2,4-D abatement from groundwater samples by photo-Fenton processes at circumneutral pH using naturally iron present. Effect of inorganic ions

Infancy of peracetic acid activation by iron, a new Fenton-based process: A review

The exacerbated global water scarcity and stricter water directives are leading to an increment in the recycled water use, requiring the development of new cost-effective advanced water treatments to provide safe water to the population. In this sense, peracetic acid (PAA, CH3C(O)OOH) is an environmentally friendly disinfectant with the potential to challenge the dominance of chlorine in large wastewater treatment plants in the near future. PAA can be used as an alternative oxidant to H2O2 to carry out the Fenton reaction, and it has recently been proven as more effective than H2O2 towards emerging pollutants degradation at circumneutral pH values and in the presence of anions. PAA activation by homogeneous and heterogeneous iron-based materials generates - besides HO• and FeO2+ - more selective CH3C(O)O• and CH3C(O)OO• radicals, slightly scavenged by typical HO• quenchers (e.g., bicarbonates), which extends PAA use to complex water matrices. This is reflected in an exponential progress of iron-PAA publications during the last few years. Although some reviews of PAA general properties and uses in water treatment were recently published, there is no account on the research and environmental applications of PAA activation by Fe-based materials, in spite of its gratifying progress. In view of these statements, here we provide a holistic review of the types of iron-based PAA activation systems and analyse the diverse iron compounds employed to date (e.g., ferrous and ferric salts, ferrate(VI), spinel ferrites), the use of external ferric reducing/chelating agents (e.g., picolinic acid, l-cysteine, boron) and of UV-visible irradiation systems, analysing the mechanisms involved in each case. Comparison of PAA activation by iron vs. other transition metals (particularly cobalt) is also discussed. This work aims at providing a thorough understanding of the Fe/PAA-based processes, facilitating useful insights into its advantages and limitations, overlooked issues, and prospects, leading to its popularisation and know-how increment.

Degradation of 2,4-Dichlorophenol by Nitrilotriacetic acid-modified photo-Fenton system: effects of organic and inorganic factors

It has proved that the photo-Fenton system modified by polycarboxylic acid is effective against the degradation of organic pollutants. Still, its effect and impact on actual water bodies are not clear. Therefore, this study mainly discussed the effect of actual water elements on the degradation of 2,4-Dichlorophenol in photo-Fenton system modified by Nitrilotriacetic acid (NTA) and its mechanism in pure water. The specific research contents were: the effect of initial concentration of 2,4-Dichlorophenol on its degradation efficiency; the effect of organic matters on the degradation of 2,4-Dichlorophenol; the effect of cations and anions; the effect of different actual water bodies. And the main results were as follows: In the effect of initial concentration, when the concentration of 2,4-Dichlorophenol was 20 mg·L^-1, the degradation efficiency was the best (reached 100%). But, with the increase of initial concentration, the degradation efficiency of the system became worse and worse; the coexistence of the same kind of organic compounds can inhibit each other's degradation, and the degradation rate of pollutants in the mixed system was slower than that in the single system; the addition of anions and cations inhibited the degradation of 2,4-Dichlorophenol, and the degradation efficiency varied with the concentration of ions, in which the effect of anions was more complex; the degradation efficiency of 2,4-Dichlorophenol in three kinds of actual water bodies was lower than in deionized water, especially in PPMW. However, the degradation rates of DSTP and NLW were the fastest in the first 20 min.

Co-catalysis of trace dissolved Fe(iii) with biochar in hydrogen peroxide activation for enhanced oxidation of pollutants

Activation of hydrogen peroxide (H₂O₂) with biochar is a sustainable and low-cost approach for advanced oxidation of organic pollutants, but faces the challenge of a low yield of hydroxyl radical (˙OH). Herein, we hypothesize that the activation efficiency of H₂O₂ can be enhanced through co-catalysis of trace dissolved iron (Fe) with biochar. Two biochar samples derived from different feedstock, namely LB from liquor-making residue and WB from wood sawdust, were tested in the co-catalytic systems using trace Fe(iii) (0.3 mg L⁻¹). The cumulative ˙OH production in [Fe(iii) + LB]/H₂O₂ was measured to be 3.28 times that in LB/H₂O₂, while the cumulative ˙OH production in [Fe(iii) + WB]/H₂O₂ was 11.9 times that in WB/H₂O₂. No extra consumption of H₂O₂ was observed in LB/H₂O₂ or WB/H₂O₂ after addition of trace Fe(iii). Consequently, the reaction rate constants (k_obs) for oxidation of pollutants (2,4-dichlorophenoxyacetic acid and sulfamethazine) were enhanced by 3.13–9.16 times. Other iron species including dissolved Fe(ii) and iron minerals showed a similar effect on catalyzing 2,4-D oxidation by biochar/H₂O₂. The interactions involved in adsorption and reduction of Fe(iii) by biochar in which the defects acted as electron donors and oxygen-containing functional groups bridged the electron transfer. The fast regeneration of Fe(ii) in the co-catalytic system resulted in the sustainable ˙OH production, thus the efficient oxidation of pollutants comparable to other advanced oxidation processes was achieved by using dissolved iron at a concentration as low as the concentration that can be found in natural water.

The yield of ˙OH and oxidation of pollutants by biochar/H₂O₂ were enhanced dramatically by trace dissolved Fe(iii).

Removal of microcontaminants by zero-valent iron solar processes at natural pH: Water matrix and oxidant agents effect

This work deals with microcontaminants (MCs) removal by natural solar zero-valent iron (ZVI) process at natural pH in actual matrices. Commercial ZVI microspheres were selected as ZVI source and hydrogen peroxide and persulfate were used as oxidant agents. The experimental plan comprised the evaluation of sulphates and carbonates/bicarbonates effect on process performance, the possibility of adding an iron chelate (EDDS) to take advantage of leached iron and the treatment of MCs in actual MWWTP secondary effluent. The presence of sulphates and EDDS addition did not lead to significant changes in the process efficiency, while the carbonates naturally present in natural water (458 mg/L) diminished the treatment time need to reach the decontamination goal. Finally, the treatment of a MCs mixture consisting of Atrazine, Carbendazim, Imidacloprid, and Thiamethoxam in the range of μg/L in actual MWWTP secondary effluent by solar/msZVI/H₂O₂ and solar/msZVI/S₂O₈^2- obtained 7 and 22% of total removal after 180 min, respectively, which indicated a moderate competitiveness of these processes with respect to other advanced oxidation processes.

Evaluation and comparison of advanced oxidation processes for the degradation of 2,4-dichlorophenoxyacetic acid (2,4-D): a review

Organochlorine pesticides have generated public concern worldwide because of their toxicity to human health and the environment, even at low concentrations, and their persistence, being mostly nonbiodegradable. The use of 2,4-dichlorophenoxyacetic acid (2,4-D) has increased in recent decades, causing severe water contamination. Several treatments have been developed to degrade 2,4-D. This manuscript presents an overview of the physicochemical characteristics, uses, regulations, environmental and human health impacts of 2,4-D, and different advanced oxidation processes (AOPs) to degrade this organic compound, evaluating and comparing operation conditions, efficiencies, and intermediaries. Based on this review, 2,4-D degradation is highly efficient in ozonation (system O₃/plasma, 99.8% in 30 min). Photocatalytic, photo-Fenton, and electrochemical processes have the optimal efficiencies of degradation and mineralization: 97%/79.67% (blue TiO₂ nanotube arrays//UV), 100%/98% (Fe²⁺/H₂O₂/UV), and 100%/84.3% (MI-meso SnO₂), respectively. The ozonation and electrochemical processes show high degradation efficiencies, but energy costs are also high, and photocatalysis is more expensive with a separation treatment used to recover the catalyst in the solution. The Fenton process is a viable economic-environmental option, but degradation efficiencies are often low (50-70%); however, they are increased when solar UV radiation is used (90-100%). AOPs are promising technologies for the degradation of organic pollutants in real wastewater, so evaluating their strengths and weaknesses is expected to help select viable operational conditions and obtain optimal efficiencies.

Developments in the intensification of photo-Fenton and ozonation-based processes for the removal of contaminants of emerging concern in Ibero-American countries

Contaminants of emerging concern (CECs), such as pharmaceuticals, personal care products, pesticides, synthetic and natural hormones and industrial chemicals, are frequently released into the environment because of the inability of conventional processes in municipal wastewater treatment plants to remove them. Some examples of alternative options to remove such pollutants are photo-Fenton and ozone-based processes, which are two techniques widely studied in Ibero-American countries. In fact, this region has been responsible for delivering frequently publications and conferences on advanced oxidation processes. This work is a critical review of recent developments in the intensification of the two aforementioned advanced oxidation techniques for CECs elimination in the Ibero-American region. Specifically for the photo-Fenton process (pF), this study analyses strategies such as iron-complexation with artificial substances (e.g., oxalic acid and ethylenediamine-N,N'-disuccinic acid) and natural compounds (such as humic-like substances, orange juice or polyphenols) and hybrid processes with ultrasound. Meanwhile, for ozonation, the enhancement of CECs degradation by adding hydrogen peroxide (i.e., peroxone), ultraviolet or solar light, and combining (i.e., photolytic ozonation) with catalysts (i.e., catalytic ozonation) was reviewed. Special attention was paid to how efficient these techniques are for removing contaminants from water matrices, and any potentialities and weak points of the intensified processes.

Effects of system parameters and residual ions on the oxidation removal of NO by Fenton method

In the present work, the effects of relevant system parameters on the oxidation removal of NO using the Fenton method were discussed in detail. Moreover, the impacts of ions remaining in the coal-fired process on the NO oxidation efficiency were investigated specifically. The experimental results showed that the oxidation efficiency of NO decreased with the increase of gas flow rate, reagent temperature, and CO₂ volume fraction in the evaluated range, while it increased first and then decreased with the increase of gas temperature, NO initial concentration, O₂ volume fraction, initial pH of reagent, and Fe²⁺/H₂O₂ molar ratio. In addition, the corresponding impact mechanism of the system parameters was discussed respectively. Although the SO₂ showed a competitive effect on the utilization of oxidative radicals, the Fenton system also showed an ability for simultaneous removal of NO and SO₂. Furthermore, the results indicated that the NO oxidation efficiency would be influenced by the residual ions, such as Ca²⁺, Mg²⁺, Na⁺, SO₄^2-, and Cl^-. The presence of the mentioned ions showed an inhibiting effect on the oxidation removal of NO in the first few minutes, while the NO oxidation efficiency would be enhanced in the bulk stage of the tests. The positive effect trended to be more obvious with the decline of the ion dosage. Subsequently, the influence mechanism of the aforesaid residual cations and anions was supposed and proposed preliminarily.

Resistance and induction of viable but non culturable states (VBNC) during inactivation of E. coli and Klebsiella pneumoniae by addition of H2O2 to natural well water under simulated solar irradiation

Inactivation of E. coli and Klebsiella pneumoniae by addition of H₂O₂ 10 mg L^-1 into natural well water samples containing natural total iron concentrations (around 0.3 mg L^-1) under simulated solar light was followed by bacterial culturability (plate count) and viability (DVC-FISH). Results showed that culturability of both bacteria was totally reduced while viability was only completely depleted for E. coli in well water samples depending of total iron concentration. Post-irradiation effects in presence of residual H₂O₂ showed that viability of both bacteria kept dropping being totally reduced for E. coli cells while K. pneumoniae decreased only 1-log. SEM micrographs showed that E. coli and K. pneumoniae cells underwent morphological changes and size reduction according to VBNC states. Different dark and photo-induced processes where physical-chemical features of groundwater samples play an important role could be responsible of bacteria abatement.

Degradation of 2,4-Dichlorophenol by Ethylenediamine-N,N′-disuccinic Acid-Modified Photo-Fenton System: Effects of Chemical Compounds Present in Natural Waters

This paper describes a study of the treatment of 2,4-dichlorophenol (2,4-DCP) with an ethylenediamine-N,N′-disuccinic-acid (EDDS)-modified photo-Fenton system in ultrapure water and different natural waters. The results showed that the EDDS-modified photo-Fenton system is adequate for 2,4-DCP degradation. Compared with a medium containing a single organic pollutant, the removal of pollutants in a more complex medium consisting of two organic compounds is slower by around 25 to 50% as a function of the organic pollutant. Moreover, 2,4-DCP can be further effectively degraded in the presence of organic materials and various inorganic ions. However, the photodegradation of 2,4-DCP in different natural waters, including natural lake water, effluent from domestic sewage treatment plants, and secondary effluent from pulp and paper mill wastewaters, is inhibited. Chemical compounds present in natural waters have different influences on the degradation of 2,4-DCP by adopting the EDDS-modified photo-Fenton system. In any case, the results obtained in this work show that the EDDS-modified photo-Fenton system can effectively degrade pollutants in a natural water body, which makes it a promising technology for treating pollutants in natural water bodies.

A phytotoxicity assessment of the efficiency 2,4-D degradation by different oxidative processes

Highly stable compounds such as dichlorophenoxyacetic acid (2,4-D), which are present in the commercial herbicide Tordon®, are difficult to degrade. Therefore, the objective of this work was to degrade 2,4-D present in Tordon® using different oxidative processes, such as anodic oxidation, Fenton reactions, electro-Fenton, photoelectro-oxidation and photoelectro-Fenton processes. An oxide electrode with the nominal composition Ti/Ru_0.3Ti_0.7O₂ was prepared and inserted into an electrochemical cell containing 100 mL of 0.05 mol L^-1 Na₂SO₄ (pH 3) and 100 mg L^-1 of Tordon®. Electrolysis was performed applying a constant current of 50 mA cm^-2 for 2 h. The concentration of Fenton's reagent was varied from 5 to 10 mg L^-1 (Fe²⁺), and from 50 to 100 mg L^-1 (H₂O₂). Chemical analyses of the total organic carbon (TOC) and the chemical oxygen demand (COD) were performed whilst high-performance liquid chromatography (HPLC) was used to monitor the degradation. A phytotoxicity assessment was performed using cucumber _{seeds as bioindicators. Germination tests were performed using cucumber seeds in the presence of the solutions collected after the application of the oxidative processes. Several analyses were carried out to determine the following: total protein content, the extent of lipid peroxidation, activity of superoxide dismutase, hydrogen peroxide content, catalase activity, and glutathione reductase activity. Oxidation of 2,4-D was observed in all of the oxidative processes, and significant results for the removal of TOC and COD were obtained. Anodic oxidation, and the Fenton and photoelectro-oxidation processes were the least efficient, affording 2,4-dichlorophenol, 2-chlorohydroquinone, and 2-chlorobenzoquinone as by-products. The electro-Fenton and photoelectro-Fenton processes were the most efficient, giving short-chain acids as the main by-products. The formation of these by-products directly affected the phytotoxicity results. The processes that formed short-chain by-products did not generate significant oxidative stress during seed growth, and therefore, seed germination tests were successful.}

Oxidation of tetracycline and oxytetracycline for the photo-Fenton process: Their transformation products and toxicity assessment

Advanced oxidation processes have gained significant attention for treating tetracycline (TC) and oxytetracycline (OTC), however, their oxidation using the photo-Fenton process has not been sufficiently studied. Although degradations of TC and OTC were enhanced by increasing H₂O₂ and Fe²⁺ within the ranges investigated (H₂O₂ = 20-50 mg/L and Fe = 1-10 mg/L) under UV irradiation, further experiments for the photo-Fenton process were conducted with 20 mg/L of H₂O₂ and 5 mg/L of Fe²⁺ to balance efficiency and cost. The photo-Fenton process (UV/H₂O₂/Fe²⁺) was shown to be more effective to remove TC and OTC than H₂O₂, ultraviolet (UV), and UV/H₂O₂ at the same doses of oxidants. Inorganic anions and cations were shown to inhibit the degradation of TC and OTC during the photo-Fenton process, in the following order: HPO₄^2- > HCO₃^- ≫ SO₄^2- > Cl^- and Cu²⁺ ≫ Ca²⁺ > Na⁺. The TC and OTC degradation are generally improved by increasing pH, which is opposite to the k_pCBA,obs values, caused by increasing the deprotonation degree of TC and OTC. Four and nine transformation products of TC and OTC, respectively, were detected over the treatment period. Among the transformation products, m/z 443.14 (C₂₂H₂₂N₂O₈) formed during TC degradation, and m/z 433.16 (C₂₀H₂₀N₂O₉) and m/z 415.15 (C₂₀H₁₈N₂O₈) formed during OTC degradation, were reported for the first time. Vibrio fischeri toxicity assessment indicated that the inhibition ratio was decreased with a decreasing TC concentration, while, OTC transformation lead to higher toxicity. The product (m/z 477.15b) was determined to be the compound causing toxicity during degradation of OTC by using the quantitative structure activity relationship (QSAR). This toxic transformation product caused higher inhibition ratios than its parental compound (OTC), but its further oxidization resulted in decreasing the inhibition ratios.

Application of heterogeneous photo-Fenton process for the mineralization of imidacloprid containing wastewater

The imidacloprid was mineralized by heterogeneous photo-Fenton process in a three-phase fluidized bed reactor using waste iron oxide as catalyst. The effects of catalyst loading, dosage of H₂O₂ and pH were investigated to determine the optimal experiments conditions. The results revealed that TOC removal efficiency increases with an increase in H₂O₂ dosage of up to 105.0 mM, an increase in catalyst dosage from 1.0 to 5.0 g L^-1, and a decrease in pH from 5.0 to 3.5. Under the optimal conditions, 97.7% TOC removal was achieved in 6 h under 254-nm UV irradiation. Moreover, recycling experiments indicated that the waste iron oxide had a good stability and the TOC removal of pesticide yielded more than 80% under the fourth recycles.

Occurrence and transformation of phenoxy acids in aquatic environment and photochemical methods of their removal: a review

The article presents the behavior of phenoxy acids in water, the levels in aquatic ecosystems, and their transformations in the water environment. Phenoxy acids are highly soluble in water and weakly absorbed in soil. These highly mobile compounds are readily transported to surface and groundwater. Monitoring studies conducted in Europe and in other parts of the world indicate that the predominant phenoxy acids in the aquatic environment are mecoprop, 4-chloro-2-methylphenoxyacetic acid (MCPA), dichlorprop, 2,4-dichlorophenoxyacetic acid (2,4-D), and their metabolites which are chlorophenol derivatives. In water, the concentrations of phenoxy acids are effectively lowered by hydrolysis, biodegradation, and photodegradation, and a key role is played by microbial decomposition. This process is determined by the qualitative and quantitative composition of microorganisms, oxygen levels in water, and the properties and concentrations of phenoxy acids. In shallow and highly insolated waters, phenoxy acids can be decomposed mainly by photodegradation whose efficiency is determined by the form of the degraded compound. Numerous studies are underway on the use of advanced oxidation processes (AOPs) to remove phenoxy acids. The efficiency of phenoxy acid degradation using AOPs varies depending on the choice of oxidizing system and the conditions optimizing the oxidation process. Most often, methods combining UV radiation with other reagents are used to oxidize phenoxy acids. It has been found that this solution is more effective compared with the oxidation process carried out using only UV.

Photo-Fenton process at natural conditions of pH, iron, ions, and humic acids for degradation of diuron and amoxicillin

Effect of ferric ions at concentrations typically found in natural waters (0.05 to 1.06 mg L^-1) and low H₂O₂ concentrations (between 0.5 and 17.9 mg L^-1) on simulated sunlight-induced (300 W m^-2) photo-Fenton degradation at initial neutral pH (7.0) of amoxicillin and diuron in Milli-Q water was studied using an rotatable central composite experimental design 2² with a central and two axial points. H₂O₂ concentration was the parameter playing the key role on the degradation of both pollutants. Despite that initial pH was 7.0 in Milli-Q water, this latter decreased rapidly in the first minutes, reaching values of 3.5 and 5.0 for diuron and amoxicillin respectively after 15 min of simulated sunlight irradiation. In contrast, in presence of bicarbonate/carbonate (HCO₃^-/CO₃⁼), fluoride (F^-), and humic acids (HAs) at concentrations found often in surface and well waters with ferric ion and H₂O₂ concentrations of 0.3 and 9.7 and 15.2 mg L^-1 respectively, both pollutants exhibited a strong degradation keeping the circumneutral pH. Amoxicillin and diuron degradation byproducts found by HPLC/MS were compatible with HO^• and/or CO₃^-• radical attack. Several photo-induced processes such as photo-Fenton (by dissolved ferric-HA complexes), heterogeneous photocatalysis (by colloidal iron), UV-B H₂O₂ photolysis, irradiated-dissolved organic matter, and their reactions with pollutants would be the main oxidative route responsible of degradations. These findings demonstrated that it could be possible using iron concentrations often found in natural waters to oxidize via photo-Fenton processes among other events, organic pollutants at natural pH conditions.

Effect of dissolved organic matters and inorganic ions on TiO2 photocatalysis of diclofenac: mechanistic study and degradation pathways

Diclofenac (DCF) exists extensively in sewage treatment plant effluent, and it is one of the most reported environmental pharmaceutical contaminants. In this work, the photocatalytic degradation of DCF by titanium dioxide (TiO₂) in pure water under visible light and the influence of humic acids (HA) (as a kind of dissolved organic matter (DOM), phosphate and ferrous ion (Fe²⁺)) were investigated. The results showed that the lower the pH was, the better the degradation effect of DCF under acidic conditions was. Different concentrations of DOM, phosphate ion, and Fe²⁺ could inhibit the degradation of DCF, and the higher the concentration was, the stronger the inhibition was. Different concentrations of chloride ions had little effect on the degradation. A slight elimination (8-12.9%) of total organic carbon (TOC) was observed during the mineralization of DCF with and without DOM and inorganic ions, indicating poor mineralization during the process of photocatalytic degradation, and DOM, phosphate, and Fe²⁺ had little effect on DCF mineralization. Furthermore, hydroxyl radicals, superoxide radicals, and singlet oxygen radicals were present during the photocatalytic degradation of DCF. DOM and inorganic ions could inhibit the intensity of hydroxyl radical and promote superoxide radicals (O₂^-) and singlet oxygen (¹O₂) to varying degrees. Finally, the degradation mechanism and main products were analyzed by liquid chromatography-mass spectrometry (LC-MS), and nine possible intermediates were detected. Hydroxylation, dechlorination, cyclization, and oxidation were the main degradation mechanisms. However, DOM, phosphate, and Fe²⁺ did not affect the type of intermediate products in terms of the mass-to-charge ratio. This paper mainly studied the mechanisms of different influencing factors in simulated environments to provide a theoretical basis for the degradation of DCF in wastewater treatment plants. Graphical abstract.

Photo-Fenton degradation of a herbicide (2,4-D) in groundwater for conditions of natural pH and presence of inorganic anions

The effects of four inorganic anions (Cl^-, SO₄²-, HCO₃^-, NO₃^-) usually present in groundwater were investigated on the photo-Fenton degradation of 2,4-dichlorophenoxyacetic acid (2,4-D). A kinetic model derived from a reaction sequence is proposed using the ferrioxalate complex as iron source at pH close to natural conditions (pH = 5). It was demonstrated that oxalate not only maintained iron in solution for the natural groundwater system, but also increased the photochemical activation of the process. Results showed that the minimum conversion of 2,4-D for the simulated groundwater after 180 min was 63.80%. This value was only 14.1% lower than the conversion achieved without anions. However, with all anions together, the consumption of hydrogen peroxide (HP) per mole of herbicide showed an increase with respect to the test without anions. Only one kinetic parameter was estimated for each anion applying a nonlinear regression method. Subsequently, these optimized kinetic constants were used to simulate the system behaviour, considering the influence of all the studied anions together. A good agreement between kinetic model predictions and experimental data was observed, with the following errors: RMSE_2,4-D = 3.98 × 10^-3 mM, RMSE_HP = 1.83 × 10^-1 mM, RMSE_OX = 1.39 × 10^-2 mM, and RMSE_2,4-DCP = 5.59 × 10^-3  mM.

Abatement of 2,4-D by H2O2 solar photolysis and solar photo-Fenton-like process with minute Fe(III) concentrations

The Photo-Fenton-like (PF-like) process with minute Fe(III) concentrations and the Hydrogen Peroxide Photolysis (HPP), using Xe-lamp or solar light as sources of irradiation, were efficiently applied to eliminate the herbicide 2,4-D from water. PF-like experiments concerning ferric and H₂O₂ concentrations of 0.6 mg L^-1 and 20 mg L^-1 respectively, using Xenon lamps (Xe-lamps) as a source of irradiation and 2,4-D concentrations of 10 mg L^-1 at pH 3.6, exhibited complete 2,4-D degradation and 77% dissolved organic carbon (DOC) removal after 30 min and 6 h of irradiation respectively whereas HPP (in absence of ferric ions) experiments showed a 2,4-D reduction and DOC removal of 90% and 7% respectively after 6 h of irradiation. At pH 7.0, HPP process achieved a 2,4-D abatement of approximately 75% and a DOC removal of 4% after 6 h. PF-like exhibited slightly improved 2,4-D and DOC removals (80% and 12% respectively) after the same irradiation time probably due to the low pH reduction (from 7.0 to 5.6). Several chlorinated-aromatic intermediates were identified by HPLC-MS. These by-products were efficiently removed by PF at pH 3.6, whereas at neutral PF-like and acid or neutral HPP, they were not efficiently degraded. With natural solar light irradiation, 10 and 1 mg L^-1 of 2,4-D were abated using minor H₂O₂ concentrations (3, 6, 10 and 20 mg L^-1) and iron at 0.6 mg L^-1 in Milli-Q water. Similar results to Xe-lamp experiments were obtained, where solar UV-B + A light H₂O₂ photolysis (HPSP) and solar photo-Fenton-like (SPF-like) played an important role and even at low H₂O₂ and ferric concentrations of 3 and 0.6 mg L^-1 respectively, 2,4-D was efficiently removed at pH 3.6. Simulated surface water at pH 3.6 containing 1 mg L^-1 2,4-D, 20 mg L^-1 H₂O₂ and 0.6 mg L^-1 Fe(III) under natural sunlight irradiation efficiently removed the herbicide and its main metabolite 2,4-DCP after 30 min of treatment while at neutral pH, 40% of herbicide degradation was achieved. In the case of very low iron concentrations (0.05 mg L^-1) at acid pH, 150 min of solar treatment was required to remove 2,4-D.