addition of B400, the decrease in Sr amounts in exchangeable fraction was detected, whereas increase occurred mainly in operationally defined carbonate phase and in the residual. High level of Sr contamination caused the increase in Zn, Ni, Co, Cu, Cd, and Mn and decrease in Ca content in exchangeable phase. Sorbent addition resulted in a migration of these cations to less soluble fractions. This effect was observed even for major soil elements such as Fe, Al, and Mn, regardless of the excessive amounts of Sr in the soil.

Conclusions

Mixing the soil with B400 resulted in reduced Sr mobility and bioavailability. B400 acted as a stabilizing agent for heavy metals, as well. Apatite distinguished selectivity towards heavy metals may interfere with the Sr immobilization and disrupt original cation distribution. Further studies should include more realistic (lower) Sr concentrations in the soil, different soil types, pH, and longer incubation times.     

Enhanced <Emphasis Type="Italic">Scirpus triqueter</Emphasis> phytoremediation of pyrene and lead co-contaminated soil with alkyl polyglucoside and nitrilotriacetic acid combined application

Purpose

This study aimed to evaluate the effect of combination of alkyl polyglucoside (APG) and nitrilotriacetic acid (NTA) on improving the efficiency of phytoremediation for pyrene and lead (Pb) co-contaminated soil by Scirpus triqueter.

Materials and methods

Seedlings of S. triqueter with a similar size and biomass (3 g/pot) were grown on 2-month aged soil contaminated with 184.5 mg kg^?1of pyrene and 454.3 mg kg^?1 of Pb at pH?=?8.3. After growth for 10 days, different doses of APG and NTA were added into the soil. After 60 days, the height of plants, Pb concentrations in plants, and pyrene amounts in soil were determined.

Results and discussion

Combined application of NTA and APG with lower dosage (1 + 1 g kg^?1 soil and 1 + 2 g kg^?1 soil) had no notable negative influence on the growth of S. triqueter. Moreover, significant synergy on Pb accumulation in S. triqueter was achieved with APG and NTA combined application. Besides, the dissipation of pyrene from soil after 60-day planting was increased in APG and NTA treatments when compared with the control treatments. Application of APG alone or combined with NTA had greater effect on enhancing dissipation of pyrene from soil than NTA alone.

Conclusions

This study demonstrated that the remediation of Pb and pyrene co-contaminated soil by S. triqueter can be enhanced by combined application of APG and NTA. Long-term evaluation of this strategy is needed in co-contaminated field sites.

     

Changes in organic carbon content and its physical and chemical distribution in paddy soils cultivated under different fertilisation practices

Purpose

Chemical protection facilitates soil organic carbon (SOC) sequestration and stabilisation due to a strong chemical binding with mineral surfaces and metal ions (e.g. iron [Fe], aluminium [Al] and calcium [Ca]). However, there is not much information regarding the role of chemical protection in SOC stabilisation in paddy soils, particularly in terms of the specific forms of organo-mineral complexes such as Fe-, Al- and Ca-bonded OC.

Materials and methods

We sampled paddy soils at the 0–20 cm soil layer from a long-term field experiment (initiated in 1981) conducted under humid subtropical conditions in China, which has five fertilisation treatments (i.e. control treatment without fertiliser [CK], chemical fertiliser only [CF], green manure [GM], Straw and Manure) with equivalent nutrient inputs (i.e. N, P₂O₅ and K₂O at the rates of 135–67.5–135 kg ha^?1, respectively, for both early and late rice) except CK. We determined the chemical binding forms of SOC and the associated soil properties in the particulate fraction (PF, >53 μm) and the mineral-associated fraction (MAF, <53 μm), which were obtained using a low-energy ultrasonic dispersion procedure, of a paddy soil in the long-term fertilisation experiment.

Results and discussion

Iron- and Al-bonded OC (Fe/Al-OC) was the dominant fraction and made up 55–70% of the total SOC in the paddy soil, while Ca-bonded OC (Ca-OC) was only a minor fraction (<4%). The Fe/Al-OC was mainly allocated in the MAF (52–67%), indicating that the chemical protection of SOC occurred mostly in the finer particle fractions. Long-term application of organic amendments increased the contents of bulk SOC by 27–34% (P < 0.05), of Fe/Al-OC by 9–16% and of Ca-OC by 35–83% (P < 0.05), whereas the sole application of chemical fertiliser had no significant effects on SOC contents of the paddy soil compared with the treatment without fertiliser inputs. Both amorphous Fe and Al extracted by ammonium oxalate (Fe_ox and Al_ox) showed significant correlations with Fe/Al-OC (r = 0.52 and 0.78, respectively), but Al_ox appeared to have a greater influence on C stabilisation in the paddy soil.

Conclusions

These results demonstrated that the dominant chemical binding forms of SOC in the paddy soils were Fe/Al-OC and amorphous Fe/Al oxyhydrates, especially amorphous Al, contributed mostly to the chemical stabilisation of SOC.

     

The effect of concentrations and properties of phenanthrene, pyrene, and benzo(a)pyrene on desorption in contaminated soil aged for 1 year

Purpose

The choice and timing of microorganisms added to soils for bioremediation is affected by the dominant bioavailable contaminants in the soil. However, changes to the concentration of bioavailable PAHs in soil are not clear, especially when several PAHs coexist. This study investigated the effects of PAH concentration and chemical properties on desorption in meadow brown soil after a 1-year aging period, which could reflect changes of PAH bioavailability during bioremediation.

Materials and methods

Based on the percentage of different molecular weights in a field investigation, high-level contaminated soil (HCS) and low-level contaminated soil (LCS) were prepared by adding phenanthrene (PHE), pyrene (PYR) and benzo(a)pyrene (BaP) to uncontaminated meadow brown soil. The concentrations of HCS and LCS were 250 mg?kg^?1 (PHE, PYR, and BaP: 100, 100, and 50 mg?kg^?1) and 50 mg?kg^?1 (PHE, PYR, and BaP: 20, 20, and 10 mg?kg^?1) respectively. The soils were aged for 1 year, after which desorption was induced by means of a XAD-2 adsorption technique over a 96-h period.

Results and discussion

The range of the rapidly desorbing fraction (F _rap) for PHE, PYR, and BaP in HCS and LCS was from 1.9 to 27.8 %. In HCS, desorption of PYR was most difficult, and the rate constant of very slow desorption (K _vs) of PYR was 8 orders of magnitude lower than that of BaP, which had similar very slow desorbing fractions (49.8 and 50.5 %, respectively). However, in LCS, desorption of PYR was the easiest; the K_vs of PYR was 8–10 orders of magnitude higher than those of PHE and BaP. In HCS, the time scale for release of 50 % of the PAHs was ranked as BaP?>?PYR?>?PHE, while in LCS this was BaP?>?PHE?>?PYR.

Conclusions

The combined effect of PAH concentrations and properties should be taken into account during desorption. The desorption of PAH did not always decrease with increasing molecular weight, and the desorption of four-ring PAHs might be special. These results are useful for screening biodegrading microbes and determining when they should be added to soils based on the dominant contaminants present during different periods, thus improving the efficiency of soil bioremediation.     

Sorption of humic acid on Fe oxides,bacteria, and Fe oxide-bacteria composites

Purpose

Sorption of humic substances on other soil components plays an important role in controlling their function and fate in soil. Sorption of humic substances by individual soil components has been studied extensively. However, few studies reported the sorption characteristic of humic substances on composites of soil components. This study aimed to investigate the sorption characteristics of humic acid on Fe oxide-bacteria composites and improve the understanding on the interaction among humic substance Fe oxide bacteria in soil.

Materials and methods

Humic acid was purchased from Sigma-Aldrich and was purified. Hematite and ferrihydrite were synthesized in the lab. Bacillus subtilis and Pseudomonas putida were cultivated in Luria-Broth medium and harvested at stationary growth phase. Batch sorption experiments were carried out at pH 5.0. Various amounts of humic acid were mixed with 20 mg of Fe oxide, bacteria, or Fe oxide-bacteria composite (oxide to bacteria of 1:1) in 10 mL of KCl (0.02 mol L^?1) to construct sorption isotherms. The effects of phosphate concentration and addition order among humic acid, Fe oxide, bacteria on the sorption of humic acid were also studied. The sorption of humic acid was calculated by the difference between the amount of humic acid added initially and that remained in the supernatant.

Results and discussion

The maximum sorption of humic acid on hematite, ferrihydrite, B. subtilis and P. putida was 73.2, 153.5, 69.1, and 56.7 mg C g^?1, respectively. The maximum sorption of humic acid on examined Fe oxide-bacteria composite was 28.2–57.2 % less than the predicted values, implying that the sorption of humic acid was reduced by the interaction between Fe oxides and bacteria. The presence of phosphate exerted negligible influence on the sorption of humic acid on bacteria while it inhibited the sorption of humic acid on Fe oxides. On Fe oxide-bacteria composites, inhibiting influences followed by promoting or weak inhibiting effects of phosphate with increasing concentration on the sorption of humic acid were found.

Conclusions

The interaction between Fe oxides and bacteria reduced the sorption of humic acid; moreover, the reduction was greater by the interaction of bacteria with ferrihydrite than that with hematite. Phosphate exerted negligible and inhibiting influence on the sorption of humic acid by bacteria and Fe oxides, respectively. On Fe oxide-bacteria composites, humic acid sorption was initially inhibited and then promoted or weakly inhibited by phosphate with increasing concentration.     

Evolution and phosphorus fractionation in saline Spolic Technosols flooded with eutrophic water

Purpose

The aim of this study was to evaluate the behaviour of P in saline Spolic Technosols flooded with eutrophic water, with and without plant rhizosphere, in order to assess the role of these soils as sinks or sources of this nutrient.

Materials and methods

Samples were taken from basic (pH?~7.8), carbonated and acidic (pH?~6.2), de-carbonated soils of salt marshes polluted by mine wastes. Three treatments were assayed: pots with Sarcocornia fruticosa, pots with Phragmites australis and pots without plants (bare soil). The pots were flooded for 15?weeks with eutrophic water (PO ₄ ^3? ~6.92?mg?L^?1) and pH, Eh and water-soluble organic carbon and PO ₄ ^3? concentrations were monitored in the soil solution. A soil P fractionation was applied before and after the flooding period.

Results and discussion

The PO ₄ ^3? concentration in the soil solution decreased rapidly in both soils, with and without plant, being diminished by 80?C90?% after 3?h of flooding. The Fe/Mn/Al oxides and the Ca/Mg compounds played an important role in soil P retention. In pots with S. fruticosa, the reductive conditions due to flooding induced P release from metal oxides and P retention to Ca/Mg compounds. In turn, P. australis may have favoured the release of P from carbonates, which was transferred to Fe/Mn/Al compounds.

Conclusions

The retention of P by the soil was the main mechanism involved in the removal of PO ₄ ^3? from the eutrophic flooding water but to evaluate the capacity of these systems as long-term P sinks, the combined effect of metals, Ca/Mg compounds and specific plant species should be considered.     

Optimization of arsenic and pentachlorophenol removal from soil using an experimental design methodology

Purpose

In this study, a soil-washing process was investigated for arsenic (As) and pentachlorophenol (PCP) removal from polluted soils. This research first evaluates the use of chemical reagents (HCl, HNO₃, H₂SO₄, lactic acid, NaOH, KOH, Ca(OH)₂, and ethanol) for the leaching of As and PCP from polluted soils.

Materials and methods

A Box–Behnken experimental design was used to optimize the main operating parameters for soil washing. A laboratory-scale leaching process was applied to treat four soils polluted with both organic ([PCP] _i ?=?2.5–30 mg kg^?1) and inorganic ([As] _i ?=?50–250 mg kg^?1, [Cr] _i ?=?35–220 mg kg^?1, and [Cu] _i ?=?80–350 mg kg^?1) compounds.

Results and discussion

Removals of 72–89, 43–62, 52–68, and 64–98 % were obtained for As, Cr, Cu, and PCP, respectively, using the optimized operating conditions ([NaOH]?=?1 N, [cocamidopropylbetaine] _i ?=?2 % w w^?1, t?=?2 h, T?=?80 °C, and PD?=?10 %).

Conclusions

The use of NaOH, in combination with the surfactant, is efficient in reducing both organic and inorganic pollutants from soils with different levels of contamination.     

Pilot-scale counter-current acid leaching process for Cu,Pb, Sb,and Zn from small-arms shooting range soil

Purpose

The main objective of this study was to evaluate the potential of a counter-current leaching process (CCLP) on 14 cycles with leachate treatment at the pilot scale for Pb, Cu, Sb, and Zn removal from the soil of a Canadian small-arms shooting range.

Materials and methods

The metal concentrations in the contaminated soil were 904?±?112 mg Cu kg^–1, 8,550?±?940 mg Pb kg^–1, 370?±?26 mg Sb kg^–1, and 169?±?14 mg Zn kg^–1. The CCLP includes three acid leaching steps (0.125 M H₂SO₄?+?4 M NaCl, pulp density (PD)?=?10 %, t?=?1 h, T?=?20 °C, total volume?=?20 L). The leachate treatment was performed using metal precipitation with a 5-M NaOH solution. The treated effluent was reused for the next metal leaching steps.

Results and discussion

The average metal removal yields were 80.9?±?2.3 % of Cu, 94.5?±?0.7 % of Pb, 51.1?±?4.8 % of Sb, and 43.9?±?3.9 % of Zn. Compared to a conventional leaching process, the CCLP allows a significant economy of water (24,500 L water per ton of soil), sulfuric acid (133 L H₂SO₄ t^–1), NaCl (6,310 kg NaCl t^–1), and NaOH (225 kg NaOH t^–1). This corresponds to 82 %, 65 %, 90 %, and 75 % of reduction, respectively. The Toxicity Characteristic Leaching Procedure test, which was applied on the remediated soil, demonstrated a large decrease of the lead availability (0.8 mg Pb L^–1) in comparison to the untreated soil (142 mg Pb L^–1). The estimated total cost of this soil remediation process is 267 US$ t^–1.

Conclusions

The CCLP process allows high removal yields for Pb and Cu and a significant reduction in water and chemical consumption. Further work should examine the extraction of Sb from small-arms shooting range.     

Effects of Single,Binary and Quinary Mixtures of Phenanthrene and Its N-PAHs on <Emphasis Type="Italic">Eisenia fetida</Emphasis> in Soil

It is now acknowledged that aromatic hydrocarbons present in contaminated soils occur in mixtures. The effect of single, binary and quinary mixtures of phenanthrene and selected nitrogen-containing polycyclic aromatic hydrocarbons (N-PAHs) were investigated on the survival, growth and behavioural index of earthworms (Eisenia fetida) over a 21-day incubation in soil. The results showed that the LC₅₀ values ranged from (not detected) ND–329.3 mg kg^?1 (single mixture), ND–219.8 mg kg^?1 (binary mixtures) to 148.4 mg kg^?1 (quinary mixture), while the EC₅₀ values (based on weight loss) ranged from 13.3–148.4 mg kg^?1 (single mixture), 63.8–148.4 mg kg^?1 (binary mixture) to 24.2 mg kg^?1 (quinary mixture). Greater impacts were recorded where N-PAHs are present with phenanthrene. Further, behavioural index of E. fetida was affected after 24-h exposure to N-PAH-amended soils. Among the N-PAHs however, benzo[h]quinoline recorded the greatest impact on the survival, growth and behavioural index of E. fetida in soil. Findings from this study showed that three ring-N-PAHs are more toxic than phenanthrene as expected from their physico-chemical properties. The binary and quinary mixtures of phenanthrene and N-PAHs in soil intensified toxicity, suggesting that PAHs-N-PAHs mixtures represent greater risk to soil biota.     

Toxicity of arsenic in relation to soil properties: implications to regulatory purposes

Purpose

The present work evaluates the influence of different soil properties and constituents on As solubility in laboratory-contaminated soils, with the aim of assessing the toxicity of this element from the use of bioassays to evaluate the soil leachate toxicity and thereby propose soil guideline values for studies of environmental risk assessment in soil contamination.

Materials and methods

Seven soils with contrasting properties were artificially contaminated in laboratory with increasing concentrations of As. Samples were incubated for 4 weeks, and afterwards, soil solution (1:1) was obtained after shaking for 24 h. The soil leachate toxicity was assessed with two commonly used bioassays (seed germination test with Lactuca sativa and Microtox ® test with Vibrio fischeri).

Results and discussion

The relationship between soluble As and soil properties indicated that iron oxides and organic matter content were the variables most closely related to the reduction of the As solubility, while pH and CaCO₃ increased As solubility in the soil solutions. Toxicity bioassays showed significant differences between soils depending on their properties, with a reduction of the toxicity in the iron-rich soil (no observed effect concentration (NOEC)?=?150 mg kg^?1) and a significant increase in the highly carbonate samples (NOEC between 15 and 25 mg kg^?1).

Conclusions

Soil guideline values for regulatory purposes usually set a single value for large areas (regions or countries) which can produce over- or underestimation of efforts in soil remediation actions. These values should consider different levels according to the main soil properties controlling arsenic mobility and the soil leachate toxicity.     

Fractionation of chromium in tannery sludge-amended soil and its availability to fenugreek plants

Purpose

Fenugreek (Trigonella foenum-graecum L.) is a medicinal plant with antidiabetic effects. Chromium has been related to better glucose tolerance in humans. The objective of this study was to determine whether tannery sludge could be used for Cr biofortification of fenugreek.

Materials and methods

Soil was mixed with tannery sludge containing 6.03 g Cr kg^?1. All Cr was in the form of Cr(III). Three treatments were disposed: control without sludge, and two treatments with 10 and 20 g sludge kg^?1, respectively. Control and the 10 g sludge kg^?1 treatments received NPK fertilizer to adjust the concentrations of major mineral nutrients to similar levels in all treatments. Soils were potted and planted with fenugreek. Plants harvested at the initial flowering stage were analysed for total Cr, Fe, Zn and Pb. Sequential soil extraction was applied to obtain operationally defined soil Cr fractions.

Results and discussion

Total Cr in all treatments was below or within the allowable range for agricultural soils (100–150 mg kg^?1). In control soils, most Cr was in the residual fraction (HF/HClO₄ digest). Tannery sludge-amended soils incorporated most Cr into the moderately reducible fraction (oxalic acid/ammonium oxalate extract). In fenugreek shoots, Cr concentrations reached 3.2 mg Cr kg^?1, a higher concentration than that reported for other leafy vegetables. Lead concentrations in plant shoots from this treatment were enhanced but hardly exceeded 1 mg Pb kg^?1.

Conclusions

Tannery sludge-amended soils containing Cr within the range of permissible concentrations can increase shoot Cr in fenugreek. Only sludge with low Pb concentrations should be used for Cr biofortification of fenugreek.     

Early detection of the effects of compaction in forested soils: evidence from selective extraction techniques

Purpose

Soil compaction resulting from mechanisation of forest operations reduces air permeability and hydraulic conductivity of soil and can result in the development of hydromorphic and/or anoxic conditions. These hydromorphic conditions can affect physico-chemical properties of the soils. However, early detection of these effects on mineralogical portion of soils is methodologically difficult.

Materials and methods

To analyse the effects of soil compaction on iron minerals in loamy Luvisol, three compacted and three non-compacted soil profiles up to the depth of 50 cm were collected from an artificially deforested and compacted soils after 2 years of treatment. Soil was compacted with the help of 25 Mg wheeler’s load to increase the dry bulk density of soil from 1.21?±?0.05 to 1.45?±?0.1 g cm^?3. Soil samples were analysed by X-ray diffraction (XRD) and were treated by citrate bicarbonate (CB) and dithionite citrate bicarbonate (DCB) under controlled conditions. Major and minor elements (Fe, Al, Mg, Si and Mn) were analysed by ICP-AES in the CB and DCB extracts.

Results and discussion

It was found that X-ray diffraction is not an enough sensitive method to detect the quick mineralogical changes due to soil compaction. Results obtained from CB-DCB extractions showed that soil compaction resulted in larger CB and smaller DCB extractable elements as compared to non-compacted soil. Labile Fe was found 30 % of total Fe oxides in compacted soil against 10–14 % in non-compacted soils. Compaction thus resulted in Fe transfer from non-labile to labile oxides (s.l.). Results showed that soil compaction leads to the reduction of Fe³⁺ to Fe²⁺. The effects of hydromorphic conditions due to soil compaction were observed up to the depth of 35 cm in forest soil profile. Furthermore, a close association of Al with Fe oxides was observed in the soil samples, while Mn and Si were mainly released from other sources, Mg showing an intermediate behaviour.

Conclusions

Hydromorphic conditions owing to soil compaction affect the mobility and crystallisation process of iron mineral. CB-DCB selective extraction technique, in contrast to XRD technique, can be effectively used to examine the possible effects of soil compaction on iron minerals.

     

Effect of application rate of a nitrification inhibitor,dicyandiamide (DCD), on nitrification rate,and ammonia-oxidizing bacteria and archaea growth in a grazed pasture soil: An incubation study

Purpose

Dicyandiamide (DCD) has been used commercially in New Zealand to reduce nitrate leaching and N₂O emissions in grazed pastures. However, there is a lack of information in the literature on the optimum rate of DCD to achieve the environmental benefits while at the same time reducing the cost of the technology. The objective of this study was to determine the effect of DCD application rate on its effectiveness to inhibit ammonia oxidizer growth and nitrification rate in a grazed pasture soil.

Materials and methods

The soil was a Templeton silt loam (Immature Pallic Soil; Udic Haplustepts) collected from Lincoln University Research Dairy Farm with a mixed pasture consisting of perennial ryegrass (Lolium perenne L.) and white clover (Trifolium repens L.) and was incubated alone (control) or with cow urine at 700 kg N/ha with 6 rates of DCD [0, 2.5, 5, 7.5, 10 (applied twice), 15 and 20 kg/ha] in incubation vessels. The incubation vessels were placed randomly in an incubator with a constant temperature of 12 °C. During 112 days of incubation, soil subsamples were taken at different time intervals to measure the concentrations of NO₃ ^?-N and NH₄ ⁺-N and the amoA gene copy numbers of ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA).

Results and discussion

DCD applied at all the different rates inhibited nitrification in urine-treated soils, but the effectiveness increased with DCD application rate. In addition, AOB growth and the amounts of nitrate-N in the soil were significantly related to the application rate of DCD. However, AOA population abundance showed no relationship to the application rate of DCD. The DCD rate at which the AOB growth rate and nitrate-N concentration were halved (effective dosage that causes 50 % reduction in nitrification rate, or ED₅₀) was about 10 kg DCD/ha.

Conclusions

These results suggest that DCD applied at relatively low rates still slowed down the nitrification rate, and the current recommended rate of 10 kg DCD/ha for DCD use in New Zealand grazed pastures would result in a 50 % reduction in nitrification rate in this soil. The actual rate of DCD application used would depend on the cost of the product and the environmental and agronomic benefits that would result from its use.