JP5777075B2 - Cleaning method for arsenic contaminated soil - Google Patents
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本発明は汚染土壌の洗浄処理技術に係わり、特に自然由来の低濃度砒素汚染土壌を対象とする洗浄方法に関する。 The present invention relates to a technique for cleaning contaminated soil, and more particularly, to a cleaning method for low-concentration arsenic contaminated soil derived from nature.
自然由来の重金属による土壌汚染は鉱床地帯を通る山岳トンネルにおいて従来より問題とされていたが、最近では市街地において海成堆積物から溶出する重金属、特に砒素による土壌汚染の問題が顕在化している。
このような自然由来の砒素による市街地での汚染濃度は低濃度ではあるものの、溶出量が０.０１５〜０.０３ｍｇ／Ｌ程度と環境基準値（０.０１ｍｇ／Ｌ）を大きく超過していることが問題であり、浄化処理が必要である。
Soil contamination with naturally-occurring heavy metals has been a problem in mountain tunnels that pass through ore deposits, but recently, in urban areas, soil contamination due to heavy metals, particularly arsenic, eluted from marine sediments has become apparent.
Although the concentration of such natural arsenic contamination in urban areas is low, the elution amount is about 0.015 to 0.03 mg / L, which greatly exceeds the environmental standard value (0.01 mg / L). This is a problem, and purification treatment is necessary.
汚染土壌中の環境汚染物質（重金属類、鉱物油、シアンなど）は、砂分や礫分などの粗粒子分よりも土壌有機物（腐植質）や粘土・シルトなどの細粒子分に多く吸着・保持されていることから、その浄化に際しては粗粒子分と細粒子分とを分離・除去することが効率的な処理を行ううえで有利である。 Environmental pollutants (heavy metals, mineral oil, cyanide, etc.) in contaminated soil are more adsorbed by fine particles such as soil organic matter (humus) and clay / silt than coarse particles such as sand and gravel. Therefore, it is advantageous to separate and remove coarse particles and fine particles from the viewpoint of efficient treatment.
たとえば特許文献１に開示されているような土壌洗浄方法は、汚染土壌をスラリーとして汚染物質を吸着している細粒子分をハイドロサイクロンにより分離したうえで、その細粒子分を含む懸濁液に凝集剤を添加・攪拌して凝集沈澱処理し、凝集スラッジを脱水して濃縮汚染土（脱水ケーキ）として処分するものである。
この場合、ハイドロサイクロンは土壌スラリーを分級する上で便利で処理能力の高い装置であるが、分級点が２０〜３０μｍよりも小さいハイドロサイクロンではサイズが小さいため処理能力が低く、したがって大量の土壌スラリーを処理するためには非常に多くのサイクロンとポンプを必要とするため現実的ではないことから、土壌洗浄の分野においては分級点を２０〜３０μｍ程度とすることが限界であって通常は分級点が６３〜１２５μｍのハイドロサイクロンが一般に用いられる。
For example, in the soil washing method disclosed in Patent Document 1, a fine particle component adsorbing a pollutant is separated by a hydrocyclone using a contaminated soil as a slurry, and then a suspension containing the fine particle component. The coagulant is added and stirred to coagulate and settle, and the coagulated sludge is dehydrated and disposed as concentrated contaminated soil (dehydrated cake).
In this case, the hydrocyclone is a device that is convenient for classifying the soil slurry and has a high processing capacity. However, a hydrocyclone having a classification point smaller than 20 to 30 μm has a small processing capacity because of its small size, and thus a large amount of soil slurry. Since it requires a large number of cyclones and pumps to treat the wastewater, it is not realistic, so in the field of soil washing, it is the limit that the classification point is about 20-30 μm. A hydrocyclone having a diameter of 63 to 125 μm is generally used.
また、特許文献２に示される汚染土壌の浄化方法は、重金属類やシアン等の有害物質に汚染された土壌を対象として、抽出剤を添加し、混合機で混練し、浸出液で浸出し、固液分離することにより有害物質を除去するものであるが、この場合もドラムウオッシャ、振動篩、ハイドロサイクロン、スパイラル分級機等を用いて１５０μｍ前後で分級し、粒径１５０μｍ以上の粒子を浄化処理するようにしている。 In addition, the method for purifying contaminated soil disclosed in Patent Document 2 is for soil contaminated with hazardous substances such as heavy metals and cyan, adding an extractant, kneading with a mixer, leaching with a leachate, In this case, harmful substances are removed by liquid separation. In this case as well, classification is performed at around 150 μm using a drum washer, vibrating sieve, hydrocyclone, spiral classifier, etc., and the particles having a particle size of 150 μm or more are purified. I am doing so.
上記のように汚染土壌を分級したうえで洗浄処理する場合においては、たとえ低濃度の汚染土壌であっても分級した細粒子分の全てを最終処分する必要があるから、処分量および処分コストを削減するためには分級点を可及的に小さくしたいという要請がある。
すなわち、日本各地の土壌には６３μｍ以下の細粒子分が乾燥比重比で２５〜４０％程度含まれていることが通常であり、粘土・シルト分を多く含む地域では細粒子分が土壌全体の５０〜６０％を示す場合もあることから、上記従来の洗浄方法において分級点を６３〜１２５μｍないし１５０μｍ程度に設定した場合には実質的に汚染土壌全体の大半を処理し処分しなければならないことになり、そのために多大なコストを要するものとなる。
When the contaminated soil is classified and washed as described above, it is necessary to finally dispose of all classified fine particles even in the case of low-concentration contaminated soil. In order to reduce it, there is a request to make the classification point as small as possible.
In other words, it is normal for soil in various parts of Japan to contain fine particles of 63 μm or less in a dry specific gravity ratio of about 25 to 40%. In regions containing a large amount of clay and silt, fine particles are contained in the entire soil. Since it may indicate 50-60%, when the classification point is set to about 63-125 μm to 150 μm in the above conventional cleaning method, it is necessary to treat and dispose of most of the contaminated soil substantially. Therefore, a large cost is required for that.
そこで、分級点をたとえば５〜１０μｍ程度にまで小さくし、それにより分級した微細粒子を対象として処理・処分するようにすれば、処分土量や処分コストを大幅に削減することが可能であるが、上述したように従来のハイドロサイクロンを始めとする一般的な分級手段では処理効率を低下させることなく分級点を６３〜１２５μｍよりも充分に小さくすることは困難であることから、それを可能とする有効適切な手段の開発が望まれているのが実状である。 Therefore, if the classification point is reduced to, for example, about 5 to 10 μm and the fine particles thus classified are processed and disposed of, the amount of disposed soil and disposal cost can be greatly reduced. As described above, it is difficult to make the classification point sufficiently smaller than 63 to 125 μm without lowering the processing efficiency with the conventional classification means including the conventional hydrocyclone. In fact, the development of effective and appropriate means is desired.
上記事情に鑑み、本発明は砒素による汚染土壌を洗浄処理するに際して、処理対象の汚染土壌をスラリーとし、該スラリーにＮａＯＨを添加してｐＨ９〜１２の条件化で攪拌するアルカリ抽出により、土粒子に吸着されている砒素を砒酸あるいは亜砒酸のオキソ陰イオンからなる溶存態としてスラリー中に抽出するアルカリ抽出工程を備え、前記アルカリ抽出工程の後、スラリーに無機凝集剤の添加を省略しながら高分子凝集剤を添加することで、スラリー中の５〜１０μｍ未満の微細粒子分以外の粒子分を凝集させてフロックを形成したうえで、前記フロックを沈降分離して中和安定化処理することで浄化土を得るとともに、微細粒子分および溶存態としての砒素を含む上澄水を、さらに無機凝集剤と高分子凝集剤とを併用する凝集沈澱処理をして、その凝集スラッジを脱水処理することにより砒素を含む濃縮汚染土を得ることを特徴とする。 In view of the above circumstances, in the present invention, when soil contaminated with arsenic is washed, soil particles are obtained by alkaline extraction in which contaminated soil to be treated is made into a slurry and NaOH is added to the slurry and stirred under conditions of pH 9-12. An alkali extraction step of extracting arsenic adsorbed on the slurry as a dissolved state composed of oxoanions of arsenic acid or arsenous acid into the slurry, and after the alkali extraction step, the addition of an inorganic flocculant to the slurry is omitted. By adding a flocculant, particles other than fine particles less than 5 to 10 μm in the slurry are agglomerated to form flocs, and then the flocs are settled and separated to neutralize and stabilize. In addition to obtaining soil, supernatant water containing fine particles and dissolved arsenic, and a coagulation-precipitation treatment using an inorganic coagulant and a polymer coagulant in combination. The was characterized by obtaining concentrated contaminated soil containing arsenic by dehydrating the aggregation sludge.
本発明では、ハイドロサイクロン等の物理的手法では達成が困難である５〜１０μｍ未満の微細粒子分の効率的な分離を化学的な手法である凝集沈澱法により分離するものであり、特に通常の凝集沈澱法の一部を省略して敢えて不完全な凝集沈澱処理を意図的に行うことによって５〜１０μｍ以上の土粒子を選択的にフロックとして沈降分離することにより５〜１０μｍ未満の微細粒子の分級を可能としたものである。
そして本発明では、汚染土壌からアルカリ抽出により砒素を溶存態としてスラリー中に抽出したうえで、上記の分級を行って５〜１０μｍ未満の微細粒子からなる濃縮汚染土を得ることにより、従来一般の洗浄方法においては濃縮汚染土に含まれてしまうことから処分せざるを得ない５〜１０μｍ以上の土粒子を再利用可能な浄化土として回収可能であり、したがって濃縮汚染土の発生量を大幅に低減し得てその処分に要するコストを大幅に削減することが可能である。
In the present invention, efficient separation of fine particles of less than 5 to 10 μm, which is difficult to achieve with a physical method such as hydrocyclone, is separated by a coagulation precipitation method that is a chemical method. By omitting a part of the coagulation precipitation method and intentionally carrying out an incomplete coagulation precipitation process, the soil particles of 5 to 10 μm or more are selectively settled and separated as flocs, so that fine particles of less than 5 to 10 μm Classification is possible.
And in this invention, after extracting arsenic from a contaminated soil by alkali extraction as a dissolved state in a slurry, the above classification is performed to obtain a concentrated contaminated soil consisting of fine particles of less than 5 to 10 μm. In the cleaning method, soil particles that have to be disposed of because they are contained in the concentrated contaminated soil can be collected as reusable purified soil, thus greatly increasing the amount of concentrated contaminated soil generated. It can be reduced and the cost required for disposal can be greatly reduced.
本発明の実施形態を説明するに先立ち、本発明が対象としている自然由来の砒素汚染土壌からの砒素の溶出機構と砒素汚染土壌のロードカーブ（砒素全含有量と砒素溶出量の分布）の特性について説明する。
（a）自然由来砒素の溶出機構
（１）ｐＨ７〜８で酸化環境にある通常の水中では、砒素はヒドロ砒酸イオン（ＨＡｓＯ４ ２−）などのオキソ陰イオンとして存在しており、水酸化鉄や粘土鉱物の微粒子に吸着される。
（２）砒素は鉄やアルミニウムなどの水酸化物、天然酸化鉱物、粘土鉱物に吸着されやすい性質がある。その吸着はｐＨに大きく依存し、低ｐＨ（酸性）であるほど高く、ｐＨの増加に従い減少する。酸化物のＺＰＣ（Ｚｅｒｏ Ｐｏｉｎｔ ｏｆ Ｃｈａｒｇｅ）より低いｐＨ条件においては表面が正（プラス）に帯電し、負の電荷を有する陰イオン種（砒酸イオンなど）は正に帯電した表面に引き寄せられる。逆に、高いｐＨ条件では表面が負（マイナス）に帯電するため、陰イオン種は脱着する。
（３）黄鉄鋼(ＦｅＳ２)に微量成分として含有される砒素は、酸化的な（酸素に富んだ）表層水や浅層地下水との接触により黄鉄鋼が酸化分解されることで溶出する。
（４）水酸化鉄に吸着した砒素は、堆積物とともに沈澱し底質中に固定される。この水酸化鉄が還元的な（酸素を含まない）地下水と接触すると、水酸化鉄の沈殿物は不安定となって分解する。この過程で水酸化鉄に吸着（固定）されていた砒素も可溶化して溶出する。
本発明が対象としている市街地の砒素溶出は上記の（１）と（２）のメカニズムで起きる場合が多く、そのような市街地の砒素汚染土壌に対しては吸着されている砒素をアルカリ抽出法により抽出することで浄化することができる。
（ｂ）自然由来の砒素汚染土壌のロードカーブ
自然由来の砒素汚染土壌のロードカーブ（含有量（底質調査法）と溶出量の両方）を図３に示す。この場合、土壌全体の含有量値は６.６ｍｇ／ｋｇであるが、図３（ａ）より粒子径が小さいほど含有量値が高いことが認められる。また、図３（ｂ）から溶出量値についても同様の傾向が認められる。したがって粗粒子分よりも細粒子分、特に微細粒子分を選択的に処理することで効率的な処理が可能である。
Prior to the description of the embodiments of the present invention, the arsenic elution mechanism from the natural arsenic-contaminated soil targeted by the present invention and the characteristics of the arsenic-contaminated soil load curve (distribution of total arsenic content and arsenic elution amount) Will be described.
(A) Elution mechanism of naturally derived arsenic (1) In normal water in an oxidizing environment at pH 7-8, arsenic exists as an oxoanion such as hydroarsenate ion (HAsO 4 2− ), and iron hydroxide And adsorbed on fine particles of clay minerals.
(2) Arsenic tends to be adsorbed by hydroxides such as iron and aluminum, natural oxide minerals, and clay minerals. Its adsorption is highly dependent on pH, it is higher at lower pH (acidic) and decreases with increasing pH. Under a pH condition lower than that of oxide ZPC (Zero Point of Charge), the surface is positively charged (positive), and negatively charged anionic species (such as arsenate ions) are attracted to the positively charged surface. On the contrary, since the surface is negatively (minus) charged under high pH conditions, the anionic species are desorbed.
(3) Arsenic contained as a trace component in pyrite steel (FeS 2 ) is eluted by oxidative decomposition of pyrite steel by contact with oxidative (oxygen-rich) surface water or shallow groundwater.
(4) Arsenic adsorbed on iron hydroxide precipitates with the sediment and is fixed in the sediment. When this iron hydroxide comes into contact with reductive (oxygen-free) groundwater, the iron hydroxide precipitate becomes unstable and decomposes. In this process, arsenic adsorbed (fixed) to iron hydroxide is also solubilized and eluted.
In many cases, arsenic elution in urban areas targeted by the present invention occurs by the mechanisms (1) and (2) described above, and arsenic adsorbed on arsenic-contaminated soil in such urban areas is obtained by alkali extraction. It can be purified by extraction.
(B) Road curve of naturally derived arsenic contaminated soil FIG. 3 shows the load curve (both content (sediment survey method) and elution amount) of naturally derived arsenic contaminated soil. In this case, although the content value of the whole soil is 6.6 mg / kg, it is recognized from FIG. 3A that the content value is higher as the particle size is smaller. Moreover, the same tendency is recognized also about the elution amount value from FIG.3 (b). Therefore, efficient processing is possible by selectively processing fine particles, particularly fine particles, over coarse particles.
本発明は、上記のような自然由来の砒素の溶出機構についての知見に基づき、低濃度の砒素汚染土壌を「アルカリ抽出法」と後述する特殊な凝集沈殿法（以下、「不完全凝集沈澱法」と称す）により洗浄処理することを主眼とする。
本発明における「不完全凝集沈澱法」は、ハイドロサイクロン等の物理的手法では達成が困難である５〜１０μｍ未満の微細粒子分の効率的な分離を化学的な手法である凝集沈澱法により可能とするものであるが、通常の凝集沈澱法の一部を省略して敢えて不完全な凝集沈澱処理を意図的に行うようにしたものである。
The present invention is based on the knowledge about the elution mechanism of naturally derived arsenic as described above, and a low concentration arsenic contaminated soil is referred to as “alkaline extraction method” and a special coagulation precipitation method (hereinafter referred to as “incomplete coagulation precipitation method”). The main purpose is cleaning treatment.
The “incomplete flocculation precipitation method” in the present invention enables efficient separation of fine particles of less than 5 to 10 μm, which is difficult to achieve with a physical method such as hydrocyclone, by the flocculation precipitation method which is a chemical method. However, a part of the ordinary coagulation precipitation method is omitted, and an incomplete coagulation precipitation process is intentionally performed.
本発明における「不完全凝集沈殿法」を説明するに先立ち、まず図４を参照して通常の凝集沈澱法について説明する。
水中の粒子が凝集せずに安定に分散している理由は、粒子表面は一般に負の電荷を帯びており、粒子どうしの荷電の反発が生じるため、および粒子周囲に存在するイオンや溶存分子が接近を阻害しているためである。
したがって、凝集作用によって大きな粒子に成長させるためには、粒子の表面電荷を反対電荷によって中和して静電的な反発を弱め、また吸着しやすい官能基（吸着基）をもった高分子凝集剤で粒子間を架橋することが必要であり、そのため通常の凝集沈澱法においては図４に示すように無機凝集剤（ＰＡＣ、硫酸バンドなど）と高分子凝集剤とを併用して凝集沈澱処理を行うことが必要である。
すなわち、無機凝集剤は粒子表面の荷電中和の作用が強く、同時に粒子や溶解物への吸着作用があるので、小さい粒子をもれなく集めるのに適しているが、強度の弱い小フロックしか形成できない。そこで、その小フロックに、さらに分子量が大きくアミド基やカルボキシル基などの吸着基をもったアニオン性や非イオン性の高分子凝集剤を添加することで、小フロックは架橋して強固な大フロックとなり、沈降が促進される。
Prior to explaining the “incomplete coagulation precipitation method” in the present invention, first, an ordinary coagulation precipitation method will be explained with reference to FIG.
The reason why particles in water are stably dispersed without agglomeration is that the particle surface is generally negatively charged and repulsion between the particles occurs, and ions and dissolved molecules present around the particles This is because access is blocked.
Therefore, in order to grow into large particles by aggregating action, the surface charge of the particles is neutralized by the opposite charge to weaken the electrostatic repulsion, and the polymer agglomerates with functional groups (adsorptive groups) that are easy to adsorb It is necessary to cross-link the particles with an agent. Therefore, in the ordinary coagulation precipitation method, as shown in FIG. 4, coagulation precipitation treatment is performed by using an inorganic coagulant (PAC, sulfate band, etc.) and a polymer coagulant together. It is necessary to do.
In other words, the inorganic flocculant has a strong charge neutralizing action on the particle surface and at the same time adsorbs to the particles and dissolved matter, so it is suitable for collecting all the small particles, but can form only small flocs with low strength. . Therefore, by adding an anionic or nonionic polymer flocculant having a larger molecular weight and an adsorbing group such as an amide group or a carboxyl group to the small flocs, the small flocs are cross-linked and become strong large flocs. And sedimentation is promoted.
このように通常の凝集沈澱法では無機凝集剤と高分子凝集剤とを併用することによって大フロックを形成して凝集沈澱作用を得るのであるが、この場合は微細粒子も含めて全ての粒子が大フロックとなって沈降して沈澱汚泥となるから、５〜１０μｍの微細粒子のみを選択的に分離することはできない。 In this way, in the ordinary coagulation precipitation method, a combination of an inorganic coagulant and a polymer coagulant forms a large floc to obtain an aggregation coagulation action. In this case, all particles including fine particles are dispersed. Since it becomes a large floc and settles to become a precipitated sludge, only fine particles of 5 to 10 μm cannot be selectively separated.
そこで本発明では、図２に示すように、無機凝集剤の添加を省略して高分子凝集剤のみを添加するに留めて、上述のように不完全な凝集沈澱処理を意図的に行う。
すなわち、無機凝集剤を添加せずに高分子凝集剤のみで凝集させる場合には凝集可能な粒子径は自ずと限界があり、５〜６μｍ以上の比較的大径の粒子どうしは従来と同様に大フロックを形成して沈降して沈澱汚泥となるが、５〜６μｍ未満の微細粒子に対しては高分子凝集剤の架橋作用が働きにくく、フロックを形成しないか、あるいはフロックを形成してもごく微細なものでしかなく、これらは沈降速度が小さいために浮遊しながらオーバーフローとして流出させることができる。
したがって、大フロックとなって沈降する沈澱汚泥（アンダーフロー）と上澄水（オーバーフロー）とを分離することが可能であり、それにより５〜６μｍ未満の微細粒子分とそれよりも大きな細粒子分とを分離できることになる。但し、現実的には上澄水を溢水流としてオーバーフローさせて回収することになるから、水平方向に流れる溢水流に伴われて１０μｍ程度の微細粒子も回収されるため、実際の分級点はやや大きくなって図中に示しているように５〜１０μｍ程度となる。
Therefore, in the present invention, as shown in FIG. 2, the addition of the inorganic flocculant is omitted and only the polymer flocculant is added, and the incomplete flocculant precipitation treatment is intentionally performed as described above.
That is, in the case of agglomerating only with the polymer aggregating agent without adding the inorganic aggregating agent, the particle size that can be agglomerated naturally has a limit. Forms flocs and settles to form precipitated sludge. However, the cross-linking action of the polymer flocculant is difficult to work on fine particles of less than 5 to 6 μm, and no flocs are formed, or even flocs are formed. These are only fine, and since they have a low sedimentation rate, they can flow out as an overflow while floating.
Therefore, it is possible to separate the sedimented sludge (underflow) and the supernatant water (overflow) that settle down as large flocs, thereby reducing the fine particle content of less than 5 to 6 μm and the fine particle content larger than that. Can be separated. However, since the supernatant water is actually overflowed and collected as an overflow stream, fine particles of about 10 μm are also collected along with the overflow stream flowing in the horizontal direction, so the actual classification point is slightly larger. As shown in the figure, it becomes about 5 to 10 μm.
なお、上記のような不完全凝集による微細粒子の分級・分離が可能となるためには、無機凝集剤の添加を省略することを前提として、高分子凝集剤の添加量およびｐＨを適正に調整する必要があるが、高分子凝集剤の添加量は通常の凝集沈澱法の場合に比べて少なくて良く、またｐＨは通常の凝集沈澱法の適正ｐＨと異なるｐＨ領域で凝集を行うように調整すると良い。
たとえば、高分子凝集剤の添加量は通常の凝集沈澱法において適正とされる量の１／２〜２／３程度で良い。また、高分子凝集剤としてアニオン系ポリマーを用いる場合における通常の凝集沈澱法ではｐＨ＝７〜９とすることが一般的であるが、本発明ではｐＨ＝９〜１１とすることが好ましい。また、高分子凝集剤の添加量およびｐＨの調整と関連づけて上記のように分級点が５〜１０μｍとなるようにオーバーフローの溢流速度を適切に設定すれば良い。
In order to be able to classify and separate fine particles by incomplete aggregation as described above, the addition amount and pH of the polymer flocculant are adjusted appropriately on the premise that the addition of the inorganic flocculant is omitted. However, the amount of the polymer flocculant added may be smaller than that of the normal coagulation precipitation method, and the pH is adjusted so that the coagulation is performed in a pH range different from the appropriate pH of the normal coagulation precipitation method. Good.
For example, the addition amount of the polymer flocculant may be about 1/2 to 2/3 of the amount that is considered appropriate in a normal coagulation precipitation method. Further, in the usual aggregation precipitation method in the case of using an anionic polymer as the polymer flocculant, the pH is generally 7 to 9, but in the present invention, the pH is preferably 9 to 11. Further, the overflow rate may be appropriately set so that the classification point is 5 to 10 μm as described above in association with the adjustment of the addition amount of the polymer flocculant and the pH.
本発明は、上記の「不完全凝集沈殿法」の前段でアルカリ抽出法を実施することを要旨とし、それによって自然由来の低濃度の砒素汚染土壌を効率的に洗浄浄化することが可能なものであり、その具体的な実施形態を図１に示す。 The gist of the present invention is that the alkali extraction method is carried out before the above-mentioned “incomplete coagulation precipitation method”, whereby it is possible to efficiently clean and purify naturally-derived low-concentration arsenic-contaminated soil. A specific embodiment thereof is shown in FIG.
処理対象の汚染土壌に水を加え、２ｍｍ以上の粗粒子分を予め除去し解泥したスラリーにＮａＯＨを添加し、ｐＨ９〜１２の条件化で攪拌することによって「アルカリ抽出法」を実施する。これにより、土壌から砒素が砒酸あるいは亜砒酸などのオキソ陰イオンの形態で抽出される。 The “alkali extraction method” is carried out by adding water to the contaminated soil to be treated, adding NaOH to the slurry obtained by removing coarse particles of 2 mm or more in advance, and stirring the slurry at a pH of 9-12. As a result, arsenic is extracted from the soil in the form of oxoanions such as arsenic acid or arsenous acid.
そこで、その砒酸が抽出された土壌スラリーに対して上記の「不完全凝集沈殿法」を実施する。これにより、５〜１０μｍから２ｍｍの土壌粒子（細粒子分と砂分）は大きなフロックを形成し、大きな沈降速度で沈殿槽底部に沈澱するから、それを引き抜いて中和・安定化処理し、脱水することによって浄化土が得られる。この浄化土は土壌環境基準を満足することから再利用が可能であり、処分の必要はない。 Therefore, the “incomplete coagulation precipitation method” is performed on the soil slurry from which the arsenic acid has been extracted. As a result, soil particles (fine particles and sand) of 5 to 10 μm to 2 mm form large flocs and settle at the bottom of the sedimentation tank at a large sedimentation rate, so that they are extracted and neutralized and stabilized. Purified soil is obtained by dehydration. This purified soil can be reused because it satisfies the soil environmental standards and does not require disposal.
一方、不完全凝集によりオーバーフローとして分離された上澄水には、５〜１０μｍ未満の微細粒子、小さな凝集フロック、および抽出された溶存態の砒素が含まれるから、このオーバーフローについてはさらに通常の凝集沈殿法（無機凝集剤を使用するいわば完全凝集沈殿法）を実施し、５〜１０μｍ未満の微細粒子、小さな凝集フロック、および抽出された溶存態の砒素を全て凝集スラッジとして水中から分離し、それをプレスして脱水する。これにより高濃度の砒素を含む濃縮汚染土が得られるから、これは外部処分する。 On the other hand, the supernatant water separated as an overflow by incomplete flocculation contains fine particles of less than 5 to 10 μm, small flocculated flocs, and extracted dissolved arsenic. (The so-called complete coagulation precipitation method using an inorganic coagulant) is carried out, and fine particles smaller than 5 to 10 μm, small coagulation flocs, and extracted dissolved arsenic are all separated from water as coagulated sludge, Press to dehydrate. As a result, a concentrated contaminated soil containing a high concentration of arsenic is obtained.
本発明によれば、従来においては濃縮汚染土に含まれてしまうことから処分せざるを得ない大量の５〜１０μｍ以上の粒子分を再利用可能な浄化土として回収可能であり、したがって濃縮汚染土の発生量を大幅に低減し得てその処分に要するコストを大幅に削減することが可能である。
一実験例によれば、ハイドロサイクロンにより６３〜１２５μｍで分級していた従来法による場合には処理土壌全体の３５％を濃縮汚染土として処分する必要があったが、上記の不完全凝集沈澱法により５〜１０μｍで分級した本発明による場合には濃縮汚染土を５％未満にまで低減させることができ、従来法に比べて濃縮汚染土量を１／８〜１／５程度に低減できることが確認されている。
According to the present invention, a large amount of particles having a size of 5 to 10 μm or more that must be disposed of because it is conventionally contained in the concentrated contaminated soil can be recovered as reusable purified soil, and thus concentrated contaminated. The amount of soil generated can be greatly reduced, and the cost required for disposal can be greatly reduced.
According to one experimental example, it was necessary to dispose of 35% of the entire treated soil as concentrated contaminated soil in the case of the conventional method in which the hydrocyclone was classified at 63 to 125 μm. In the case of the present invention classified by 5 to 10 μm, the concentrated contaminated soil can be reduced to less than 5%, and the amount of concentrated contaminated soil can be reduced to about 1/8 to 1/5 compared with the conventional method. It has been confirmed.
また、図３に示されるように砒素の含有量や溶出量は粒子径が小さい成分ほど高いことから、本発明のように微細粒子分を選択的に処理することで従来法に比べてより効率的な処理が可能である。
しかも、不完全凝集沈澱法においては５〜１０μｍから２ｍｍまでの土壌粒子が大きなフロックを形成するため、フロックの沈降速度が非常に大きく、そのため沈殿槽の面積を小さくできるし、その際には自ずと大きな圧密がかかって固形分率が高い（約５０％ｄｓ）堆積物となり、後段の脱水処理が容易である。
さらに、不完全凝集沈澱法によるオーバーフローに対しては後段で無機凝集剤（ＰＡＣや硫酸バンドなど）を使用した通常の凝集沈殿法を行うので、その際にはアルカリ助剤が必要となるが、不完全凝集沈殿法の前段のアルカリ抽出工程で添加するＮａＯＨはそのアルカリ助剤として有効利用できるので無駄になることはない。
以上のことから、本発明の洗浄方法は「アルカリ抽出法」と「不完全凝集沈澱法」を有機的に組み合わせたことで自然由来の低濃度の砒素汚染土壌を効率的に処理可能なものであり、極めて合理的であり有効である。
In addition, as shown in FIG. 3, the arsenic content and elution amount are higher as the particle size is smaller, so that it is more efficient than the conventional method by selectively processing the fine particles as in the present invention. Processing is possible.
In addition, in the incomplete flocculation precipitation method, soil particles from 5 to 10 μm to 2 mm form large flocs, so the floc settling speed is very large, so the area of the precipitation tank can be reduced, and in that case A large compaction is applied to form a deposit having a high solid content (about 50% ds), and subsequent dehydration is easy.
Furthermore, since an ordinary coagulation precipitation method using an inorganic coagulant (PAC, sulfuric acid band, etc.) is performed in the latter stage against overflow due to the incomplete coagulation precipitation method, an alkaline auxiliary is required in that case. Since NaOH added in the alkali extraction step preceding the incomplete coagulation precipitation method can be effectively used as the alkali auxiliary agent, it is not wasted.
From the above, the cleaning method of the present invention can efficiently treat low-concentration arsenic-contaminated soil derived from nature by organically combining the “alkali extraction method” and the “incomplete flocculation precipitation method”. Yes, very reasonable and effective.
Claims (1)
処理対象の汚染土壌をスラリーとし、該スラリーにＮａＯＨを添加してｐＨ９〜１２の条件化で攪拌するアルカリ抽出により、土粒子に吸着されている砒素を砒酸あるいは亜砒酸のオキソ陰イオンからなる溶存態としてスラリー中に抽出するアルカリ抽出工程を備え、
前記アルカリ抽出工程の後、スラリーに無機凝集剤の添加を省略しながら高分子凝集剤を添加することで、スラリー中の５〜１０μｍ未満の微細粒子分以外の粒子分を凝集させてフロックを形成したうえで、
前記フロックを沈降分離して中和安定化処理することで浄化土を得るとともに、微細粒子分および溶存態としての砒素を含む上澄水を、さらに無機凝集剤と高分子凝集剤とを併用する凝集沈澱処理をして、その凝集スラッジを脱水処理することにより砒素を含む濃縮汚染土を得ることを特徴とする砒素汚染土壌の洗浄方法。 A method for cleaning soil contaminated with arsenic,
Dissolved arsenic or arsenous acid oxo-anions are converted into arsenic adsorbed on the soil particles by alkaline extraction in which the contaminated soil to be treated is made into slurry and NaOH is added to the slurry and stirred under conditions of pH 9-12. As an alkali extraction step to extract into the slurry as
After the alkali extraction step, by adding a polymer flocculant while omitting the addition of an inorganic flocculant to the slurry, particles other than fine particles less than 5-10 μm in the slurry are agglomerated to form a flock. And then
The floc is settled and separated and neutralized and stabilized to obtain purified soil, and the supernatant water containing fine particles and arsenic as a dissolved state is further agglomerated using an inorganic flocculant and a polymer flocculant in combination. A method for cleaning arsenic-contaminated soil, characterized in that a concentrated contaminated soil containing arsenic is obtained by performing a precipitation treatment and dewatering the aggregated sludge.
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