#soil remediation

Bioremediation

Phytostabilization results in:

Phytostabilization aims to reduce mobility and bioavailability of heavy metals not readily degradable without removing mass, establishing persistent vegetative cover immobilizing pollutants in root zone and preventing off-site migration via wind or water. Tolerant grasses Festuca rubra, Agrostis stolonifera and Buchloe dactyloides plus legumes fix nitrogen achieve stabilization through raising rhizosphere pH via carbonates causing precipitation as hydroxides and phosphates, stimulating sulfate-reducing Desulfovibrio producing sulfide precipitating PbS and ZnS with Ksp 10^-28, binding to root cell wall pectic carboxyl and vacuolar storage as phytochelatin complexes via synthase, accumulation of humic matter complexing metals, and hydraulic control limiting infiltration. Accumulation mainly in roots with translocation factor below 0.1 avoiding grazing chain. Outcome is decreased dust, runoff and leaching to groundwater, suitable for large mine tailings and smelter sites where excavation cost-prohibitive, differing from phytoextraction harvesting metals and phytovolatilization releasing to atmosphere, often implemented with soil amendments lime and compost enhancing immobilization long term.

Ref: Mendez & Maier Phytostabilization of mine tailings 2008 Environmental Health; EPA phytostabilization guidance 2000; PubMed immobilization mechanisms.

View Details

Bioremediation

Phytoextraction refers to:

Phytoextraction also termed phytoaccumulation focuses on removal of inorganics heavy metals nickel, zinc, cadmium, lead and metalloids arsenic and selenium from soils via plant uptake into harvestable shoots. Hyperaccumulators Alyssum bertolonii accumulating nickel up to 10,000 mg per kg, Thlaspi caerulescens Noccaea concentrating zinc above 30,000, Brassica juncea for lead achieve concentrations 10 to 500 times soil via high-affinity transporters ZIP ZNT1 and HMA ATPase, chelation with citrate, malate, histidine and sequestration in leaf vacuoles via MTP1 and HMA3. Shoots harvested at flowering when accumulation peaks, biomass ashed at 500 Celsius or processed for phytomining metal recovery via smelting, progressively depleting soil inventory over sequential cropping 5 to 20 seasons. Unlike phytostabilization immobilizing metals in situ reducing leaching or phytovolatilization releasing to atmosphere, phytoextraction physically extracts and concentrates metals into manageable biomass offering sustainable low-disturbance alternative to excavation but requiring long timeframe and careful handling to prevent food chain entry, often combined with soil amendments enhancing bioavailability.

Ref: Baker & Brooks 1989 Terrestrial higher plants hyperaccumulation; Chaney et al. Current Opinion Biotechnology 1997 phytoextraction; EPA extraction guidance.

View Details

Bioremediation

Land farming is characterized by:

Land farming represents oldest, simplest ex situ bioremediation for oily sludges from refineries and crude pits where biodegradable fraction high. Approach involves excavation, spreading soil in thin layer 30 to 50 cm over lined area with berms preventing runoff, periodic tilling every one to two weeks improving aeration 10-fold, breaking clods and mixing nutrients like urea, diammonium phosphate correcting deficiency. Indigenous degraders Pseudomonas aeruginosa, Arthrobacter, Rhodococcus and Aspergillus carrying alkane hydroxylases and catechol cleavage proliferate 10 to 100 fold oxidizing hydrocarbons aerobically with irrigation moisture. Low energy using farm equipment, solar heating, minimal infrastructure yields costs $30 to 60 per ton versus incineration $300 to 1000 per ton or slurry reactors. Limitations include large land requirement 1 acre per 10,000 tons, volatile emissions requiring BTEX air monitoring, unsuitability for high toxic metal or highly chlorinated solvent soils inhibiting biology, and seasonal dependence on temperature and rainfall limiting year-round operation in temperate zones.

Ref: EPA Landfarming Guidance 1994; Khan et al. Journal Environmental Management 2004 landfarm cost; PubMed low-cost remediation.

View Details

Bioremediation

Biosparging primarily promotes:

Principal benefit of biosparging lies in stimulating rapid aerobic biodegradation within otherwise anaerobic groundwater by resolving oxygen limitation that constrains metabolic energetics. Most petroleum hydrocarbon catabolic routes depend on iron-containing monooxygenases and Rieske dioxygenases inserting molecular oxygen into inert C-H bonds of alkanes forming alcohols and aromatic rings forming catechols, reactions impossible without O2 as cosubstrate. Products funneled via beta-oxidation and ortho cleavage into TCA cycle generating high ATP yield. Under anoxia, degradation relies on slower anaerobic activation like fumarate addition to toluene by benzylsuccinate synthase coupled to nitrate or sulfate reduction with lower energy and half-lives years. Oxygen injection raises dissolved oxygen to 6 to 10 mg per L, redox to plus 100 to 200 mV, supports aerobes with low oxygen half-saturation 0.1 mg per L, accelerates mineralization 5 to 20 fold versus natural attenuation, documented by in situ microcosm measuring oxygen utilization, carbon dioxide production and decrease in BTEX half-life from years to months without chemical oxidants.

Ref: Vidali Bioremediation review 2001 aerobic pathways; EPA Biosparging enhances aerobic degradation; PubMed PMID 14767740 rates.

View Details

Bioremediation

Biosparging involves:

Biosparging constitutes in situ groundwater remediation targeting dissolved and sorbed petroleum hydrocarbons in saturated zone where natural oxygen typically depleted below 1 mg per L. Technology injects air, pure oxygen or microbubble oxygen-emitters at pressures slightly above hydrostatic 5 to 15 psi through wells placed 5 to 10 feet below plume. Bubbles rising dissolve oxygen up to 8 to 10 mg per L per Henry's law and partition volatile BTEX upward to vadose zone for capture by soil vapor extraction. Aerobic bacteria Pseudomonas putida and Rhodococcus carrying xylene monooxygenase and catechol pathways exploit oxygen for initial ring activation otherwise rate-limiting. Design accounts for air channeling due to heterogeneity by using closely spaced points 20 to 50 feet apart, pulsed injection preventing biofouling and monitoring dissolved oxygen, oxidation-reduction potential and contaminant concentrations to verify radius of influence. Unlike bioventing treating unsaturated zone, biosparging prevents plume migration and protects downgradient receptors while promoting aerobic mineralization rather than slower anaerobic routes.

Ref: EPA Biosparging Design Guidance 1994; Leeson et al. Air Sparging Principles; PubMed biosparging review PMID 10463012.

View Details

Bioremediation

Bioslurping is limited to approximately:

Bioslurping practical effectiveness limited by physics of lifting liquid column against gravity using available vacuum, which imposes maximum operational depth. Field experience compiled in EPA engineering bulletins and US Army Corps of Engineers guidance indicates product recovery feasible only where water table depth less than approximately 25 feet below ground surface when using typical regenerative blowers 15 to 25 horsepower generating 20 to 30 inches mercury vacuum. Beyond this, hydrostatic head exceeding vacuum capability, increased friction losses in riser pipe, product viscosity particularly weathered diesel, and capillary retention in fine-grained sediments trapping residual NAPL prevent efficient slurping and cause water upconing rather than product collection. Radius of influence declines exponentially with depth due to reduced pressure differential, leading to channeling. Performance curves from demonstration sites at Hill Air Force Base show recovery rate dropping from 10 gallons per day at 10 feet to less than 0.5 at 30 feet. Shallower sites with thick floating layer yield higher initial rates then transition to bioventing polishing phase once free product depleted. Engineering calculations using modified Theis equation for multiphase flow incorporate vacuum limit, guiding selection between bioslurping and more energy-intensive dual-pump or skimmer systems for deeper aquifers exceeding threshold.

Ref: EPA Bioslurping depth limitations 1996; Khan et al. Bioslurping design considerations Bioslurping; USACE depth guidance.

View Details

Bioremediation

Bioventing works by:

Bioventing achieves remediation by supplying air as oxygen source directly into unsaturated vadose zone above capillary fringe where soil pores partially air-filled but contaminated with sorbed fuel in smear zone. Operational layout uses injection wells screened in contaminated interval with oxygen, carbon dioxide and hydrocarbon analyzers plus extraction vents creating pressure gradient drawing atmospheric air at low velocity 0.5 to 5 cfm, ensuring oxygen above 2 percent while limiting volatilization below 20 percent. Indigenous heterotrophs use oxygen for alkane hydroxylase and aromatic dioxygenase initiating mineralization and as electron acceptor for complete oxidation to carbon dioxide via TCA cycle. Biological activity evidenced by O2 drop from 21 to 5 percent and CO2 rise to 5 to 10 percent above background, plus increased microbial counts. Moisture sensors prevent clogging. Unlike biosparging injecting air below water table, bioventing specifically targets unsaturated zone avoiding aggressive stripping, focusing on biological transformation as primary removal, proven cost-effective across thousands of fuel sites under EPA guidance since 1990s.

Ref: EPA Bioventing Manual 1994; Hinchee et al. Bioventing case studies; PubMed bioventing mechanism EA/2.

View Details

Bioremediation

Bioventing is mainly used for remediation of:

Bioventing has emerged over past three decades as leading cost-effective in situ technology for vadose zone soils contaminated with mid-range and light petroleum hydrocarbons such as diesel, jet fuel, gasoline and BTEX compounds frequently leaked from underground storage tanks. Unsaturated soils typically harbor indigenous populations of hydrocarbonoclastic genera Alcanivorax, Pseudomonas, Rhodococcus and Mycobacterium carrying genes alkB alkane monooxygenase, CYP153 cytochrome P450, and catechol dioxygenases requiring molecular oxygen both as terminal electron acceptor and as cosubstrate for initial activation via insertion of one oxygen atom into alkane forming alcohol and aromatic ring forming catechol. Low-flow air injection typically 1 to 2 cubic feet per minute per well provides just enough oxygen to maintain aerobic respiration rates measured by oxygen utilization 0.1 to 1 percent per hour while minimizing stripping and need for off-gas treatment, contrasting with soil vapor extraction designed primarily for volatilization. Nutrients such as ammonium nitrate and phosphate salts and moisture optimization to 40 to 70 percent field capacity further accelerate rates. Heavy metals, radionuclides and chlorinated plastics lacking oxidizable C-H bonds not amenable to oxidative bioventing and require alternative strategies.

Ref: Leeson & Hinchee EPA Bioventing Principles and Practice 1997; EPA Bioventing design manual; PubMed BTEX bioventing.

View Details