JP2004121993A

JP2004121993A - Method of cleaning soil

Info

Publication number: JP2004121993A
Application number: JP2002290188A
Authority: JP
Inventors: Shintaro Nakaie; 仲家　新太郎; Tokio Kamoshita; 鴨下　時男; Masahiro Tokuda; 徳田　昌弘; Takeshi Sakurai; 櫻井　健
Original assignee: Mitsubishi Materials Corp
Current assignee: Mitsubishi Materials Corp
Priority date: 2002-10-02
Filing date: 2002-10-02
Publication date: 2004-04-22

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of cleaning soil which has a high removal efficiency of heavy metal from a soil contaminated with heavy metal and realizes an inexpensive treatment of the soil contaminated with the heavy metal by employing a low-cost post-treatment method with a minimum scale treatment as the post-treatment of the soil and treated water after the heavy metal removal. <P>SOLUTION: Water, halogenated substances of calcium, magnesium and strontium and an acid are added to the soil contaminated with the heavy metal to obtain a weakly acidic aqueous solution and the heavy metal is eluted and removed. The halogenated substances of calcium, magnesium and strontium are made to be an aqueous solution thereof having a concentration of 0.5∼1.5mol/l and the quantity of the aqueous solution 5∼10 times in weight ratio for the soil contaminated with the heavy metal is used. The cleaned soil after the heavy metal elution is recyclable after pH adjustment and desalting. The aqueous solution containing eluted heavy metal is disposed of after being rendered harmless by well-known treatment. <P>COPYRIGHT: (C)2004,JPO

Description

【０００１】
【発明の属する技術分野】
この発明は、重金属で汚染された土壌の浄化方法に関するものである。
【０００２】
【従来の技術】
従来、鉛、亜鉛、ヒ素、カドミウム、六価クロム、セレン、水銀、アンチモン、銅などの重金属で汚染された土壌の浄化方法としては、例えば重金属で汚染された土壌を流動化溶液で洗浄し、先ず大粒子を機械的に水洗分離し、次いで汚染物質とともに微粒子を流動化溶液で中程度粒子から分離し、さらに中程度粒子をアトリッション研磨して付着微粒子を脱落させ、得られた微粒子を中程度粒子から分離し、重金属を微粒子として捕捉する分級法が知られている（例えば、特許文献１参照）。
あるいはまた、分級と化学的抽出を併用する土壌浄化法として、分級法によって得られた重金属を含む微粒子を、酸、アルカリあるいはキレート剤等の薬品を用いて処理する方法も知られている（例えば、非特許文献１参照）。
さらに、重金属で汚染された土壌に対して水と塩酸、硫酸、硝酸あるいは燐酸等の強酸を加え攪拌し、これに鉄粉を加えて強酸性下で土壌中の重金属を鉄粉に担持させて土壌から分離する方法が知られている（例えば、特許文献２参照）
【０００３】
【特許文献１】
特開平６−３４３９４８号公報
【特許文献２】
特開２０００−５１８３５号公報
【非特許文献１】
「Ｈａｚａｒｄｏｕｓ　Ｗａｓｔｅ　Ｒｅｍｅｄｉａｔｉｏｎ」，Ｔｅｃｈｎｏｍｉｃ　Ｐｕｂｌｉｃａｔｉｏｎ　Ｃｏｍｐａｎｙ　Ｉｎｃ．，１９９５，Ｐ．１０３−Ｐ．１１２
【０００４】
【発明が解決しようとする課題】
しかしながら、上記の分級による方法は、土壌と重金属物質の存在粒度の偏りによって土壌を浄化するため、広範囲の土壌粒度にわたって重金属汚染されている土壌や微粒子を多く含む土壌に対しては、除去効率が極めて低くなる欠点がある。
また、化学抽出による方法では、土壌からの重金属の除去効率は高くなるものの、重金属抽出後に土壌中に残留する抽出剤を除去する付加工程が必要となる上に、重金属を移行させた抽出溶液の無害化処理に大きなコストがかかるため、土壌の浄化処理コストの増大を招くこととなる難点がある。
さらに、強酸性下で重金属汚染土壌を処理する方法は、処理後の浄化土壌や水溶液を中和する処理が必要となり、酸性が強いために中和剤の必要量が多く、処理コストが増大する難点がある。
従って、本発明の目的は重金属汚染土壌を浄化処理するにあたり、土壌からの重金属の除去効率が高く、重金属除去後の土壌及び処理水の後処理において、なるべく薬品コストのかからない後処理方法を採用することにより、重金属汚染土壌処理のコスト負担を軽減することを目的とする。
【０００５】
【課題を解決するための手段】
上記課題を解決するために、本発明の土壌の浄化方法は、重金属で汚染された土壌を浄化する方法であって、重金属で汚染された土壌に水と、カルシウム（Ｃａ）やマグネシウム（Ｍｇ）やストロンチウム（Ｓｒ）のハロゲン化物と、酸とを加えた後、弱酸性の水溶液にして重金属を溶出除去する土壌の浄化方法を採用した。
このような方法とすることにより、ハロゲン化物イオンの錯体形成作用により、弱酸性下でも重金属の錯化合物を形成するので、重金属は溶液中に効
率良く溶出して土壌から分離除去され、しかも重金属を含んだ水溶液は弱酸性なので土壌処理後の中和処理をするコストを大幅に節減することが可能となる。
【０００６】
本発明の土壌の浄化方法では、重金属イオンに対して錯体形成作用があり、かつ毒性の低いイオンを含む塩類を重金属の抽出剤として使用する。このような抽出剤としては、カルシウムやマグネシウムやストロンチウムのハロゲン化物が利用できる。特に、塩化カルシウムや塩化マグネシウムは、特に有害なイオンを含まず、且つ比較的安価に入手できるので好んで利用できる。
【０００７】
本発明の土壌の浄化方法では、前記ハロゲン化物の濃度が増加するに伴い重金属の除去率も上昇するが、０．５〜１．５モル／リットルで最大の除去率に達し、ハロゲン化物濃度をそれ以上増加しても、除去率は増加しない。よって、前記ハロゲン化物を０．５〜１．５モル／リットル、より好ましくは１モル／リットルの濃度の水溶液として加えるのが好ましい。前記ハロゲン化物は水に可溶であり、水は重金属物質移動媒体として作用するので、重金属の溶出が一層容易となるからである。
また、本発明の土壌の浄化方法では、前記ハロゲン化物の水溶液を、重量比で土壌１に対して５〜１０倍添加するのが好ましい。
汚染土壌中の重金属濃度は通常１００〜１０００ｍｇ／ｋｇ程度であり、この程度の濃度の重金属と効率よく反応させて錯体化合物を生成させるためである。水溶液を使用すれば重金属がイオンとして溶出し、土壌から効率よく分離できる利点がある。水溶液に溶出した重金属イオンは公知の手段を利用して無害化処理を行えば良い。
水溶液の添加量は処理すべき汚染土壌の粒子径にもよるが、モルタル状ないしはスラリー状にして十分攪拌接触させるようにするためである。
【０００８】
さらに、本発明の土壌の浄化方法では、前記ハロゲン化物を加えた後の水溶液のｐＨを、３ないし５の弱酸性とするのが好ましい。水溶液の酸性度が低いほど、処理後の土壌及び処理水の中和処理にかかる負担が軽減されて有利となるからである。
本発明の土壌の浄化方法では、土壌中の重金属を酸性の水溶液中にイオンとして溶出させ、ハロゲン化物イオンの錯体形成作用を利用して、水溶液中に溶出した重金属イオンから錯化合物を生成する反応を促進させ、弱酸性下でも十分水溶液中に溶出した重金属イオンを補足することが可能である。
【０００９】
さらに、本発明の土壌の浄化方法では、重金属を弱酸性の水溶液中に溶出させた後、浄化土壌を沈澱させ、次いで固液分離を行って土壌から重金属を分離除去する方法を採用する。
弱酸性下で土壌中の重金属は水溶液中に溶出し、重金属が溶出した後の土壌は静置することにより沈降し、容易に固液分離することができる。重金属を含んだ弱酸性の水溶液は、公知の水処理方法を利用して重金属の無害化処理をすれば良い。
【００１０】
【発明の実施の形態】
本発明は、鉛、亜鉛、ヒ素、カドミウム、六価クロム、セレン、水銀、アンチモン、銅などの重金属で汚染された土壌を、化学抽出により浄化する方法に関するものである。本発明の土壌の浄化方法の基本フローを図１に示す。
本発明の土壌の浄化方法は、重金属で汚染された土壌に水と、カルシウムやマグネシウムやストロンチウムのハロゲン化物と、酸とを加えて混合した後、ｐＨ３〜５の弱酸性水溶液にして重金属を水溶液中に溶出させて除去し、浄化された土壌を沈澱させて固液分離を行って土壌から重金属を分離除去する方法である。浄化された土壌はｐＨ調整、洗浄による脱塩の後、埋め戻して再利用することも可能である。また、重金属を含んだ水溶液は、別途公知の無害化処理を施し、重金属を除去した後ｐＨ調整すれば、排水基準を満たしたものとなり排水可能となる。
本発明の方法によれば、化学抽出法を採用することにより効果的に重金属を除去することが可能となり、しかも処理工程中はｐＨ値が弱酸性となる領域で処理するので、処理後の土壌や排水のｐＨ調整に要するコストを低く抑えることができる利点を有する。
【００１１】
対象となる汚染土壌は通常局所的に埋設されているので、掘り出した後夾雑物を除去して適当な粒径に整粒する前処理を行っておくのが好ましい。
先ず、汚染土壌に水と重金属イオンに対して錯体形成作用を有し、かつ毒性の低い薬剤と酸とを加えて攪拌混合して、重金属の溶出を行う。
重金属イオンに対して錯体形成作用を有する化合物としては、カルシウムやマグネシウムやストロンチウムのハロゲン化物が挙げられる。前記ハロゲン化物のうち無害なものとしては、塩化カルシウム（ＣａＣｌ_２）や塩化マグネシウム（ＭｇＣｌ_２）が挙げられる。これらのハロゲン化物は、最大の除去率で重金属を除去するために、予め０．５〜１．５モル／リットル、より好ましくは１．０モル／リットルの濃度の水溶液としておくのが好ましい。これらのハロゲン化物は水に可溶であり、水は重金属を移動させる物質移動媒体として作用するからである。
汚染土壌中の重金属濃度は通常１００〜１０００ｍｇ／ｋｇ程度であり、この程度の濃度の重金属を水溶液を使用して重金属イオンとして水溶液中に溶出させ、錯体化合物を生成させて重金属イオンの溶出を促進させる。錯体化合物となった重金属は公知の手段を利用して無害化処理を行えば良い。
【００１２】
汚染土壌はカルシウムやマグネシウムやストロンチウムのハロゲン化物を含む弱酸性の水溶液中で良く攪拌混合して、重金属の付着した土壌粒子と水溶液とを充分に接触させることが好ましい。このため前記ハロゲン化物の水溶液は、重量比で土壌１に対して５〜１０倍添加するのが好ましい。土壌粒子が数ミリメートルより大きな砂礫状の場合には、ミキサーやブレンダー等を用いて攪拌するので水分を少なめに、重量比で土壌１に対して５〜７倍程度添加するのが好ましい。また、土壌粒子が数ミリメートルより小さな砂質又は粘土質の場合には、水の添加量はスラリーとして扱えるように、重量比で土壌１に対して８〜１０倍程度添加するのが好ましい。
【００１３】
汚染土壌に添加したカルシウムやマグネシウムやストロンチウムのハロゲン化物の水溶液は、酸を加えてｐＨが３〜５の弱酸性になるように調整する。酸はｐＨを３〜５の弱酸性に調整するためのもので特に制限はなく、塩酸、硫酸、硝酸、燐酸、酢酸等が使用できる。このような弱酸性の条件下では、土壌中の重金属が前記ハロゲン化物の水溶液中に溶け易く、土壌中からの溶出が容易となるからである。前記ハロゲン化物の弱酸性水溶液を添加した汚染土壌は、良く攪拌して重金属イオンの溶出を促進させるのが好ましい。
【００１４】
攪拌手段は汚染土壌の粒子径や添加するハロゲン化物の水溶液量等を考慮して、各種ミキサー、ブレンダー、ニーダーあるいはアジテーター等から適宜選択すればよい。
攪拌時間にも特に制限はなく、汚染土壌中の重金属が十分溶出すれば良く、例えば１時間ないし数時間攪拌するのが好ましい。
【００１５】
重金属を充分溶出させて浄化した土壌は、沈澱槽で静置して土壌を沈降させて固液分離を行う。静置時間は土壌の粒子径等を考慮して適宜決定すればよい。
静置して沈降した後の沈澱槽の澄水を抜き取れば、浄化された土壌を分離することができる。この浄化土壌は弱酸性の水分を含んでいるので、アルカリ成分を加えてほぼ中性になるまでｐＨ調整をする。アルカリ成分には特に制限はないが、消石灰、苦土石灰、石灰石、ドロマイト等のアルカリ性の薬剤や小砕石を使用することができる。
この際、浄化土壌のｐＨはせいぜい３〜４程度の弱酸性なので、ｐＨ調整に要するアルカリの使用量はごく少なくて良い。従ってｐＨ調整のコストも最少限に抑えることができる。
【００１６】
以上のような浄化処理を施した土壌は、有害な重金属がほとんど取り除かれて浄化されており、ｐＨ調整も施されているのでこの土壌を脱塩することにより、全く無害となり、人体への影響はもちろんのこと農作物への影響も無く、埋め戻し用土壌として再利用することが可能となる。
【００１７】
一方、重金属を錯体化合物として含んだ弱酸性のハロゲン化物水溶液は、公知の廃水処理技術を使用して、重金属を水に不溶な化合物として固定し、固液分離して水中から取り除く。水中の重金属の除去方法は特に制限するものではなく、重金属の種類や濃度に従って、公知の技術を適宜選択して使用すればよい。
これら公知技術としては、例えば化学的還元法、光電気化学的還元法、電解還元法等の還元法や、酸化法、吸着法、イオン交換法あるいは生物処理方法等が利用できる。これらの手段を利用して水に不溶性の沈澱物とし、濾過分離して有害物として最終処理する。
【００１８】
重金属を除去した後の澄液は、弱酸性を示すのでほぼ中性にｐＨ調整をすれば、浄水として排水処分することができる。
この際、浄水のｐＨはせいぜい３〜４程度の弱酸性なので、ｐＨ調整に要するアルカリ成分の使用量はごく少なくて良い。従ってｐＨ調整のコストも最少限に抑えることができる。ｐＨ調整には消石灰のような弱アルカリの他に、苛性ソーダのような強アルカリを使用することも可能である。いずれにしても使用量はわずかで良く、処理コストが嵩むこともない。
【００１９】
【実施例】
次に、実施例と比較例を挙げて本発明を説明する。
（実施例１）
３５３０ｍｇ／ｋｇのカドミウム（Ｃｄ）を含む汚染土壌に、濃度１モル／リットルの塩化カルシウム（ＣａＣｌ_２）溶液を重量比で１０倍加えてスラリー状とし、さらに硫酸（Ｈ_２ＳＯ_４）を加えてこのスラリーのｐＨを３に調整し、ミキサー中でゆっくりと１時間攪拌した。その後スラリーを２時間放置し、土壌を沈澱させ上澄水を分離した。このような処理をした土壌のカドミウム含有量を測定したところ表１に示すように、５２５ｍｇ／ｋｇと除去率は８５％以上に達していた。
【００２０】
（実施例２）
３５３０ｍｇ／ｋｇのカドミウム（Ｃｄ）を含む汚染土壌に、濃度１モル／リットルの塩化カルシウム（ＣａＣｌ_２）溶液を重量比で１０倍加えてスラリー状とし、さらに塩酸（ＨＣｌ）を加えてこのスラリーのｐＨを３〜５に調整し、ミキサー中でゆっくりと１〜５時間攪拌した。その後スラリーを２〜５時間放置し、土壌を沈澱させ澄水を分離した。なお、攪拌時間や放置時間等は、適切な反応性が得られるように適宜調整すればよい。なお、ｐＨを３〜５とすることによりアルカリ消費量が少なくなる。ｐＨを２とするとＣｄ除去率は９０％となるがアルカリ消費量が多くなる。
このような処理をした土壌のカドミウム含有量を測定したところ表１に示すように５４０ｍｇ／ｋｇ程度となり、除去率は７１．０〜８５．１％を推移することが確認できた。
なお、硝酸、燐酸等、他の酸を用いた場合についても、攪拌時間や放置時間等の条件について適切な反応性が得られるように適宜調整し、上記実施例と同様に処理すれば、上記実施例と同様に良好な浄化結果が得られる。
【００２１】
【表１】

【００２２】
次いで、上記のようにして浄化した土壌に消石灰（Ｃａ（ＯＨ）_２）を重量比で１．２％添加し、ｐＨを６．５に調整した。また、土壌分離後の上澄水には苛性ソーダ（ＮａＯＨ）を重量比で０．１％添加し、ｐＨを６．５に調整した。ｐＨ調整に必要なアルカリ量を表２に示す。
【００２３】
【表２】

【００２４】
（比較例１）
比較のため、実施例と同じ３５３０ｍｇ／ｋｇのカドミウムを含む土壌に、塩化カルシウムを使用せずに、汚染土壌に水と硫酸だけを加えてスラリーのｐＨを３に調整し、ミキサー中でゆっくりと１時間攪拌した。その後スラリーを２時間放置し、土壌を沈澱させ上澄水を分離した。このような処理をした土壌のカドミウム含有量を測定したところ、土壌のカドミウム含有量は１４３０ｍｇ／ｋｇで、除去率は約５９％であった。土壌中のカドミウム含有量の変化を表１に併記する。
また、上記のように処理した土壌のｐＨを消石灰（Ｃａ（ＯＨ）_２）を使用して６．５に調整するには、重量比で１．０％添加する必要があった。また、苛性ソーダを使用して土壌分離後の上澄水のｐＨを６．５に調整するには、重量比で０．１％を添加する必要があった。ｐＨ調整に必要なアルカリ量を表２に併記する。
【００２５】
（比較例２）
実施例と同じ汚染土壌に、重量比で１０倍の濃度１規定の硫酸水を加えて、ミキサー中でゆっくりと１時間攪拌した。その後スラリーを２時間放置し、土壌を沈澱させ上澄水を分離した。このような処理をした土壌のカドミウム含有量を測定したところ、土壌のカドミウム含有量は３８７ｍｇ／ｋｇで、除去率は８９％に達していた。土壌中のカドミウム含有量の変化を表１に併記する。
また、上記のように処理した土壌のｐＨを消石灰（Ｃａ（ＯＨ）_２）を使用して６．５に調整するには、重量比で３．７％添加する必要があった。また、苛性ソーダを使用して土壌分離後の上澄水のｐＨを６．５に調整するには、重量比で０．６％を添加する必要があった。ｐＨ調整に必要なアルカリ量を表２に併記する。
【００２６】
実施例１，実施例２及び比較例１、比較例２から明らかなとおり、重金属汚染土壌を弱酸性下でカルシウムやマグネシウムやストロンチウムのハロゲン化物水溶液を使用することにより、従来の強酸性下で土壌を洗浄した場合とほぼ同等の重金属除去効果が得られ、しかも弱酸性下で処理をするので、浄化後の土壌及び処理水のｐＨを調整するのに要するアルカリ物質の使用量が少なくてすみ、処理コストを大幅に削減することが可能となる。
【００２７】
【発明の効果】
以上詳細に説明したように、本発明の土壌の浄化方法によれば、重金属汚染土壌を弱酸性にした上でカルシウムやマグネシウムやストロンチウムのハロゲン化物を使用することにより、土壌中の重金属を９０％近く除去することが可能となり、しかも弱酸性下で処理をするので、浄化後の土壌及び処理水のｐＨを調整するのに要するアルカリ物質の使用量が少なくてすみ、処理コストを大幅に削減することが可能となる。
【図面の簡単な説明】
【図１】本発明の土壌の浄化方法の基本フローを示す図である。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for purifying soil contaminated with heavy metals.
[0002]
[Prior art]
Conventionally, as a method of purifying soil contaminated with heavy metals such as lead, zinc, arsenic, cadmium, hexavalent chromium, selenium, mercury, antimony, and copper, for example, washing soil contaminated with heavy metals with a fluidizing solution, First, the large particles are mechanically washed and separated by water, then the fine particles together with the contaminants are separated from the medium particles by a fluidizing solution, and the medium particles are subjected to attrition polishing to remove the attached fine particles. There is known a classification method in which particles are separated from particles and heavy metals are captured as fine particles (for example, see Patent Document 1).
Alternatively, as a soil purification method using both classification and chemical extraction, a method of treating fine particles containing a heavy metal obtained by the classification method with a chemical such as an acid, an alkali, or a chelating agent is also known (for example, , Non-Patent Document 1).
In addition, water and a strong acid such as hydrochloric acid, sulfuric acid, nitric acid or phosphoric acid are added to the soil contaminated with heavy metals and stirred, and iron powder is added thereto, and heavy metals in the soil are supported on iron powder under strong acidity. A method of separating from soil is known (for example, see Patent Document 2).
[0003]
[Patent Document 1]
JP-A-6-343948 [Patent Document 2]
Japanese Patent Application Laid-Open No. 2000-51835 [Non-Patent Document 1]
"Hazardous Waste Remediation," Technological Publication Company Inc. , 1995, p. 103-P. 112
[0004]
[Problems to be solved by the invention]
However, the method based on the classification described above purifies the soil by biasing the existing particle size of the soil and heavy metal substances, so that the removal efficiency is high for soils that are contaminated with heavy metals and soils that are rich in fine particles over a wide range of soil particle sizes. There is a disadvantage that it is extremely low.
In addition, in the method using chemical extraction, although the removal efficiency of heavy metals from soil is increased, an additional step of removing the extractant remaining in soil after heavy metal extraction is required, and the extraction solution in which heavy metals have been transferred is required. Since a large cost is required for the detoxification treatment, there is a problem that the cost of the soil purification treatment is increased.
Furthermore, the method for treating heavy metal-contaminated soil under strong acid requires a treatment for neutralizing the purified soil and the aqueous solution after the treatment. Since the acidity is strong, the required amount of the neutralizing agent is large, and the treatment cost is increased. There are difficulties.
Accordingly, an object of the present invention is to purify a soil contaminated with heavy metals, in which the efficiency of removing heavy metals from soil is high, and in the post-treatment of soil and treated water after heavy metal removal, a post-treatment method that does not require as much chemical cost is adopted. This aims to reduce the cost burden of treating heavy metal contaminated soil.
[0005]
[Means for Solving the Problems]
In order to solve the above-mentioned problems, a soil purification method of the present invention is a method of purifying soil contaminated with heavy metals, wherein water, calcium (Ca) and magnesium (Mg) are added to soil contaminated with heavy metals. After adding a halide of strontium and strontium (Sr) and an acid, a method of purifying soil was adopted in which a weakly acidic aqueous solution was used to elute and remove heavy metals.
By adopting such a method, a complex compound of a heavy metal is formed even under a weak acid by a complex forming action of a halide ion, so that the heavy metal is efficiently eluted into the solution and separated and removed from the soil, and further, the heavy metal is removed. Since the contained aqueous solution is weakly acidic, the cost of neutralization treatment after soil treatment can be greatly reduced.
[0006]
In the method for purifying soil of the present invention, salts containing ions having a complex forming action with respect to heavy metal ions and having low toxicity are used as heavy metal extractants. As such an extractant, halides of calcium, magnesium and strontium can be used. In particular, calcium chloride and magnesium chloride do not contain particularly harmful ions and can be preferably used because they can be obtained at relatively low cost.
[0007]
In the soil purification method of the present invention, the removal rate of heavy metals also increases as the halide concentration increases. However, the maximum removal rate is reached at 0.5 to 1.5 mol / L, and the halide concentration is reduced. Any further increase does not increase the removal rate. Therefore, it is preferable to add the halide as an aqueous solution having a concentration of 0.5 to 1.5 mol / l, more preferably 1 mol / l. This is because the halide is soluble in water, and water acts as a heavy metal mass transfer medium, so that the elution of heavy metals is further facilitated.
Further, in the soil purification method of the present invention, it is preferable that the aqueous solution of the halide is added 5 to 10 times in weight ratio to 1 of the soil.
The concentration of heavy metals in the contaminated soil is usually about 100 to 1000 mg / kg, and this is because a complex compound is formed by efficiently reacting with heavy metals at such a concentration. Use of an aqueous solution has the advantage that heavy metals are eluted as ions and can be efficiently separated from soil. The heavy metal ions eluted in the aqueous solution may be subjected to detoxification treatment using known means.
Although the amount of the aqueous solution to be added depends on the particle size of the contaminated soil to be treated, it is in the form of a mortar or a slurry to allow sufficient contact with stirring.
[0008]
Further, in the soil purification method of the present invention, it is preferable that the pH of the aqueous solution after the addition of the halide is set to a weak acidity of 3 to 5. This is because the lower the acidity of the aqueous solution, the less the burden on the neutralization treatment of the soil and treated water after treatment becomes, which is advantageous.
In the soil purification method of the present invention, the heavy metal in the soil is eluted into the acidic aqueous solution as an ion, and a complex compound is formed from the heavy metal ion eluted in the aqueous solution by utilizing the complex forming action of the halide ion. It is possible to sufficiently capture heavy metal ions eluted in an aqueous solution even under weak acidity.
[0009]
Further, the soil purification method of the present invention employs a method in which heavy metals are eluted into a weakly acidic aqueous solution, the purified soil is precipitated, and then solid-liquid separation is performed to separate and remove the heavy metals from the soil.
Under weakly acidic conditions, heavy metals in the soil elute into the aqueous solution, and after the heavy metals elute, the soil sediments by standing, and can be easily separated into solid and liquid. The weakly acidic aqueous solution containing a heavy metal may be subjected to detoxification treatment of the heavy metal using a known water treatment method.
[0010]
BEST MODE FOR CARRYING OUT THE INVENTION
The present invention relates to a method for purifying soil contaminated with heavy metals such as lead, zinc, arsenic, cadmium, hexavalent chromium, selenium, mercury, antimony, and copper by chemical extraction. FIG. 1 shows a basic flow of the soil purification method of the present invention.
The method for purifying soil according to the present invention comprises the steps of: adding water, a halide of calcium, magnesium, or strontium, and an acid to soil contaminated with heavy metals; This is a method in which heavy metals are separated and removed from the soil by performing solid-liquid separation by precipitating the purified soil by eluting it into the soil and performing solid-liquid separation. The purified soil can be reused by re-filling after pH adjustment and desalination by washing. In addition, the aqueous solution containing heavy metals is subjected to a separately known detoxification treatment, and if the pH is adjusted after removing the heavy metals, the aqueous solution satisfies the drainage standard and can be drained.
According to the method of the present invention, it is possible to effectively remove heavy metals by employing the chemical extraction method, and since the treatment is performed in a region where the pH value is weakly acidic during the treatment step, the soil after treatment is treated. It has the advantage that the cost required for pH adjustment of wastewater and wastewater can be kept low.
[0011]
Since the target contaminated soil is usually buried locally, it is preferable to carry out a pretreatment for removing contaminants after digging and sizing to an appropriate particle size.
First, a contaminated soil is mixed with water and a heavy metal ion and has a low toxicity and an acid, and the mixture is stirred and mixed to elute the heavy metal.
Examples of the compound having a complex forming action on heavy metal ions include halides of calcium, magnesium, and strontium. Harmless ones among the halides include calcium chloride (CaCl ₂ ) and magnesium chloride (MgCl ₂ ). These halides are preferably prepared in advance as an aqueous solution having a concentration of 0.5 to 1.5 mol / l, more preferably 1.0 mol / l, in order to remove heavy metals at the maximum removal rate. This is because these halides are soluble in water and water acts as a mass transfer medium for transferring heavy metals.
The concentration of heavy metals in contaminated soil is usually about 100 to 1000 mg / kg, and heavy metals having such a concentration are eluted into aqueous solution as heavy metal ions using an aqueous solution to form a complex compound to promote elution of heavy metal ions. Let it. The heavy metal that has become the complex compound may be subjected to detoxification treatment by using a known means.
[0012]
It is preferable that the contaminated soil is mixed well with a weakly acidic aqueous solution containing a halide of calcium, magnesium, or strontium so that the soil particles to which the heavy metal is attached sufficiently contact the aqueous solution. For this reason, it is preferable to add the aqueous solution of the halide in an amount of 5 to 10 times the weight of soil 1 by weight. When the soil particles are in the form of gravel larger than several millimeters, it is preferable to add a small amount of water to the soil 1 by a factor of 5 to 7 in a weight ratio, since the mixture is stirred using a mixer or a blender. In addition, when the soil particles are sandy or clayey smaller than several millimeters, the amount of water is preferably added about 8 to 10 times the weight of soil 1 so as to be handled as a slurry.
[0013]
The aqueous solution of the halide of calcium, magnesium or strontium added to the contaminated soil is adjusted to a weak acidity of pH 3 to 5 by adding an acid. The acid is used to adjust the pH to a weak acidity of 3 to 5 and is not particularly limited, and hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, acetic acid, and the like can be used. Under such weakly acidic conditions, heavy metals in the soil are easily dissolved in the aqueous solution of the halide, and are easily eluted from the soil. The contaminated soil to which the weakly acidic aqueous solution of the halide is added is preferably stirred well to promote the elution of heavy metal ions.
[0014]
The stirring means may be appropriately selected from various mixers, blenders, kneaders, agitators and the like in consideration of the particle size of the contaminated soil, the amount of the aqueous halide solution to be added, and the like.
The stirring time is not particularly limited as long as the heavy metal in the contaminated soil can be sufficiently eluted. For example, stirring is preferably performed for 1 hour to several hours.
[0015]
The soil that has been sufficiently eluted and purified is left standing in a sedimentation tank to settle the soil and perform solid-liquid separation. The standing time may be appropriately determined in consideration of the soil particle size and the like.
If the clear water of the sedimentation tank after settling by standing still is extracted, purified soil can be separated. Since the purified soil contains weakly acidic water, the pH is adjusted until it becomes almost neutral by adding an alkaline component. Although there is no particular limitation on the alkali component, alkaline chemicals such as slaked lime, formic lime, limestone, dolomite and small crushed stones can be used.
At this time, the pH of the purified soil is at most about 3 to 4 slightly acidic, so that the amount of alkali used for pH adjustment may be very small. Therefore, the cost of pH adjustment can be minimized.
[0016]
The soil that has been subjected to the above purification treatment has been purified by removing most of the harmful heavy metals, and has been subjected to pH adjustment, so by desalinating this soil, it is completely harmless and has no effect on the human body. Of course, there is no effect on crops, and it can be reused as backfill soil.
[0017]
On the other hand, a weakly acidic halide aqueous solution containing a heavy metal as a complex compound is fixed using a known wastewater treatment technique as a compound insoluble in water, and is separated from water by solid-liquid separation. The method for removing heavy metals in water is not particularly limited, and a known technique may be appropriately selected and used according to the type and concentration of heavy metals.
As these known techniques, for example, reduction methods such as a chemical reduction method, a photoelectrochemical reduction method, and an electrolytic reduction method, an oxidation method, an adsorption method, an ion exchange method, and a biological treatment method can be used. Using these means, a water-insoluble precipitate is formed, separated by filtration and finally treated as a harmful substance.
[0018]
The supernatant from which the heavy metals have been removed shows a weak acidity, so if the pH is adjusted to almost neutral, it can be disposed of as wastewater as purified water.
At this time, since the pH of the purified water is weakly acidic, at most, about 3 to 4, the amount of the alkali component required for pH adjustment may be very small. Therefore, the cost of pH adjustment can be minimized. For pH adjustment, in addition to a weak alkali such as slaked lime, a strong alkali such as caustic soda can be used. In any case, the amount used is small and the processing cost does not increase.
[0019]
【Example】
Next, the present invention will be described with reference to Examples and Comparative Examples.
(Example 1)
To a contaminated soil containing 3530 mg / kg of cadmium (Cd), a calcium chloride (CaCl ₂ ) solution having a concentration of 1 mol / liter was added 10 times by weight to make a slurry, and sulfuric acid (H ₂ SO ₄ ) was further added. The pH of this slurry was adjusted to 3 and stirred slowly for 1 hour in a mixer. Thereafter, the slurry was left for 2 hours to settle the soil and separate the supernatant water. When the cadmium content of the soil subjected to such treatment was measured, as shown in Table 1, it was 525 mg / kg, and the removal rate reached 85% or more.
[0020]
(Example 2)
To a contaminated soil containing 3530 mg / kg of cadmium (Cd), a calcium chloride (CaCl ₂ ) solution having a concentration of 1 mol / liter is added 10 times by weight to form a slurry, and hydrochloric acid (HCl) is further added thereto to form a slurry. The pH was adjusted to 3-5 and stirred slowly in the mixer for 1-5 hours. Thereafter, the slurry was left for 2 to 5 hours to sediment the soil and separate the clear water. Note that the stirring time, the standing time, and the like may be appropriately adjusted so as to obtain appropriate reactivity. In addition, alkali consumption is reduced by setting the pH to 3 to 5. When the pH is 2, the Cd removal rate becomes 90%, but the alkali consumption increases.
When the cadmium content of the soil subjected to such treatment was measured, it was about 540 mg / kg as shown in Table 1, and it was confirmed that the removal rate was in the range of 71.0 to 85.1%.
In the case of using other acids such as nitric acid and phosphoric acid, the conditions such as the stirring time and the standing time are appropriately adjusted so as to obtain appropriate reactivity, and if the treatment is performed in the same manner as in the above example, the above Good purification results are obtained as in the embodiment.
[0021]
[Table 1]

[0022]
Subsequently, 1.2% by weight of slaked lime (Ca (OH) ₂ ) was added to the soil purified as described above, and the pH was adjusted to 6.5. Further, 0.1% by weight of caustic soda (NaOH) was added to the supernatant water after the soil separation to adjust the pH to 6.5. Table 2 shows the amount of alkali required for pH adjustment.
[0023]
[Table 2]

[0024]
(Comparative Example 1)
For comparison, to the same soil containing 3530 mg / kg of cadmium as in the example, the pH of the slurry was adjusted to 3 by adding only water and sulfuric acid to the contaminated soil without using calcium chloride, and slowly mixed in the mixer. Stir for 1 hour. Thereafter, the slurry was left for 2 hours to settle the soil and separate the supernatant water. When the cadmium content of the soil subjected to such treatment was measured, the cadmium content of the soil was 1,430 mg / kg, and the removal rate was about 59%. Table 1 also shows the change in cadmium content in soil.
Further, in order to adjust the pH of the soil treated as described above to 6.5 using slaked lime (Ca (OH) ₂ ), it was necessary to add 1.0% by weight. Further, in order to adjust the pH of the supernatant water after soil separation to 6.5 using caustic soda, it was necessary to add 0.1% by weight. Table 2 also shows the amount of alkali required for pH adjustment.
[0025]
(Comparative Example 2)
To the same contaminated soil as in the example, a 1N sulfuric acid solution having a concentration 10 times by weight was added, and the mixture was slowly stirred in a mixer for 1 hour. Thereafter, the slurry was left for 2 hours to settle the soil and separate the supernatant water. When the cadmium content of the soil subjected to such treatment was measured, the cadmium content of the soil was 387 mg / kg, and the removal rate reached 89%. Table 1 also shows the change in cadmium content in soil.
Further, to adjust the pH of the soil treated as described above to 6.5 using slaked lime (Ca (OH) ₂ ), it was necessary to add 3.7% by weight. Further, in order to adjust the pH of the supernatant water after soil separation to 6.5 using caustic soda, it was necessary to add 0.6% by weight. Table 2 also shows the amount of alkali required for pH adjustment.
[0026]
As is clear from Examples 1 and 2 and Comparative Examples 1 and 2, heavy soils contaminated with heavy metals were soiled under a conventional strong acidity by using an aqueous halide solution of calcium, magnesium or strontium under weak acidity. It is possible to obtain almost the same heavy metal removal effect as when washing, and because the treatment is carried out under weak acidity, the amount of alkali substances required for adjusting the pH of the soil and treated water after purification is small, Processing costs can be greatly reduced.
[0027]
【The invention's effect】
As described above in detail, according to the soil purification method of the present invention, the heavy metal contaminated soil is made weakly acidic, and calcium, magnesium and strontium halides are used. Since it is possible to remove it nearer, and because the treatment is performed under weak acidity, the amount of alkaline substances required for adjusting the pH of the soil and treated water after purification is small, and the treatment cost is greatly reduced. It becomes possible.
[Brief description of the drawings]
FIG. 1 is a diagram showing a basic flow of a soil purification method of the present invention.

Claims

A method for purifying soil contaminated with heavy metals, comprising adding one or more halides of Sr, Mg, Ca and an acid to soil contaminated with heavy metals to form a weakly acidic aqueous solution. A method for purifying soil, comprising eluting and removing heavy metals from an acidic aqueous solution.

The soil purification method according to claim 1, wherein the halide is calcium chloride or magnesium chloride.

The soil purification method according to claim 1 or 2, wherein the halide is added as an aqueous solution having a concentration of 0.5 to 1.5 mol / liter.

The soil purification method according to any one of claims 1 to 3, wherein the halide aqueous solution is added in a weight ratio of 5 to 10 times the soil 1.

The soil purification method according to any one of claims 1 to 4, wherein the pH of the aqueous solution containing the contaminated soil after the addition of the halide is set to a weak acidity of 3 to 5.

6. The heavy metal is eluted into a weakly acidic aqueous solution, the purified soil is precipitated, and then solid-liquid separation is performed to separate and remove the heavy metal from the soil. 3. The method for purifying soil according to item 1.